II BSC ECS
SUBJECT NAME: IC’S AND INSTRUMENTATION
UNIT I IC FABRICATION TECHNOLOGY
Fundamentals of Monolithic IC technology – Basic planar process – Wafer preparation – Epitaxial growth
– Oxidation – Photolithography – Diffusion of impurities – Isolation techniques – Metallization – Monolithic
transistors –Integrated resistors – Integrated capacitors- integrated Inductors- Thin and Thick film technology
UNIT II TIMER AND PLL
Functional block diagram of 555 timer – Monostable operation – Applications: – Linear ramp generator –
Pulse width modulator – Astable operation – Applications: Schemitt trigger – FSK Generator
Phase locked loop: Functional block diagram – Phase detector / Comparator –Voltage Controlled Oscillator
– Low pass filter – Applications: Frequency multiplier/Division – AM detection
UNIT III OPERATIONAL AMPLIFIER
Inverting and non inverting amplifier – Op-amp parameters – Summing Amplifier – Differential Amplifier
– Integrator – Differentiator – Instrumentation Amplifier – Voltage to current converter – Current to Voltage
converter – Precision half wave rectifiers – Precision full wave rectifiers.
UNIT IV TRANSDUCERS
Introduction – Electrical Transducer – Basic requirements of Transducer – Classification of transducers –
selection of transducers – resistive transducers – potentiometers – Thermistors – Thermocouple – LVDT – RVDT –
Piezoelectric transducers – hall effect transducers – Photoelectric transducers – digital displacement transducers.
UNIT V ELECTRONIC INSTRUMENTS
Q Meters- CRO: Block Diagram – cathode ray tube – Measurement of frequency – Measurement of voltage
and current – Digital Oscilloscope – digital voltmeter: Ramp type DVM – dual slope integrating type DVM –
Digital multimeter – Humidity and humidity measurement – Measurement of PH.
TEXT BOOKS
1. D.Roy Choudhury and Shahil B Jain, “Linear Integrated Circuits”, Second Edition New Age International
Publishers 2004.
2. K.R.Botkar, “Integrated Circuits”, 10 th Edition Khanna Publishers 2006.
3. J.B.GUPTA “A course in electronic and electrical measurements and instrumentation”, 12th Edition, S.K
Kataria & sons
Unit I
Section A
1. Crucible is made op quartz or graphite
2. Integrated circuits that are present in everyday electrical and electronic devices.
3. A typical wafer is made out of extremely pure silicon
4. Photolithography (or "optical lithography") is a process used in microfabrication
5. Photolithography uses light to transfer a geometric pattern from a photo mask to a lightsensitive chemical "photoresist",
Section B
1.Explain Czochralski Crystal Growth
Czochralski Crystal Growth
The Czochralski(CZ) process, which accounts for 80% to 90% of worldwide silicon
consumption, consists of dipping a small single-crystal seed into molten silicon and slowly
withdrawing the seed while rotating it simultaneously.
The crucible is usually made of quartz or graphite with a fused silica lining. After the seed is dipped into
the EGS melt, the crystal is pulled at a rate that minimizes defects and yields a constant ingot diameter.
2.Explain Epitaxial growth
Semiconductor device fabrication is the process used to create the integrated circuits that are
present in everyday electrical and electronic devices. It is a multiple-step sequence of
photographic and chemical processing steps during which electronic circuits are gradually
created on a wafer made of pure semiconducting material. Silicon is almost always used, but
various compound semiconductors are used for specialized applications.
The entire manufacturing process, from start to packaged chips ready for shipment, takes six to
eight weeks and is performed in highly specialized facilities referred to as fabs.
When feature widths were far greater than about 10 micrometres, purity was not the issue that it
is today in device manufacturing. As devices became more integrated, cleanrooms became even
cleaner. Today, the fabs are pressurized with filtered air to remove even the smallest particles,
which could come to rest on the wafers and contribute to defects. The workers in a
semiconductor fabrication facility are required to wear cleanroom suits to protect the devices
from human contamination.
In an effort to increase profits, semiconductor device manufacturing has spread from Texas and
California in the 1960s to the rest of the world, such as Europe, Middle East, and Asia. It is a
global business today.[citation needed]
3. Explain about Photomasks?
The image for the mask originates from a computerized data file. This data file is converted to a
series of polygons and written onto a square fused quartz substrate covered with a layer of
chrome using a photolithographic process. A laser beam (laser writer) or a beam of electrons (ebeam writer) is used to expose the pattern defined by the data file and travels over the surface of
the substrate in either a vector or raster scan manner. Where the photoresist on the mask is
exposed, the chrome can be etched away, leaving a clear path for the illumination light in the
stepper/scanner system to travel through.
4. Explain the steps of IC fabrication.
1. Wafer preparation
2. Epitaxial growth
3. Oxidisation
4. Photolithographic process
5. Isolation diffusion
6. Base and emitter diffusion
7. Pre-ohmic etch
8. Metallisation
9. circuit probing
10. Scribing and separating into chips
11. Mounting and packing
12. Encapsulation
5. Explain about integrated transistors and integrated diodes
Transistors and Diodes.
Monolithic IC-Transistor fabrication
Transistors and diodes are usually formed using the epitaxial planar diffusion process described in
previous blog post.Collector, base, and emitter regions are diffused into a P-type silicon substrate,as
shown in figure, and surface terminals are provided for connection. In discrete transistors the substrate
is normally employed as collector. If this were done with transistors in monolithic IC, all transistors
fabricated on one substrate would have their collectors connected together. This is the reason that
separate collector regions are diffused into the substrate. Even though separate collector regions are
formed, they are not completely isolated from the substrate. For proper functioning of the circuit it is
necessary that the P-type substrate is always kept negative with respect to the transistor collector. This
is achieved by connecting the substrate to the most negative terminal of the circuit supply.The
unwanted or parasitic junctions, even when reverse-biased, can still affect the circuit performance
adversely. The junction reverse leakage current can cause a serious problem in circuits operating at very
low current levels. The capacitance of the reverse-biased junction may affect the circuit high-frequency
performance, and the junction break down voltage imposes limits on the usable level of supply voltage.
All these adverse effects can be reduced to the minimum if highly resistive material is
employed for the substrate. If the substrate is very lightly doped, it will behave almost
as an insulator.
Monolithic IC-Diode Fabrication
Integrated circuit diodes
They are usually fabricated by diffusion exactly as transistors. Only two of the regions are used to form
one P-N junction. In figure, collector-base junction of the transistor is used as a diode. Anode of the
diode is formed during the base diffusion of the transistor and the collector region of the transistor
becomes the cathode of the diode. For high speed switching emitter base junction is used as a diode.
Monolithic IC Fabrication-Diffused resistors
6. Explain About Ion Implantation?
An ion implantation system at LAAS technological facility in Toulouse, France.
Ion implantation is a materials engineering process by which ions of a material are accelerated
in an electrical field and impacted into another solid. This process is used to change the physical,
chemical, or electrical properties of the solid. Ion implantation is used in semiconductor device
Dry etching refers to the removal of material, typically a masked pattern of semiconductor
material, by exposing the material to a bombardment of ions (usually a plasma of reactive gases
such as fluorocarbons, oxygen, chlorine, boron trichloride; sometimes with addition of nitrogen,
argon, helium and other gases) that dislodge portions of the material from the exposed surface.
Unlike with many (but not all, see isotropic etching) of the wet chemical etchants used in wet
etching, the dry etching process typically etches directionally or anisotropically.
Section C
1. Explain photolithography?
Photolithography (or "optical lithography") is a process used in microfabrication to selectively
remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern
from a photo mask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate.
A series of chemical treatments then either engraves the exposure pattern into, or enables
deposition of a new material in the desired pattern upon, the material underneath the photo resist.
In complex integrated circuits, for example a modern CMOS, a wafer will go through the
photolithographic cycle up to 50 times.
Photolithography shares some fundamental principles with photography in that the pattern in the
etching resist is created by exposing it to light, either directly (without using a mask) or with a
projected image using an optical mask. This procedure is comparable to a high precision version
of the method used to make printed circuit boards. Subsequent stages in the process have more in
common with etching than with lithographic printing. It is used because it can create extremely
small patterns (down to a few tens of nanometers in size), it affords exact control over the shape
and size of the objects it creates, and because it can create patterns over an entire surface costeffectively. Its main disadvantages are that it requires a flat substrate to start with, it is not very
effective at creating shapes that are not flat, and it can require extremely clean operating
conditions.
The wafer track portion of an aligner that uses 365 nm ultraviolet light.
A single iteration of photolithography combines several steps in sequence. Modern cleanrooms
use automated, robotic wafer track systems to coordinate the process. The procedure described
here omits some advanced treatments, such as thinning agents or edge-bead removal.[1]
Cleaning
If organic or inorganic contaminations are present on the wafer surface, they are usually removed
by wet chemical treatment, e.g. the RCA clean procedure based on solutions containing
hydrogen peroxide.
Preparation
The wafer is initially heated to a temperature sufficient to drive off any moisture that may be
present on the wafer surface. Wafers that have been in storage must be chemically cleaned to
remove contamination. A liquid or gaseous "adhesion promoter", such as
Bis(trimethylsilyl)amine ("hexamethyldisilazane", HMDS), is applied to promote adhesion of the
photoresist to the wafer. The surface layer of silicon dioxide on the wafer reacts with HMDS to
form tri-methylated silicon-dioxide, a highly water repellent layer not unlike the layer of wax on
a car's paint. This water repellent layer prevents the aqueous developer from penetrating between
the photoresist layer and the wafer's surface, thus preventing so-called lifting of small photoresist
structures in the (developing) pattern.
Photoresist application
The wafer is covered with photoresist by spin coating. A viscous, liquid solution of photoresist is
dispensed onto the wafer, and the wafer is spun rapidly to produce a uniformly thick layer. The
spin coating typically runs at 1200 to 4800 rpm for 30 to 60 seconds, and produces a layer
between 0.5 and 2.5 micrometres thick. The spin coating process results in a uniform thin layer,
usually with uniformity of within 5 to 10 nanometres. This uniformity can be explained by
detailed fluid-mechanical modelling, which shows that the resist moves much faster at the top of
the layer than at the bottom, where viscous forces bind the resist to the wafer surface. Thus, the
top layer of resist is quickly ejected from the wafer's edge while the bottom layer still creeps
slowly radially along the wafer. In this way, any 'bump' or 'ridge' of resist is removed, leaving a
very flat layer. Final thickness is also determined by the evaporation of liquid solvents from the
resist. For very small, dense features (<125 or so nm), thinner resist thicknesses (<0.5
micrometres) are needed to overcome collapse effects at high aspect ratios; typical aspect ratios
are <4:1.
The photo resist-coated wafer is then prebaked to drive off excess photoresist solvent, typically
at 90 to 100 °C for 30 to 60 seconds on a hotplate.
Exposure and developing
After prebaking, the photoresist is exposed to a pattern of intense light. Optical lithography
typically uses ultraviolet light (see below). Positive photoresist, the most common type, becomes
soluble in the basic developer when exposed; exposed negative photoresist becomes insoluble in
the (organic) developer. This chemical change allows some of the photoresist to be removed by a
special solution, called "developer" by analogy with photographic developer. To learn more
about the process of exposure and development of positive resist, see, for example: Ralph
Dammel, "Diazonaphtoquinone-based resists", SPIE Optical Engineering Press, Vol TT11
(1993).
A PEB (post-exposure bake) is performed before developing, typically to help reduce standing
wave phenomena caused by the destructive and constructive interference patterns of the incident
light. In DUV (deep ultraviolet, or shorter than 300 nm exposure wavelength) lithography, CAR
(chemically amplified resist) chemistry is used. This process is much more sensitive to PEB
time, temperature, and delay, as most of the "exposure" reaction (creating acid, making the
polymer soluble in the basic developer) actually occurs in the PEB.[2]
The develop chemistry is delivered on a spinner, much like photoresist. Developers originally
often contained sodium hydroxide (NaOH). However, sodium is considered an extremely
undesirable contaminant in MOSFET fabrication because it degrades the insulating properties of
gate oxides (specifically, sodium ions can migrate in and out of the gate, changing the threshold
voltage of the transistor and making it harder or easier to turn the transistor on over time). Metalion-free developers such as tetramethylammonium hydroxide (TMAH) are now used.
The resulting wafer is then "hard-baked" if a non-chemically amplified resist was used, typically
at 120 to 180 °C[citation needed] for 20 to 30 minutes. The hard bake solidifies the remaining
photoresist, to make a more durable protecting layer in future ion implantation, wet chemical
etching, or plasma etching.
Etching
Main article: Etching (microfabrication)
In etching, a liquid ("wet") or plasma ("dry") chemical agent removes the uppermost layer of the
substrate in the areas that are not protected by photoresist. In semiconductor fabrication, dry
etching techniques are generally used, as they can be made anisotropic, in order to avoid
significant undercutting of the photoresist pattern. This is essential when the width of the
features to be defined is similar to or less than the thickness of the material being etched (i.e.
when the aspect ratio approaches unity). Wet etch processes are generally isotropic in nature,
which is often indispensable for microelectromechanical systems, where suspended structures
must be "released" from the underlying layer.
The development of low-defectivity anisotropic dry-etch process has enabled the ever-smaller
features defined photolithographically in the resist to be transferred to the substrate material.
Photoresist removal
After a photoresist is no longer needed, it must be removed from the substrate. This usually
requires a liquid "resist stripper", which chemically alters the resist so that it no longer adheres to
the substrate. Alternatively, photoresist may be removed by a plasma containing oxygen, which
oxidizes it. This process is called ashing, and resembles dry etching.
Exposure ("printing") systems
Exposure systems typically produce an image on the wafer using a photomask. The light shines
through the photomask, which blocks it in some areas and lets it pass in others. (Maskless
lithography projects a precise beam directly onto the wafer without using a mask, but it is not
widely used in commercial processes.) Exposure systems may be classified by the optics that
transfer the image from the mask to the wafer.
2. Explain about Wafer Preparation?
Wafer Preparation Processes:

Ingot Growth

Wafer Type and Orientation

Slicing, Lapping and Polishing

Dicing
1. Ingot Growth
Two methods are presently used to grow single-crystal
silicon. These two methods are called Czochralski and float
zone crystal growth, respectively. (These two crystal growth
methods are often abbreviated as CZ and FZ) (Gise and
Blanchard, 1979). Approximately 75 percent of the singlecrystal silicon used today is produced by the Czochralski
process of crystal pulling; 25 percent is produced by the
float zone refining technology (Veronis, 1979).
Fig. 1.1
Czochralski crystal growth utilized a crucible in which pieces of polycrystalline silicon have been heated
to their melting point of 1415?C (Figure 1.1). The crucible containing the silicon is made of quartz (SiO 2)
and is heated by either induction (RF) or thermal resistance methods. The crucible rotates during the
growth process to prevent the formation of local hot or cold regions. The atmosphere around the
crystal-growth apparatus or crystal puller is controlled to prevent contamination of the molten silicon.
Argon is often used as the ambient gas. When the temperature of the silicon has stabilized, an arm with
a piece of silicon mounted on the end is slowly lowered until it comes into contact with the surface of
the molten silicon. This piece of silicon is called as the seed crystal. As the bottom of the seed crystal
begins to melt in the molten silicon, the downward motion of the rod holding the silicon is reversed. As
the seed crystal is slowly withdrawn from the melt, the molten silicon adhering to the crystal freezes or
solidifies, taking on the crystal structure of the seed crystal. The rod continues its upward movement,
forming an even large crystal. The crystal growth terminates when the silicon in the crucible is depleted.
By carefully controlling the temperature of the crucible and the rotation speeds of the crucible and the
rod, precise control of the diameter of the crystal is maintained. The desired impurity concentration is
obtained by adding the impurities to the melt in the form of heavily doped silicon prior to crystal
growth.
2. Wafer Type and Orientation
The silicon crystals are first ground perfectly round (if necessary), and the rotational orientation of the
crystal is ascertained. The seed crystal has determined which crystal face will be presented on the wafer
surface, but the rotational position of the rod determines other axes of the crystal. Since the bar of
silicon is one crystal, it has preferential break or cleavage planes. It is critical for later device separation
to align the circuits precisely with respect to their cleavage planes. This precise alignment is
accomplished by grinding a flat along the crystal (prior to slicing) that is used as a reference during all
subsequent processing steps. X-ray diffraction provides a fast and accurate method of determining the
crystal orientation prior to grinding the flat.
The crystal is usually doped p- or n-type while grown,
depending on wafer specification. For coarse wafer alignment
and future identification, a notch and one or more flats are
ground along the length of the crystal. The primary flat is
typically located on a (01 1 ) surface and is used (with a
secondary flat, when present) to quickly identify the type and
Fig. 2.1
3. Slicing, Lapping and Polishing
crystal orientation of the wafer.
The silicon crystal is then sawed into thin slices of thickness 0.5-1 mm called wafers. The process of cut
the silicon ingot into thin slices is called slicing as shown in Fig. 3.1. Extreme care is taken to minimize
the amount of the single crystal silicon that is lost in the slicing process by using the inside diameter of a
ring-shaped saw blade. The blade is coated with diamond powder to enable it to cut through the hard
silicon. The slicing process leaves wafers with saw marks on both sides that must be removed. A silicon
etchant is used to remove the saw marks and any accompanying damage from both sides of the wafer.
Stainless steel core
Silicon ingot
Diamond ID saw
Damage
Fig. 3.1 Schematic diagram of the slicing process.
The wafers are then lapped, polished, and cleaned to remove the damage caused by slicing. Care must
be taken to remove any crystal damage introduced by the slicing operation, or the damage may prevent
the successful fabrication of devices. The wafers are next mounted on large circular polishing plates
using either wax or a vacuum to hold them. The polishing plates are mounted on a polisher, and one
side of the wafer receives a mirrorlike finish. The polishing operation uses a polishing solution that
simultaneously chemically etches and mechanically polishes the wafers. The polishing pad must be
tough and durable. When the wafers have reached the proper thickness rang and surface quality, the
polishing plates are removed and the wafers are dismounted. The wafers are thoroughly cleaned to
remove any residual contamination, and inspected to insure that wafers with imperfect surfaces are not
shipped. Wafers that pass the finial inspection are ready to start on their journey to become devices
(Gise and Blanchard, 1979).
4. Dicing
After integrated circuit (IC) fabrication and before chip package, the wafer with well-done IC is diced into
individual die with a diamond-tipped wheel, as shown in Fig. 4.1. This process is called dicing. Dicing by
grinding causes fracture and cracks to the edges of the dies, which are to be removed by a chemical
etching process.
Fig. 4.1 Schematic diagram of the dicing process.
3.Explain about isolation techniques.
Isolation Techniques
 Diffusion isolation with reverse biased diodes.
 Historically used for bipolar
 Currently used to isolate NMOS from PMOS through a well
2D effects in thermal oxidation of Si
Typical experimental result from Kao, et.al. Silicon wafers were plasma etched to
produce a variety of shaped structures including the cylinder illustrated in the top
drawing. These structures were oxidized and the oxide thickness was measured. The
drawing at the right labels the structure shown experimentally on the left. The oxide is
thinner on both concave and convex corners than it is on flat regions.
4. Explain the fabrication process:?
Layout is designed from a point of view where the designer is looking down onto the layout. This is the
best way to design layout, but it does make visualizing the physical device rather difficult.
This layout is slightly strange, but is still perfectly valid. The horizontal line will act as the reference for
the cross sectional view. A cross sectional view is what the layout would look like if it were cut along the
line, and the edge of the cut was examined.
The exact process steps change dramatically from one fab to another. However, they all follow the
same basic idea. Layout designers don't need to know the exact details, just a general idea of how a
chip is made. The specific details is the job of the process engineers.
The die is covered with a photoresist that will react with the pattern of shadows cast onto the chip from
the mask. The sections that are still soluble are washed away, leaving those sections unprotected from
that process step. In a standard N-Well process, one of the first things made is the N-Well.
Once the N-Well is created, the P-type diffusions can be created. Boron is the most popular element
used for this step.
The N-type diffusions must also be created. Phosphorous and Arsenic can both be used for this step.
A very thin layer of silicon dioxide is created on the chip. This will be used to insulate the gate from the
surface of the chip.
The first deposit of polysilicon now takes place to act as the gate of each transistor. The silicon dioxide
under each polysilicon gate is known as gate oxide.
After the gate polysilicon is created, an additional layer of silicon dioxide is added. Next, the polysilicon
used for routing is created. This polysilicon is known as field polysilicon since it is used in the field of a
chip and not for a gate. Likewise, the layers of insulating silicon dioxide not directly under the gate of a
transistor is known as field oxide.
Additional oxide is created, then the contact holes are cut in the oxide down to the diffusions and field
polysilicon. Depending on the process, these contacts can either be filled by having small plugs of metal
inserted into the holes, or the metal during the metal1 step is permitted to flow into the holes.
Tungsten is the most popular metal used in a "plug" process.
The first layer of metal is now placed on the chip. The most popular metal to use is Aluminum.
However, the most advanced processes now use copper when dealing with very high speed
microprocessors
5. Explain about integrated resistors, capacitors and inductors
Resistors.
In IC resistors, the resistance value can be controlled by varying the concentration of doping
impurity and depth of diffusion. The range of resistor values that may be produced by the
diffusion process varies from ohms to hundreds of kilohms. The typical tolerance, however, may
be no better than ± 5%, and may even be as high as ± 20%. On the other hand, if all the resistors
are diffused at the same time, then the tolerance ratio may be good. Most resistors are formed
during the base diffusion of the integrated transistor, as shown in figure. This is because it is the
highest resistivity region. For low resistance values, emitter region is used as it has much lower
resistivity.Alternatively, resistors for ICs can be produced by using thin-film technique. In this
process a metal film is deposited on a glass or Si02 surface. The resistance value can
be controlled by varying thickness, width and length of the film. Since diffused resistors can be
processed while diffusing transistors, the diffusion technique is the cheapest and, therefore, the
most widely used.
Monolithic IC Fabrication-Diffused Capacitor
Capacitors.
All P-N junctions have capacitance so capacitors may be produced by fabricating suitable
junctions. As shown in figure, P- and N-regions form the capacitor plates and depletion region
between them is the dielectric. The depletion region width and, therefore, the junction
capacitance also varies with change in reverse bias. The value is limited to 100 p F. IC capacitors
may also be fabricated by utilizing the Si02 surface layer as a dielectric. A heavily-doped N-region
is diffused to form one plate of the capacitor. The other plate is formed by depositing a film of
aluminium on the Si02 formed, on the wafer surface. The breakdown voltage of such a capacitor
is much larger than for diffused capacitor and voltage of any polarity can be used.
Inductors.
There is no feasible process for fabrication of inductors as part of monolithic structure. It is added
externally as a discrete component, when required.
6. Explain about Thick and Thin Film Technology?
Thick and Thin Film Technology:
 Film Ics are broadly classified as Thick film and thin film circuits. Thin films are the ones
whose thickness vary from 50 to 20000 A and the thick films have thickness that vary
between 125000 to 625000A. However, the thickness of the film is not a critical tool for
classifying, but the technology for fabricating the film classifies whether the film is thin or
thick film.
 Only passive components like resistors and capacitors can be fabricated using film
technology. Conventional film circuits are made by depositing film capacitors on a nonconducting substrate like glass or ceramic and pre-fabricated active components.
Thin Film Technology:
Thin films provide greater precision in component values. Thin film deposition can be
done using any of the following methods.
1. Vacuum Evaporation
2. Sputtering
3. Gas plating
4. Electroplating
5. Electroless plating
6. Silk Screening.
Vacuum Evaporation:
Vacuum Evaporation technique is same as vapour deposition technique.
Cathode Sputtering:
Sputtering system
 Sputtering uses a system that is identical to that used for vacuum evaporation. The process of
Sputtering is carried out at a very low pressure.
 The material to be deposited on the substrate is subjected to heavy bombardment by the ions of a
heavy inert gas , Viz argon.
 These ions are accelerated by making the source material as the cathode. The atoms are given out
from the cathode through a low pressure inert gas and finally deposited on the substrate.
 The high energy possed by the particles, while landing on the substrate results in a uniform coating
over the substrate with good crystal structure.
 A D.C. potential of about 3KV is applied between the cathode and the anode, Which produces a
glow discharge from the cathode that fills completely the interelectrode space.
UNIT II
Section A
1. Frequency of astable multivibrator is f =
1.4 (R1 + 2R2) × C1
2. Time period of monostable operation is time period, T = 1.1 × R1 × C1
3. An audio frequency (astable) = 20Hz to 20kHz
4. Astable mode puts out a continuous stream of rectangular pulses
5. Bistable mode acts as a basic flip-flop
Section B
1.Explain about timer 555.
The 555 Timer IC is an integrated circuit (chip) implementing a variety of timer and multivibrator
applications. The IC was designed by Hans R. Camenzind in 1970 and brought to market in 1971 by
Signetics (later acquired by Philips). The original name was the SE555 (metal can)/NE555 (plastic DIP)
and the part was described as "The IC Time Machine".[1] It has been claimed that the 555 gets its name
from the three 5 kΩ resistors used in typical early implementations,[2] but Hans Camenzind has stated
that the number was arbitrary.[3] The part is still in wide use, thanks to its ease of use, low price and
good stability. As of 2003, it is estimated that 1 billion units are manufactured every year.[3]
Depending on the manufacturer, the standard 555 package includes over 20
transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini
dual-in-line package (DIP-8).[4] Variants available include the 556 (a 14-pin DIP
combining two 555s on one chip), and the 558 (a 16-pin DIP combining four
slightly modified 555s with DIS & THR connected internally, and TR falling
edge sensitive instead of level sensitive).
Ultra-low power versions of the 555 are also available, such as the 7555 and TLC555.[5] The 7555
requires slightly different wiring using fewer external components and less power.
The 555 has three operating modes:



Monostable mode: in this mode, the 555 functions as a "one-shot". Applications include timers,
missing pulse detection, bouncefree switches, touch switches, frequency divider, capacitance
measurement, pulse-width modulation (PWM) etc
Astable - free running mode: the 555 can operate as an oscillator. Uses include LED and lamp
flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position
modulation, etc.
Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not
connected and no capacitor is used. Uses include bouncefree latched switches, etc.

Pinout diagram
The connection of the pins is as follows:
Nr. Name
Purpose
1
GND
Ground, low level (0 V)
2
TRIG
A short pulse high-to-low on the trigger starts the timer
3
OUT
During a timing interval, the output stays at +VCC
4
RESET A timing interval can be interrupted by applying a reset pulse to low (0 V)
5
CTRL
Control voltage allows access to the internal voltage divider (2/3 VCC)
6
THR
The threshold at which the interval ends (it ends if the voltage at THR is at least 2/3 VCC)
7
DIS
Connected to a capacitor whose discharge time will influence the timing interval
8
V+, VCC The positive supply voltage which must be between 3 and 15 V
2.Explain Monostable mode operation?
Schematic of a 555 in monostable mode
The relationships of the trigger signal, the voltage on C and the pulse width in monostable mode
In the monostable mode, the 555 timer acts as a “one-shot” pulse generator. The pulse begins when the
555 timer receives a trigger signal. The width of the pulse is determined by the time constant of an RC
network, which consists of a capacitor (C) and a resistor (R). The pulse ends when the charge on the C
equals 2/3 of the supply voltage. The pulse width can be lengthened or shortened to the need of the
specific application by adjusting the values of R and C.[6]
The pulse width of time t, which is the time it takes to charge C to 2/3 of the supply voltage, is given by
where t is in seconds, R is in ohms and C is in farads. See RC circuit for an explanation of this effect.
3. .Explain Bistable Mode
In bistable mode, the 555 timer acts as a basic flip-flop. The trigger and reset inputs (pins 2 and 4
respectively on a 555) are held high via pull-up resisters while the threshold input (pin 6) is simply
grounded. Thus configured, pulling the trigger momentarily to ground acts as a 'set' and transitions the
output pin (pin 3) to Vcc (high state). Pulling the reset input to ground acts as a 'reset' and transitions
the output pin to ground (low state). No capacitors are required in a bistable configuration. Pin 8 (Vcc)
is, of course, tied to Vcc while pin 1 (Gnd) is grounded. Pins 5 and 7 (control and discharge) are left
floating.
4. .Explain Astable mode
Standard 555 Astable Circuit.
In astable mode, the '555 timer ' puts out a continuous stream of rectangular pulses having a specified
frequency. Resistor R1 is connected between VCC and the discharge pin (pin 7) and another resistor (R2) is
connected between the discharge pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share
a common node. Hence the capacitor is charged through R1 and R2, and discharged only through R2,
since pin 7 has low impedance to ground during output low intervals of the cycle, therefore discharging
the capacitor.
In the astable mode, the frequency of the pulse stream depends on the values of R1, R2 and C:
5. Explain about IC Linear Ramp Generator.
555 IC Linear Ramp (Sawtooth) Generator/Oscillator
The Vc1 increases linearly when the pull-up resistor RA in the monostable circuit is replaced
with constant current source, generating a linear ramp. The linear ramp generating circuit
and the generated linear ramp waveforms illustration is shown in figures below.
Current source is created by PNP transistor Q1 and resistor R1, R2, and Re.
Ic= (Vcc-Ve)/Re
Ve= Vbe + (R2/(R1+R2))Vcc
For example, if Vcc=15V, RE=20k, R1=5kW, R2=10k, and VBE=0.7V, VE=0.7V+10V=10.7V,
Ic=(15-10.7)/20k=0.215mA
The current flowing through capacitor C1 becomes a constant current generated by PNP
transistor and resistor when the trigger starts in a timer configured as shown in figure
below.
Hence, the Vc is linear function. The gradient S of the linear ramp function is defined as:
S= (Vp-p)/T
The Vp-p is the peak to peak voltage. The Vc comes out as follows is the electric charge
amount accumulated in the capacitor is divided by the capacitance.
V= Q/C
The above equation divided on both sides by T gives us
V/T= (Q/T)/C and may be simplified into the following equation.
S=I/C
In other words, we can obtained the gradient of the linear ramp function appearing across
the capacitor by using the constant current flowing through the capacitor. The gradient of
the ramp function at both ends of the capacitor is S = 0.215m/0.022? = 9.77V/ms if the
constant current flow through the capacitor is 0.215mA and the capacitance
Section c
1. Explain about Astable operation?
555 Astable
An astable circuit produces a 'square wave', this is a
digital waveform with sharp transitions between low
(0V) and high (+Vs). Note that the durations of the low
and high states may be different. The circuit is called
an astable because it is not stable in any state: the
output is continually changing between 'low' and
'high'.
555 astable output, a square wave
(Tm and Ts may be different)
The time period (T) of the square wave is the time
for one complete cycle, but it is usually better to
consider frequency (f) which is the number of
cycles per second.
1.4
T = 0.7 × (R1 + 2R2) × C1 and f =
(R1 + 2R2) × C1
T = time period in seconds (s)
f = frequency in hertz (Hz)
R1 = resistance in ohms ( )
R2 = resistance in ohms ( )
C1 = capacitance in farads (F)
555 astable circuit
The time period can be split into two parts: T = Tm + Ts
Mark time (output high): Tm = 0.7 × (R1 + R2) × C1
Space time (output low): Ts = 0.7 × R2 × C1
Many circuits require Tm and Ts to be almost equal; this is achieved if R2 is much larger than
R1.
For a standard astable circuit Tm cannot be less than Ts, but this is not too restricting because the
output can both sink and source current. For example an LED can be made to flash briefly with
long gaps by connecting it (with its resistor) between +Vs and the output. This way the LED is
on during Ts, so brief flashes are achieved with R1 larger than R2, making Ts short and Tm long.
If Tm must be less than Ts a diode can be added to the circuit as explained under duty cycle
below.
Choosing R1, R2 and C1
555 astable frequencies
R1 and R2 should be in the range 1k to 1M . It is best
to choose C1 first because capacitors are available in
just a few values.


C1
0.001µF
Choose C1 to suit the frequency range you
require (use the table as a guide).
0.01µF
Choose R2 to give the frequency (f) you require.
Assume that R1 is much smaller than R2 (so that
0.1µF
Tm and Ts are almost equal), then you can use:
R2 = 10k
R1 = 1k
R2 = 100k
R1 = 10k
R2 = 1M
R1 = 100k
68kHz
6.8kHz
680Hz
6.8kHz
680Hz
68Hz
680Hz
68Hz
6.8Hz
6.8Hz
0.68Hz
0.68Hz
0.068Hz
0.7
1µF
68Hz
f × C1
10µF
6.8Hz
R2 =



(41 per min.) (4 per min.)
Choose R1 to be about a tenth of R2 (1k min.)
unless you want the mark time Tm to be significantly longer than the space time Ts.
If you wish to use a variable resistor it is best to make it R2.
If R1 is variable it must have a fixed resistor of at least 1k in series
(this is not required for R2 if it is variable).
Astable operation
With the output high (+Vs)
the capacitor C1 is charged
by current flowing through
R1 and R2. The threshold
and trigger inputs monitor
the capacitor voltage and
when it reaches 2/3Vs
(threshold voltage) the
output becomes low and
the discharge pin is connected to 0V.
The capacitor now discharges with current flowing through R2 into the discharge pin. When the
voltage falls to 1/3Vs (trigger voltage) the output becomes high again and the discharge pin is
disconnected, allowing the capacitor to start charging again.
This cycle repeats continuously unless the reset input is connected to 0V which forces the output
low while reset is 0V.
An astable can be used to provide the clock signal for circuits such as counters.
A low frequency astable (< 10Hz) can be used to flash an LED on and off, higher frequency
flashes are too fast to be seen clearly. Driving a loudspeaker or piezo transducer with a low
frequency of less than 20Hz will produce a series of 'clicks' (one for each low/high transition)
and this can be used to make a simple metronome.
An audio frequency astable (20Hz to 20kHz) can be used to produce a sound from a
loudspeaker or piezo transducer. The sound is suitable for buzzes and beeps. The natural
(resonant) frequency of most piezo transducers is about 3kHz and this will make them produce a
particularly loud sound.
Duty cycle
The duty cycle of an astable circuit is the proportion of the
complete cycle for which the output is high (the mark time). It is
usually given as a percentage.
For a standard 555/556 astable circuit the mark time (Tm)
must be greater than the space time (Ts), so the duty cycle
must be at least 50%:
Tm
Duty cycle =
R1 + R2
=
Tm + Ts
R1 + 2R2
To achieve a duty cycle of less than 50% a diode
can be added in parallel with R2 as shown in the
diagram. This bypasses R2 during the charging
(mark) part of the cycle so that Tm depends only
on R1 and C1:
Tm = 0.7 × R1 × C1 (ignoring 0.7V across diode)
Ts = 0.7 × R2 × C1 (unchanged)
Tm
Duty cycle with diode =
R1
=
Tm + Ts
R1 + R2
555 astable circuit with diode across R2
Use a signal diode such as 1N4148.
2. Explain about Monostable operation?
555 Monostable
A monostable circuit produces a single output
pulse when triggered. It is called a monostable
because it is stable in just one state: 'output low'.
The 'output high' state is temporary.
555 monostable output, a single pulse
The duration of the pulse is called the time
period (T) and this is determined by resistor
R1 and capacitor C1:
time period, T = 1.1 × R1 × C1
T = time period in seconds (s)
R1 = resistance in ohms ( )
C1 = capacitance in farads (F)
The maximum reliable time period is about 10
minutes.
Why 1.1? The capacitor charges to 2/3 = 67% so it is a
bit longer than the time constant (R1 × C1) which is the
time taken to charge to 63%.




555 monostable circuit with manual trigger
Choose C1 first (there are relatively few values available).
Choose R1 to give the time period you need. R1 should be in the range 1k to 1M , so use a
fixed resistor of at least 1k in series if R1 is variable.
Beware that electrolytic capacitor values are not accurate, errors of at least 20% are common.
Beware that electrolytic capacitors leak charge which substantially increases the time period if
you are using a high value resistor - use the formula as only a very rough guide!
For example the Timer Project should have a maximum time period of 266s (about 4½ minutes), but many
electrolytic capacitors extend this to about 10 minutes!
Monostable
operation
The timing period is
triggered (started) when
the trigger input (555 pin
2) is less than 1/3 Vs, this
makes the output high
(+Vs) and the capacitor C1 starts to charge through resistor R1. Once the time period has started further
trigger pulses are ignored.
The threshold input (555 pin 6) monitors the voltage across C1 and when this reaches 2/3 Vs the
time period is over and the output becomes low. At the same time discharge (555 pin 7) is
connected to 0V, discharging the capacitor ready for the next trigger.
The reset input (555 pin 4) overrides all other inputs and the timing may be cancelled at any time
by connecting reset to 0V, this instantly makes the output low and discharges the capacitor. If the
reset function is not required the reset pin should be connected to +Vs.
Power-on reset or trigger
It may be useful to ensure that a monostable circuit is reset or triggered
automatically when the power supply is connected or switched on. This is
achieved by using a capacitor instead of (or in addition to) a push switch as
shown in the diagram.
The capacitor takes a short time to charge, briefly holding the input close
to 0V when the circuit is switched on. A switch may be connected in
parallel with the capacitor if manual operation is also required.
Power-on reset or
trigger circuit
This arrangement is used for the trigger in the Timer Project.
Edge-triggering
If the trigger input is still less than 1/3 Vs at the end of the time period the
output will remain high until the trigger is greater than 1/3 Vs. This situation
can occur if the input signal is from an on-off switch or sensor.
The monostable can be made edge triggered, responding only to
changes of an input signal, by connecting the trigger signal through a
capacitor to the trigger input. The capacitor passes sudden changes
(AC) but blocks a constant (DC) signal. For further information
please see the page on capacitance. The circuit is 'negative edge
triggered' because it responds to a sudden fall in the input signal.
edge-triggering circuit
The resistor between the trigger (555 pin 2) and +Vs ensures that the trigger is normally high
(+Vs).
3. Explain about Schmitt trigger?
Application of Astable:
The buffer circuit's input has a very high impedance (about
1M ) so it requires only a few µA, but the output can sink or
source up to 200mA. This enables a high impedance signal
source (such as an LDR) to switch a low impedance output
transducer (such as a lamp).
It is an inverting buffer or NOT gate because the output
logic state (low/high) is the inverse of the input state:


Input low (< 1/3 Vs) makes output high, +Vs
Input high (> 2/3 Vs) makes output low, 0V
When the input voltage is between 1/3 and 2/3 Vs the output
remains in its present state. This intermediate input region is a
deadspace where there is no response, a property called
hysteresis, it is like backlash in a mechanical linkage. This type
of circuit is called a Schmitt trigger.
If high sensitivity is required the hysteresis is a problem,
but in many circuits it is a helpful property. It gives the
input a high immunity to noise because once the circuit
output has switched high or low the input must change
back by at least 1/3 Vs to make the output switch back.
555 inverting buffer circuit
(a NOT gate)
NOT gate symbol
4. Frequency multiplier operations of 565 PLL?
Frequency Multiplication or Frequency Synthesis
NE 565 Frequency Multiplier
The block diagram of a frequency muliplier (or synthesizer) is shown in figure. In this circuit, a
frequency divider is inserted between the output of the VCO and the phase comparator (PC) so
that the loop signal to the PC is at frequency fOUT while the output of VCO is N fOUT. This output
is a multiple of the input frequency as long as the loop is in lock. The desired amount of
multiplication can be obtained by selecting a proper divide- by N network where N is an integer.
Figure shows this function performed by a 7490 configured as a divide-by-4 circuit.
In this case the input Vin at frequency /in is compared with the output frequency fOUT at pin 5. An
output at N fOUT (4 fOUT in this case) is connected through an inverter circuit to give an input at
pin 14 of the 7490, which varies between 0 and + 5 V. Using the output at pin 9, which is onefourth of that at the input to the 7490, the signal at pin 4 of the PLL is four times the input
frequency as long as the loop remains in lock.
Since the VCO can be adjusted over a limited range from its centre frequency, it may become
necessary to change the VCO frequency whenever the divider value is changed.
For verification of the circuit operation, one must determine the input frequency range and then
adjust the free running fOUT of the VCO by means of R1 and C1 so that the output frequency of
the 7490 divider is midway within the predetermined input frequency range. The output of VCO
should now be equal to 4 fin.
5. Explain about a phase-locked loop(PLL)?








A phase-locked loop or phase lock loop (PLL) is a control system that tries to generate
an output signal whose phase is related to the phase of the input "reference" signal.
A phase detector compares two input signals and produces an error signal which is
proportional to their phase difference. The error signal is then low-pass filtered and used
to drive a VCO which creates an output phase. The output is fed through an optional
divider back to the input of the system, producing a negative feedback loop. If the output
phase drifts, the error signal will increase, driving the VCO phase in the opposite
direction so as to reduce the error. Thus the output phase is locked to the phase at the
other input. This input is called the reference.
Analog phase locked loops are generally built with an analog phase detector, low pass
filter and VCO placed in a negative feedback configuration. A digital phase locked loop
uses a digital phase detector; it may also have a divider in the feedback path or in the
reference path, or both, in order to make the PLL's output signal frequency a rational
multiple of the reference frequency. A non-integer multiple of the reference frequency
can also be created by replacing the simple divide-by-N counter in the feedback path with
a programmable pulse swallowing counter. This technique is usually referred to as a
fractional-N synthesizer or fractional-N PLL.[dubious – discuss]
The oscillator generates a periodic output signal. Assume that initially the oscillator is at
nearly the same frequency as the reference signal. If the phase from the oscillator falls
behind that of the reference, the phase detector changes the control voltage of the
oscillator so that it speeds up. Likewise, if the phase creeps ahead of the reference, the
phase detector changes the control voltage to slow down the oscillator. Since initially the
oscillator may be far from the reference frequency, practical phase detectors may also
respond to frequency differences, so as to increase the lock-in range of allowable inputs.
Depending on the application, either the output of the controlled oscillator, or the control
signal to the oscillator, provides the useful output of the PLL system.
Phase detector
The two inputs of the phase detector are the reference input and the feedback from the VCO.
The PD output controls the VCO such that the phase difference between the two inputs is held
constant, making it a negative feedback system. There are several types of phase detectors in
the two main categories of analog and digital.
Different types of phase detectors have different performance characteristics.


For instance, the frequency mixer produces harmonics that adds complexity in applications
where spectral purity of the VCO signal is important. The resulting unwanted (spurious)
sidebands, also called "reference spurs" can dominate the filter requirements and reduce the
capture range and lock time well below the requirements. In these applications the more
complex digital phase detectors are used which do not have as severe a reference spur
component on their output. Also, when in lock, the steady-state phase difference at the inputs
using this type of phase detector is near 90 degrees. The actual difference is determined by the
DC loop gain.
A bang-bang charge pump phase detector must always have a dead band where the phases of
inputs are close enough that the detector detects no phase error. For this reason, bang-bang
phase detectors are associated with significant minimum peak-to-peak jitter, because of drift
within the dead band.[citation needed] However these types, having outputs consisting of very narrow
pulses at lock, are very useful for applications requiring very low VCO spurious outputs. The
narrow pulses contain very little energy and are easy to filter out of the VCO control voltage.
This results in low VCO control line ripple and therefore low FM sidebands on the VCO.[citation
needed]

In PLL applications it is frequently required to know when the loop is out of lock. The more
complex digital phase-frequency detectors usually have an output that allows a reliable
indication of an out of lock condition.


Filter





The block commonly called the PLL loop filter (usually a low pass filter) generally has two distinct
functions.
The primary function is to determine loop dynamics, also called stability. This is how the loop
responds to disturbances, such as changes in the reference frequency, changes of the feedback
divider, or at startup. Common considerations are the range over which the loop can achieve
lock (pull-in range, lock range or capture range), how fast the loop achieves lock (lock time, lockup time or settling time) and damping behavior. Depending on the application, this may require
one or more of the following: a simple proportion (gain or attenuation), an integral (low pass
filter) and/or derivative (high pass filter). Loop parameters commonly examined for this are the
loop's gain margin and phase margin. Common concepts in control theory including the PID
controller are used to design this function.
The second common consideration is limiting the amount of reference frequency energy (ripple)
appearing at the phase detector output that is then applied to the VCO control input. This
frequency modulates the VCO and produces FM sidebands commonly called "reference
spurious". The low pass characteristic of this block can be used to attenuate this energy, but at
times a band reject "notch" may also be useful.
The design of this block can be dominated by either of these considerations, or can be a
complex process juggling the interactions of the two. Typical trade-offs are: increasing the
bandwidth usually degrades the stability or too much damping for better stability will reduce
the speed and increase settling time. Often also the phase-noise is affected.
Oscillator
All phase-locked loops employ an oscillator element with variable frequency capability. This can
be an analog VCO either driven by analog circuitry in the case of an APLL or driven digitally
through the use of a digital-to-analog converter as is the case for some DPLL designs. Pure
digital oscillators such as a numerically-controlled oscillator are used in ADPLLs.




Feedback path and optional divider
An Example Digital Divider (by 4) for use in the Feedback Path of a Multiplying PLL
PLLs may include a divider between the oscillator and the feedback input to the phase detector
to produce a frequency synthesizer. A programmable divider is particularly useful in radio
transmitter applications, since a large number of transmit frequencies can be produced from a
single stable, accurate, but expensive, quartz crystal–controlled reference oscillator.
Some PLLs also include a divider between the reference clock and the reference input to the
phase detector. If the divider in the feedback path divides by N and the reference input divider


divides by M, it allows the PLL to multiply the reference frequency by N / M. It might seem
simpler to just feed the PLL a lower frequency, but in some cases the reference frequency may
be constrained by other issues, and then the reference divider is useful.
Frequency multiplication in a sense can also be attained by locking the PLL to the 'N'th harmonic
of the signal
It should also be noted that the feedback is not limited to a frequency divider. This element can
be other elements such as a frequency multiplier, or a mixer. The multiplier will make the VCO
output a sub-multiple (rather than a multiple) of the reference frequency. A mixer can translate
the VCO frequency by a fixed offset. It may also be a combination of these. An example being a
divider following a mixer; this allows the divider to operate at a much lower frequency than the
VCO without a loss in loop gain.
ICs & INSTUMENTATION
UNIT-III
OPERATIONAL AMPLIFIERS
Section-A(2-marks) :
1 .What does the negative sign indicate in inverting amplifier?
A. The negative sign indicates a phase shift of 180 between the I/P voltage and O/P voltage.
2. The Input resistance of the non-inverting Amplifier is Extremely larger than the non-inverting
amplifier. give reasons.
A. This is because ,in non-inverting amplifier, the op-amp draws negligible current from the signal
source.
3. Name any difference between integrator and differentiator.
A.As the gain of the integrator decreases with increasing frequency ,integrator circuit does not have any
frequency problem as faced in differentiator.
4. Explain why IC 741 is used for integrator and differentiator circuits?
A.

When an resistor is connected between I/P and O/P terminal IC 741 works as
differentiator.

When an capacitor is connected between I/P & O/P terminal this works as integrator.
5. Define summing amplifier.
A. OP-AMP may be used to design a circuit whose output is the sum of several inputs.such a circuit is
called as a summing amplifier.
SECTION-B (5 marks)
1. Explain about the trans resistance amplifier.
A.

Trans resistance amplifier is otherwise called as current to voltage converter.

Photo cell, photodiode and photo voltaic cell give an output current that is proportional to
an incident radiant energy or light.

The current can be converted to voltage by using current- to- voltage converter.

Thus, the amount of light or radiant energy incident on the photo diode can be measured.

Circuit diagram and its explanation.

The feedback resistor is sometimes shunted with a capacitor to reduce high frequency
noise and the possibility of oscillations.
2. Write a short note on instrumentation amplifier.
A.

The transducers are used for measuring the physical quantities like temp, humidity, light
intensity, water flow etc.

The instrumentation amplifier amplifies the o/p of the transducer so that it can drive the
indicator or display system.

Important features of instrumentation amplifier:-

1.high gain accuracy.
2.high CMRR.
3.high gain stability with low temp coefficient.
4.low dc offset.
5.low o/p impedance.
Specially designed op-amps – to meet the requirements of a good amplifier.

Commercially available amplifiers – monolithic(single-chip)instrumentation amplifiers.

Circuit diagram & its explanation.

Instrumentation amplifiers are highly significant in industrial and consumer applications.
3. Give a detailed description of differentiator with circuit diagram.
A.

Simplest of the op-amp circuits that contain capacitor.

Circuit performs the mathematical operation of differentiation.

The o/p wave form is the derivative of the i/p wave form.

Circuit diagram & its explanation.

Differentiator becomes unstable at high frequency & therefore break into oscillation.

DRAWBACKS:The input impedance decreases with increasing frequency, thereby making the circuit
sensitive to high frequency noise.

REMEDY:A practical differentiator can eliminate the problem of stability and high frequency noise.
4. Give an account of inverting summing amplifier with neatly labeled circuit diagram.
A.

Inverting summing amplifier consists of three i/p voltages V1,V2 & V3.

Consists of a feedback resistor Rf.

Labeled Circuit diagram & its explanation.

Output is the average of the input signals(inverted).

Practical circuit:-I/P bias current compensates the Rcomp resistor.

The effective input resistance Ri = R1||R2||R3.

Therefore, Rcomp = Ri||Rf = R1||R2||R3||Rf.
5. Write a short note on voltage-to- current amplifier.
A.

Trans conductance amplifier – also called as voltage-to-current amplifier.

Conversion of a voltage signal to a proportional output current.

2 types:V-I converter with floating load.

V-I converter with grounded load.

Circuit diagram & its explanation.

Op-amp is used in non- inverting mode.

ADVANTAGE:The input impedance of the non – inverting amplifier is very high.Therefore, the circuit
draws very little current from the source.
SECTION – C: (10 marks)
1. Give a detailed account on integrator with the circuit diagram & explanations.
A.

Circuit in which o/p voltage is the integral of the i/p voltage wave form – integrator or
integration amplifier.

The circuit is obtained by – by using a basic inverting amplifier configuration if the
feedback resistor Rf is replaced by a capacitor Cf.

Expression for o/p voltage is obtained by applying Kirchoff’s Current Law.

O/p voltage- directly proportional to the negative integral of the input voltage.

O/p voltage is also inversely proportional to the time constant.

Neatly labeled circuit diagram.

If the i/p is a sine wave, then the o/p will be a cosine wave.

If the i/p is a square wave, then the o/p will be a triangular wave.

Practical integrator – reduces the error voltage at the output, a feedback resistor is
connected across the feedback capacitor.

Thus, the feedback resistor limits the low- frequency gain & hence minimizes the
variations in the o/p voltage.

Stability & the low- frequency roll- off problems can be corrected by the addition of a
feedback resistor in the practical integrator. Stability – Constant gain as frequency of an
i/p signal is varied over a certain range.

The integrator is most commonly used in analog computers & analog-m to digital(ADC)
& signal – wave shaping circuits.
2. Write a detailed description about the voltage to current converter with grounded load with a
neatly labeled circuit diagram.
A.

A grounded load is connected to the voltage to current converter circuit.

In this circuit, one terminal of the load is grounded, and the load current is controlled by
an input voltage.

Neatly labeled circuit diagram.

Analysis of the circuit :-

Accomplished by first determining the voltage V1 at the non inverting input terminal and
then establishing the relationship between V1 and the load current.

Kirchoff’s current equation is applied at node V1.

Finally, V1 = Vin + Vo –IlR/2.

Output voltage Vo = 2V1.

Therefore, Vo = Vin + Vo – IlR.

This means that the load current depends on the input voltage Vin and resistor R.

The main point to be noted – all resistors must be equal in value.

APPLICATIONS :Used for testing devices such as zeners and LEDs forming a ground load.

On the whole, the circuit will perform satisfactorily provided the load size <= R value.
3. Explain about Non inverting and inverting amplifier?
We have said that the operational amplifiers in an analog computer are always
connected so that they invert the output signal even as they add input signals together. This
is necessary, so that the summing junction remains at a virtual ground, thus preventing
various input resistors from affecting other input signals. But what if we have only a single
input signal? Can we apply it to an op amp in such a way as to obtain a controllable gain
without signal inversion?
The circuit to the right shows a non-inverting op amp circuit. In this circuit, the input
signal is effectively used as the reference voltage at the "+" input to the differential
amplifier, while the "-" input is indirectly referenced to ground. In order to keep the two
input voltages to the amplifier the same, the amplifier must set Vout to whatever voltage is
required to make the feedback voltage to the "-" input match the input voltage to the "+"
input.
Since Rf and Rin form a voltage divider, the feedback voltage will be VoutRin/(Rf + Rin). The
gain of this circuit, then, calculated as Vout/Vin, is (Rf + Rin)/Rin, or (Rf/Rin) + 1. Resistor Rz has
no effect on the gain of the circuit. However, to balance out variations caused by the small
input current to the amplifier, Rz should be made equal to the parallel combination of Rf and
Rin.
As we said at the top of this page, it is not generally a good idea to apply multiple input
signals and resistors to the non-inverting amplifier. That doesn't mean it can't be done;
there are some special-purpose circuits that use the interactions quite effectively. Indeed, a
notable application of multiple inputs to a non-inverting amplifier is the R-2R ladder
network used in some digital to analog conversion circuits. But unless you're prepared to do
some very careful analysis and circuit testing, you should avoid multiple inputs to this
circuit.
Inverting amplifier
An op-amp connected in the inverting amplifier configuration
In an inverting amplifier, the output voltage changes in an opposite direction to the input voltage.
As with the non-inverting amplifier, we start with the gain equation of the op-amp:
This time, V– is a function of both Vout and Vin due to the voltage divider formed by Rf and Rin. Again,
the op-amp input does not apply an appreciable load, so:
Substituting this into the gain equation and solving for Vout:
If AOL is very large, this simplifies to
. A resistor is often inserted between the non-inverting input and ground (so both inputs "see"
similar resistances), reducing the input offset voltage due to different voltage drops due to bias
current, and may reduce distortion in some op-amps.
A DC-blocking capacitor may be inserted in series with the input resistor when a frequency response
down to DC is not needed and any DC voltage on the input is unwanted. That is, the capacitive
component of the input impedance inserts a DC zero and a low-frequency pole that gives the circuit a
bandpass or high-pass characteristic.
The potentials at the operational amplifier inputs remain virtually constant (near ground) in the
inverting configuration. The constant operating potential typically results in distortion levels that are
lower than those attainable with the non-inverting topology.
Positive feedback configurations
Another typical configuration of op-amps is with positive feedback, which takes a fraction of the output
signal back to the non-inverting input. An important application of it is the comparator with hysteresis,
the Schmitt trigger.
Positive voltage level detector
A positive reference voltage Vref is applied to one of the op-amp's inputs. This means that the op-amp is
set up as a comparator to detect a positive voltage. If the voltage to be sensed, Ei, is applied to op amp's
(+) input, the result is a noninverting positive-level detector. When Ei is above Vref, VO equals +Vsat. When
Ei is below Vref, VO equals -Vsat.
If Ei, is applied to the inverting input, the circuit is an inverting positive-level detector: When Ei is above
Vref, VO equals -Vsat.
Negative voltage level detector
A negative voltage detector is a circuit that detects when input signal Ei crosses the negative voltage Vref. When Ei is above -Vref, VO equals +Vsat. When Ei is below -Vref, VO equals -Vsat. When Ei is above -Vref,
VO equals -Vsat, and when Ei is below -Vref, VO equals +Vsat.
Sine to square wave converter
The zero detector will convert the output of a sine-wave from a function generator into a variablefrequency square wave. If Ei is a sine wave, triangular wave, or wave of any other shape that is
symmetrical around zero, the zero-crossing detector's output will be square.
Because of the wide slew-range and lack of positive feedback, the response of all the level detectors
described above will be relatively slow. Using a general-purpose op-amp, for example, the frequency of
Ei for the sine to square wave converter should probably be below 100 Hz.
4. Explain about precision Half wave and precision Full wave rectifiers?
A Precision Half-Wave Rectifier
One of the non-linear behaviors that is sometimes required in analog circuits is
rectification. Rectification is a process of separating the positive and negative portions of a
waveform from each other and selecting from them what part of the signal to retain. In the
case of half-wave rectification, we can choose to keep one polarity while discarding the
other.
The circuit above accepts an incomimng waveform and as usual with op amps, inverts
it. However, only the positive-going portions of the output waveform, which correspond to
the negative-going portions of the input signal, actually reach the output. The direct
feedback diode shunts any negative-going output back to the "-" input directly, preventing
it from being reproduced. The slight voltage drop across the diode itself is blocked from the
output by the second diode.
The second diode allows positive-going output voltage to reach the output.
Furthermore, since the output voltage is taken from beyond the output diode itself, the op
amp will necessarily compensate for any non-linear characteristics of the diode itself. As a
result, the output voltage is a true and accurate (but inverted) reproduction of the negative
portions of the input signal. Thus, this circuit operates as a precision half-wave rectifier. If Rf
is equal to Rin as is the usual case, the output voltage will have the same amplitude as the
input voltage.
If you want to keep the positive-going portion of the input signal instead of the
negative-going portion, simply reverse the two diodes. The result will be a negative-going
copy of the positive part of the input signal.
A Precision Full-Wave Rectifier
The half-wave rectifier kept only those parts of the original input signal that were positive
(or negative). Is there a way to keep both halves of the input signal, and yet render them both
with the same output polarity? This is the behavior of a full-wave rectifier.
The circuit shown above performs full-wave rectification on the input signal, as shown. If
you wish the final output to be positive instead of negative, simply reverse the two diodes in
the half-wave rectifier section.
The full-wave rectifier depends on the fact that both the half-wave rectifier and the
summing amplifier are precision circuits. It operates by producing an inverted half-waverectified signal and then adding that signal at double amplitude to the original signal in the
summing amplifier. The result is a reversal of the selected polarity of the input signal.
The resistor values shown are reasonable; the resistors themselves must be of high
precision in order to keep the rectification process accurate. If for some reason you must
build such a circuit with a different set of resistance values, you must maintain the indicated
2:1 resistance ratio, and you must still use precision resistors in order to obtain accurate
results.
5. Explain about inverting integrator?
Inverting integrator
Integrates the (inverted) signal over time
(where Vin and Vout are functions of time, Vinitial is the output voltage of the integrator at time t
= 0.)

Note that this can also be viewed as a low-pass electronic filter. It is a filter with a single pole at
DC (i.e., where ω

= 0) and gain.
There are several potential problems with this circuit.
o
It is usually assumed that the input Vin has zero DC component (i.e., has a zero average
value). Otherwise, unless the capacitor is periodically discharged, the output will drift
outside of the operational amplifier's operating range.
o
Even when Vin has no offset, the leakage or bias currents into the operational amplifier
inputs can add an unexpected offset voltage to Vin that causes the output to drift.
Balancing input currents and replacing the non-inverting ( + ) short-circuit to ground
with a resistor with resistance R can reduce the severity of this problem.
o
Because this circuit provides no DC feedback (i.e., the capacitor appears like an open
circuit to signals with ω
= 0), the offset of the output may not agree with expectations
(i.e., Vinitial may be out of the designer's control with the present circuit).
Many of these problems can be made less severe by adding a large resistor RF in parallel with
the feedback capacitor. At significantly high frequencies, this resistor will have negligible effect.
However, at low frequencies where there are drift and offset problems, the resistor provides the
necessary feedback to hold the output steady at the correct value. In effect, this resistor reduces
the DC gain of the "integrator" – it goes from infinite to some finite value RF
/ R.
****************************************************************
UNIT – IV
TRANSDUCERS
Section A
1. A transducer is a device that converts one type of energy to another
2. A transducer conversion can be to/from electrical, electro-mechanical, electromagnetic,
photonic, photovoltaic, or any other form of energy
3. A thermocouple is a junction between two different metals that produces a voltage
related to a temperature difference
4. Thermocouples are widely used in science and industry
5. Full form of RTD resistance temperature detectors
Section B
1. Requirements of transducers ?
Basic Requirements of Transducers
Transducers should meet the basic requirements of
1. Ruggedness
2. Linearity
3. Repeatability
4. High output signal quality
5. High reliability and stability
6. Good dynamic response
7. No hysteresis
8. No residual deformation.
2. What is Transducers?
A transducer is a device that converts one type of energy to another. The conversion can
be to/from electrical, electro-mechanical, electromagnetic, photonic, photovoltaic, or
any other form of energy. While the term transducer commonly implies use as a
sensor/detector, any device which converts energy can be considered as a transducer.
3. Explain about thermocouple ?
Thermocouple
A thermocouple is a junction between two different metals that produces a voltage related to a
temperature difference. Thermocouples are a widely used type of temperature sensor for
measurement and control[1] and can also be used to convert heat into electricity. They are
inexpensive[2] and interchangeable, are supplied fitted with standard connectors, and can measure
a wide range of temperatures. The main limitation is accuracy: system errors of less than one
degree Celsius (C) can be difficult to achieve.[3]
Any junction of dissimilar metals will produce an electric potential related to temperature.
Thermocouples for practical measurement of temperature are junctions of specific alloys which
have a predictable and repeatable relationship between temperature and voltage. Different alloys
are used for different temperature ranges. Properties such as resistance to corrosion may also be
important when choosing a type of thermocouple. Where the measurement point is far from the
measuring instrument, the intermediate connection can be made by extension wires which are
less costly than the materials used to make the sensor. Thermocouples are usually standardized
against a reference temperature of 0 degrees Celsius; practical instruments use electronic
methods of cold-junction compensation to adjust for varying temperature at the instrument
terminals. Electronic instruments can also compensate for the varying characteristics of the
thermocouple, and so improve the precision and accuracy of measurements.
Thermocouples are widely used in science and industry; applications include temperature
measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.
Principle of operation
In 1821, the German–Estonian physicist Thomas Johann Seebeck discovered that when any conductor is
subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric
effect or Seebeck effect. Any attempt to measure this voltage necessarily involves connecting another
conductor to the "hot" end. This additional conductor will then also experience the temperature
gradient, and develop a voltage of its own which will oppose the original. Fortunately, the magnitude of
the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit
in which the two legs generate different voltages, leaving a small difference in voltage available for
measurement. That difference increases with temperature, and is between 1 and 70 microvolts per
degree Celsius (µV/°C) for standard metal combinations.
The voltage is not generated at the junction of the two metals of the thermocouple but rather along that
portion of the length of the two dissimilar metals that is subjected to a temperature gradient. Because
both lengths of dissimilar metals experience the same temperature gradient, the end result is a
measurement of the temperature at the thermocouple junction.
4. Explain thermistor?
NTC thermistor, bead type, insulated wires
A thermistor is a type of resistor whose resistance varies significantly with temperature, more
so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are
widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors,
and self-regulating heating elements.
Thermistors differ from resistance temperature detectors (RTD) in that the material used in a
thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature
response is also different; RTDs are useful over larger temperature ranges, while thermistors
typically achieve a higher precision within a limited temperature range [usually −90 °C to
130 °C].
Thermistor symbol
Assuming, as a first-order approximation, that the relationship between resistance and
temperature is linear, then:
where
ΔR = change in resistance
ΔT = change in temperature
k = first-order temperature coefficient of resistance
Thermistors can be classified into two types, depending on the sign of k. If k is positive, the
resistance increases with increasing temperature, and the device is called a positive temperature
coefficient (PTC) thermistor, or posistor. If k is negative, the resistance decreases with
increasing temperature, and the device is called a negative temperature coefficient (NTC)
thermistor. Resistors that are not thermistors are designed to have a k as close to zero as
possible(smallest possible k), so that their resistance remains nearly constant over a wide
temperature range.
Instead of the temperature coefficient k, sometimes the temperature coefficient of resistance α
(alpha) or αT is used. It is defined as[1]
For example, for the common PT100 sensor, α = 0.00385 or 0.385 %/°C. This αT coefficient
should not be confused with the a parameter below.
5. Applications of Thermistor

PTC thermistors can be used as current-limiting devices for circuit protection, as replacements
for fuses. Current through the device causes a small amount of resistive heating. If the current is
large enough to generate more heat than the device can lose to its surroundings, the device
heats up, causing its resistance to increase, and therefore causing even more heating. This
creates a self-reinforcing effect that drives the resistance upwards, reducing the current and
voltage available to the device.

PTC thermistors are used as timers in the degaussing coil circuit of most CRT displays and
televisions. When the display unit is initially switched on, current flows through the thermistor
and degaussing coil. The coil and thermistor are intentionally sized so that the current flow will
heat the thermistor to the point that the degaussing coil shuts off in under a second. For
effective degaussing, it is necessary that the magnitude of the alternating magnetic field
produced by the degaussing coil decreases smoothly and continuously, rather than sharply
switching off or decreasing in steps; the PTC thermistor accomplishes this naturally as it heats
up. A degaussing circuit using a PTC thermistor is simple, reliable (for its simplicity), and
inexpensive.

NTC thermistors are used as resistance thermometers in low-temperature measurements of the
order of 10 K.

NTC thermistors can be used as inrush-current limiting devices in power supply circuits. They
present a higher resistance initially which prevents large currents from flowing at turn-on, and
then heat up and become much lower resistance to allow higher current flow during normal
operation. These thermistors are usually much larger than measuring type thermistors, and are
purposely designed for this application.

NTC thermistors are regularly used in automotive applications. For example, they monitor things
like coolant temperature and/or oil temperature inside the engine and provide data to the ECU
and, indirectly, to the dashboard.

NTC thermistors can be also used to monitor the temperature of an incubator.

Thermistors are also commonly used in modern digital thermostats and to monitor the
temperature of battery packs while charging.
SECTION C
1. Explain The Classification On Transducers?
CLASSIFICATION OF TRANSDUCERS
A transducer can be defined as a device capable of converting energy from one form into
another. Transducers can be found both at the input as well as at the output stage of a measuring
system.
The input transducer is called the sensor, because it senses the desired physical quantity and
converts it into another energy form.
The output transducer is called the actuator, because it converts the energy into a form to which
another independent system can react, whether it is a biological system or a technical system. So, for a
biological system the actuator can be a numerical display or a loudspeaker to which the visual or aural
senses react respectively. For a technical system the actuator could be a recorder or a laser, producing
holes in a ceramic material. The results can be interpreted by humans.
Types of energy form
Figure The six different energy domains at the input of a measuring system.
We can distinguish six different energy domains: (1) radiant, (2) mechanical, (3) thermal, (4)
electrical, (5) magnetic and (6) chemical.
If certain information is already available in the electrical domain it can be claimed that it
requires no energy conversion, but in general there is 'shape' conversion left and this is just the domain
which belongs to the field of electronics and electrical science and engineering. A good example of such
a sensor only sensitive to electrical energy is the probe of an oscilloscope, with which a good adaptation
to the signal source is realized. In the modifier stage we meet other examples of shape converters, for
instance the A/D and D/A converters.
Figure The six different energy conversions at the output stage.
In the same way, the six different domain conversions at the output can be drawn. This is
illustrated in figure above, where compared with previous figure the only difference is the
reversed direction of the arrows.
Types of energy source
If the energy sources at the input, or the actuators are acting on at the output, are considered, we
can distinguish between technical systems and biological systems.
Technical systems can produce all six energy forms. Hence at the input side the six different
types of energy source can always be recognized.
For biological systems this is not so clear, but a more careful consideration will reveal the same
Six different types of energy form.
1. Radiant energy is produced by all biological systems. This is normally infrared radiation and can be
detected although it is not visible, for instance, with thermographic cameras.
2. Biological systems can also produce mechanical energy as a result of movements or the liquid
pressure in the vessels.
3. Thermal energy is produced by all systems in which oxidation takes place.
4. Electrical energy for instance is produced by the heart muscle at a potential of several mV.
5. Magnetic energy is also produced by the human heart muscle. Also, magnetic brain
activities can be monitored with the help of superconducting quantum interference devices, so-called
SQUIDs.
6. In biological systems chemical energy is produced in all types of process and they can act an energy
source also.
2 . Explain about LVDT?
Cutaway view of an LVDT. Current is driven through the primary coil at A, causing an induction current
to be generated through the secondary coils at B.
The linear variable differential transformer (LVDT) is a type of electrical transformer used
for measuring linear displacement. The transformer has three solenoidal coils placed end-to-end
around a tube. The center coil is the primary, and the two outer coils are the secondaries. A
cylindrical ferromagnetic core, attached to the object whose position is to be measured, slides
along the axis of the tube.
An alternating current is driven through the primary, causing a voltage to be induced in each
secondary proportional to its mutual inductance with the primary. The frequency is usually in the
range 1 to 10 kHz.
As the core moves, these mutual inductances change, causing the voltages induced in the
secondaries to change. The coils are connected in reverse series, so that the output voltage is the
difference (hence "differential") between the two secondary voltages. When the core is in its
central position, equidistant between the two secondaries, equal but opposite voltages are
induced in these two coils, so the output voltage is zero.
When the core is displaced in one direction, the voltage in one coil increases as the other
decreases, causing the output voltage to increase from zero to a maximum. This voltage is in
phase with the primary voltage. When the core moves in the other direction, the output voltage
also increases from zero to a maximum, but its phase is opposite to that of the primary. The
magnitude of the output voltage is proportional to the distance moved by the core (up to its limit
of travel), which is why the device is described as "linear". The phase of the voltage indicates the
direction of the displacement.
Because the sliding core does not touch the inside of the tube, it can move without friction,
making the LVDT a highly reliable device. The absence of any sliding or rotating contacts
allows the LVDT to be completely sealed against the environment.
LVDTs are commonly used for position feedback in servomechanisms, and for automated
measurement in machine tools and many other industrial and scientific applications.
2. Explain Hall effect sensor?
The magnetic piston (1) in this pneumatic cylinder will cause the Hall effect sensors (2 and 3)
mounted on its outer wall to activate when it is fully retracted or extended.
Clutch with Hall Effect sensor.
A Hall effect sensor is a transducer that varies its output voltage in response to changes in
magnetic field. Hall sensors are used for proximity switching, positioning, speed detection, and
current sensing applications.
In its simplest form, the sensor operates as an analogue transducer, directly returning a voltage.
With a known magnetic field, its distance from the Hall plate can be determined. Using groups of
sensors, the relative position of the magnet can be deduced.
Electricity carried through a conductor will produce a magnetic field that varies with current, and
a Hall sensor can be used to measure the current without interrupting the circuit. Typically, the
sensor is integrated with a wound core or permanent magnet that surrounds the conductor to be
measured.
Frequently, a Hall sensor is combined with circuitry that allows the device to act in a digital
(on/off) mode, and may be called a switch in this configuration. Commonly seen in industrial
applications such as the pictured pneumatic cylinder, they are also used in consumer equipment;
for example some computer printers use them to detect missing paper and open covers. When
high reliability is required, they are used in keyboards.

Hall sensors are commonly used to time the speed of wheels and shafts, such as for internal
combustion engine ignition timing, tachometers and anti-lock braking systems. They are used in
brushless DC electric motors to detect the position of the permanent magnet. In the pictured wheel
with two equally spaced magnets, the voltage from the sensor will peak twice for each revolution.
This arrangement is commonly used to regulate the speed of disc drives.
Hall probe
A hall probe contains an indium compound crystal such as indium antimonide, mounted on an
aluminum backing plate, and encapsulated in the probe head. The plane of the crystal is
perpendicular to the probe handle. Connecting leads from the crystal are brought down through
the handle to the circuit box.
When the Hall Probe is held so that the magnetic field lines are passing at right angles through
the sensor of the probe, the meter gives a reading of the value of magnetic flux density (B). A
current is passed through the crystal which, when placed in a magnetic field has a “Hall Effect”
voltage developed across it. The Hall Effect is seen when a conductor is passed through a
uniform magnetic field. The natural electron drift of the charge carriers causes the magnetic field
to apply a Lorentz force (the force exerted on a charged particle in an electromagnetic field) to
these charge carriers. The result is what is seen as a charge separation, with a build up of either
positive or negative charges on the bottom or on the top of the plate. The crystal measures 5 mm
square. The probe handle, being made of a non-ferrous material, has no disturbing effect on the
field.
A Hall Probe is enough to measure the Earth's magnetic field. It must be held so that the Earth's
field lines are passing directly through it. It is then rotated quickly so the field lines pass through
the sensor in the opposite direction. The change in the flux density reading is double the Earth's
magnetic flux density. A hall probe must first be calibrated against a known value of magnetic
field strength. For a solenoid the hall probe is placed in the center.
UNIT - V
Section A
1.
Full form of CRT Cathode Ray Tube
2. An oscilloscope (also known as a scope, CRO, DSO or, an O-scope) is a type of
electronic test instrument
3. Types of digital voltmeter Ramp, Stair case ramp, integrating, Successive Approximation
4. A LCR meter (Inductance (L), Capacitance (C), and Resistance (R)) is a piece of
electronic test equipment
5. In CRT anode and cathode are used in electronics as synonyms for positive and
negative terminals
Section B
1. Explain about LCR meter?
A LCR meter (Inductance (L), Capacitance (C), and Resistance (R)) is a piece of electronic
test equipment used to measure the inductance, capacitance and, resistance of a component.
In the usual versions of this instrument these quantities are not measured directly, but
determined from a measurement of impedance. The necessary calculations are, however,
incorporated in the instrument's circuitry; the meter reads L, C and R directly with no human
calculation required.
Usually the device under test (DUT) is subjected to an AC voltage source. The meter detects
the voltage over, and the current through the DUT. From the ratio of these the meter can
determine the magnitude of the impedance. The phase angle between the voltage and current
is also detected and between that and the impedance magnitude the DUT can be represented
as an L and R or a C and R. The meter must assume either a parallel or a series model for
these two elements. The most useful assumption, and the one usually adopted, is that LR
measurements have the elements in series (as would be encountered in an inductor coil) and
that CR measurements have the elements in parallel (as would be encountered in measuring a
capacitor with a leaky dielectric).It can also be used to judge the inductance variation with
respect to the rotor position in permanent magnet machines.
.Explain CRT?
The Cathode Ray Tube
Almost all TVs in use today rely on a device known as the cathode ray tube, or CRT, to display their
images. LCDs and plasma displays are sometimes seen, but they are still rare when compared to CRTs. It
is even possible to make a television screen out of thousands of ordinary 60-watt light bulbs! You may
have seen something like this at an outdoor event like a football game. Let's start with the CRT,
however, because CRTs are the most common way of displaying images today.
The terms anode and cathode are used in electronics as synonyms for positive and negative terminals.
For example, you could refer to the positive terminal of a battery as the anode and the negative
terminal as the cathode.
In a cathode ray tube, the "cathode" is a heated filament (not unlike the filament in a normal light bulb).
The heated filament is in a vacuum created inside a glass "tube." The "ray" is a stream of electrons that
naturally pour off a heated cathode into the vacuum.
Electrons are negative. The anode is positive, so it attracts the electrons pouring off the cathode. In a
TV's cathode ray tube, the stream of electrons is focused by a focusing anode into a tight beam and then
accelerated by an accelerating anode. This tight, high-speed beam of electrons flies through the vacuum
in the tube and hits the flat screen at the other end of the tube. This screen is coated with phosphor,
which glows when struck by the beam.
3. Explain about pH meter?
pH meter
A pH meter

A pH meter is an electronic instrument used for measuring the pH (acidity or alkalinity)
of a liquid (though special probes are sometimes used to measure the pH of semi-solid
substances).

A typical pH meter consists of a special measuring probe (a glass electrode) connected to
an electronic meter that measures and displays the pH reading.
The probe
The pH probe measures pH as the activity of hydrogen cations surrounding a thin-walled glass
bulb at its tip. The probe produces a small voltage (about 0.06 volt per pH unit) that is measured
and displayed as pH units by the meter. For more information about pH probes, see glass
electrode.
The meter
The meter circuit is no more than a voltmeter that displays measurements in pH units instead of
volts. The input impedance of the meter must be very high because of the high resistance —
approximately 20 to 1000 MΩ — of the glass electrode probes typically used with pH meters.
The circuit of a simple pH meter usually consists of operational amplifiers in an inverting
configuration, with a total voltage gain of about −17. The inverting amplifier converts the small
voltage produced by the probe (+0.059 volt/pH) into pH units, which are then offset by seven
volts to give a reading on the pH scale. For example:

At neutral pH (pH 7) the voltage at the probe's output is 0 volts. 0 · 17 + 7 = 7.

At basic pH, the voltage at the probe's output ranges from +0 to +0.41 volts (7 · 0.059 = 0.41). So
for a sample of pH 10 (3 pH units above neutral), 3 · 0.059 = 0.18 volts), the output of the
meter's amplifier is 0.18 · 17 + 7 = 10.

At acid pH, the voltage at the probe's output ranges from −0.41 volts to −0. So for a sample of
pH 4 (3 pH units below neutral), −3 · 0.059 = −0.18 volts, the output of the meter's amplifier is
−0.18 · 17 + 7 = 4.
The two basic adjustments performed at calibration (see below) set the gain and offset of the
inverting amplifier.
4. Explain about humidity?
Tropical forests and high-altitude regions often have high humidity.
Humidity is a term for the amount of water vapor in air, and can refer to any one of several
measurements of humidity. Formally, humid air is not "moist air" but a mixture of air and
water vapor, and humidity is defined in terms of the water content of this mixture, called the
Absolute humidity. In everyday usage, it commonly refers to relative humidity, expressed as a
percent in weather forecasts and on household humidistats; it is so called because it measures
the current absolute humidity relative to the maximum. Specific humidity is a ratio of the
water vapor content of the mixture to the dry air content. The water vapor content of the
mixture can be measured either as mass per volume or as a partial pressure, depending on the
usage.
In meteorology, humidity indicates the likelihood of precipitation, dew, or fog. High relative
humidity reduces the effectiveness of sweating in cooling the body by reducing the rate of
evaporation of moisture from the skin. This effect is calculated in a heat index table, used
during summer weather.
Section c
1.Explain about Digital Oscilloscope?
This article is about current oscilloscopes, providing general information. For history of oscilloscopes,
see Oscilloscope history. For detailed information about various types of oscilloscopes, see Oscilloscope
types.
Illustration showing the interior of a cathode-ray tube for use in an oscilloscope. Numbers in the picture
indicate: 1. Deflection voltage electrode; 2. Electron gun; 3. Electron beam; 4. Focusing coil; 5.
Phosphor-coated inner side of the screen
A Tektronix model 475A portable analog oscilloscope, a very typical instrument of the late 1970s
An oscilloscope (also known as a scope, CRO, DSO or, an O-scope) is a type of electronic test
instrument that allows observation of constantly varying signal voltages, usually as a twodimensional graph of one or more electrical potential differences using the vertical or 'Y' axis,
plotted as a function of time, (horizontal or 'x' axis). Although an oscilloscope displays voltage
on its vertical axis, any other quantity that can be converted to a voltage can be displayed as well.
In most instances, oscilloscopes show events that repeat with either no change, or change slowly.
Oscilloscopes are commonly used to observe the exact wave shape of an electrical signal. In
addition to the amplitude of the signal, an oscilloscope can show distortion, the time between
two events (such as pulse width, period, or rise time) and relative timing of two related signals.[1]
Oscilloscopes are used in the sciences, medicine, engineering, and telecommunications industry.
General-purpose instruments are used for maintenance of electronic equipment and laboratory
work. Special-purpose oscilloscopes may be used for such purposes as analyzing an automotive
ignition system, or to display the waveform of the heartbeat as an electrocardiogram.
Originally all oscilloscopes used cathode ray tubes as their display element and linear amplifiers
for signal processing, (commonly referred to as CROs) however, modern oscilloscopes have
LCD or LED screens, fast analog-to-digital converters and digital signal processors. Although
not as commonplace, some oscilloscopes used storage CRTs to display single events for a
limited time. Oscilloscope peripheral modules for general purpose laptop or desktop personal
computers use the computer's display, allowing them to be used as test instruments.

Features and uses
Display and general external appearance
The basic oscilloscope, as shown in the illustration, is typically divided into four sections: the
display, vertical controls, horizontal controls and trigger controls. The display is usually a CRT
or LCD panel which is laid out with both horizontal and vertical reference lines referred to as the
graticule. In addition to the screen, most display sections are equipped with three basic controls,
a focus knob, an intensity knob and a beam finder button.
The vertical section controls the amplitude of the displayed signal. This section carries a Voltsper-Division (Volts/Div) selector knob, an AC/DC/Ground selector switch and the vertical
(primary) input for the instrument. Additionally, this section is typically equipped with the
vertical beam position knob.
The horizontal section controls the time base or “sweep” of the instrument. The primary control
is the Seconds-per-Division (Sec/Div) selector switch. Also included is a horizontal input for
plotting dual X-Y axis signals. The horizontal beam position knob is generally located in this
section.
The trigger section controls the start event of the sweep. The trigger can be set to automatically
restart after each sweep or it can be configured to respond to an internal or external event. The
principal controls of this section will be the source and coupling selector switches. An external
trigger input (EXT Input) and level adjustment will also be included.
In addition to the basic instrument, most oscilloscopes are supplied with a probe as shown. The
probe will connect to any input on the instrument and typically has a resistor of ten times the
oscilloscope's input impedance. This results in a .1 (-10X) attenuation factor, but helps to isolate
the capacitive load presented by the probe cable from the signal being measured. Some probes
Size and portability
Most modern oscilloscopes are lightweight, portable instruments that are compact enough to be
easily carried by a single person. In addition to the portable units, the market offers a number of
miniature battery-powered instruments for field service applications. Laboratory grade
oscilloscopes, especially older units which use vacuum tubes, are generally bench-top devices or
may be mounted into dedicated carts. Special-purpose oscilloscopes may be rack-mounted or
permanently mounted into a custom instrument housing.
Inputs
The signal to be measured is fed to one of the input connectors, which is usually a coaxial
connector such as a BNC or UHF type. Binding posts or banana plugs may be used for lower
frequencies. If the signal source has its own coaxial connector, then a simple coaxial cable is
used; otherwise, a specialised cable called a "scope probe", supplied with the oscilloscope, is
used. In general, for routine use, an open wire test lead for connecting to the point being
observed is not satisfactory, and a probe is generally necessary. General-purpose oscilloscopes
usually present an input impedance of 1 megohm in parallel with a small but known capacitance
such as 20 picofarads.[2] This allows the use of standard oscilloscope probes.[3] Scopes for use
with very high frequencies may have 50-ohm inputs, which must be either connected directly to
a 50-ohm signal source or used with Z0 or active probes.
Less-frequently-used inputs include one (or two) for triggering the sweep, horizontal deflection
for X-Y mode displays, and trace brightening/darkening, sometimes called "Z-axis" inputs.
Timebase Controls
These select the horizontal speed of the CRT's spot as it creates the trace; this process is
commonly referred to as the sweep. In all but the least-costly modern oscilloscopes, the sweep
speed is selectable and calibrated in units of time per major graticule division. Quite a wide
range of sweep speeds is generally provided, from seconds to as fast as picoseconds (in the
fastest) per division. Usually, a continuously-variable control (often a knob in front of the
calibrated selector knob) offers uncalibrated speeds, typically slower than calibrated. This
control provides a range somewhat greater than that of consecutive calibrated steps, making any
speed available between the extremes.
Horizontal sensitivity control
This control is found only on more elaborate oscilloscopes; it offers adjustable sensitivity for e
Vertical position control
The vertical position control moves the whole displayed trace up and down. It is used to set the
no-input trace exactly on the center line of the graticule, but also permits offsetting vertically by
a limited amount. With direct coupling, adjustment of this control can compensate for a limited
DC component of an input.
Horizontal position control
The horizontal position control moves the display sidewise. It usually sets the left end of the
trace at the left edge of the graticule, but it can displace the whole trace when desired. This
control also moves the X-Y mode traces sidewise in some instruments, and can compensate for a
limited DC component as for vertical position.
Dual-trace controls
Each input channel usually has its own set of sensitivity, coupling, and position controls,
although some four-trace oscilloscopes have only minimal controls for their third and fourth
channels.
Dual-trace oscilloscopes have a mode switch to select either channel alone, both channels, or (in
some) an X-Y display, which uses the second channel for X deflection. When both channels are
displayed, the type of channel switching can be selected on some oscilloscopes; on others, the
type depends upon timebase setting. If manually selectable, channel switching can be freerunning (asynchronous), or between consecutive sweeps. Some Philips dual-trace analog
oscilloscopes had a fast analog multiplier, and provided a display of the product of the input
channels.
Multiple-trace oscilloscopes have a switch for each channel to enable or disable display of that
trace's signal.
Basic types of sweeps
Triggered sweeps
To display events with unchanging or slowly (visibly) changing waveforms, but occurring at
times that may not be evenly spaced, modern oscilloscopes have triggered sweeps. Compared to
simpler oscilloscopes with sweep oscillators that are always running, triggered-sweep
oscilloscopes are markedly more versatile.
A triggered sweep starts at a selected point on the signal, providing a stable display. In this way,
triggering allows the display of periodic signals such as sine waves and square waves, as well as
nonperiodic signals such as single pulses, or pulses that don't recur at a fixed rate.
With triggered sweeps, the scope will blank the beam and start to reset the sweep circuit each
time the beam reaches the extreme right side of the screen. For a period of time, called holdoff,
(extendable by a front-panel control on some better oscilloscopes), the sweep circuit resets
completely and ignores triggers. Once holdoff expires, the next trigger starts a sweep. The trigger
event is usually the input waveform reaching some user-specified threshold voltage (trigger
level) in the specified direction (going positive or going negative—trigger polarity).
In some cases, variable holdoff time can be really useful to make the sweep ignore interfering
triggers that occur before the events one wants to observe. In the case of repetitive, but quitecomplex waveforms, variable holdoff can create a stable display that can't otherwise practically
be obtained.
Automatic sweep mode
Triggered sweeps can display a blank screen if there are no triggers. To avoid this, these sweeps
include a timing circuit that generates free-running triggers so a trace is always visible. Once
triggers arrive, the timer stops providing pseudo-triggers. Automatic sweep mode can be deselected when observing low repetition rates.
Types of trigger include:

external trigger, a pulse from an external source connected to a dedicated input on the scope.

edge trigger, an edge-detector that generates a pulse when the input signal crosses a specified
threshold voltage in a specified direction. These are the most-common types of triggers; the
level control sets the threshold voltage, and the slope control selects the direction (negative or
positive-going). (The first sentence of the description also applies to the inputs to some digital
logic circuits; those inputs have fixed threshold and polarity response.)

video trigger, a circuit that extracts synchronizing pulses from video formats such as PAL and
NTSC and triggers the timebase on every line, a specified line, every field, or every frame. This
circuit is typically found in a waveform monitor device, although some better oscilloscopes
include this function.

delayed trigger, which waits a specified time after an edge trigger before starting the sweep. As
described under delayed sweeps, a trigger delay circuit (typically the main sweep) extends this
delay to a known and adjustable interval. In this way, the operator can examine a particular
pulse in a long train of pulses.
Some recent designs of oscilloscopes include more sophisticated triggering schemes; these are
described toward the end of this article.
X-Y mode
Most modern oscilloscopes have several inputs for voltages, and thus can be used to plot one
varying voltage versus another. This is especially useful for graphing I-V curves (current versus
voltage characteristics) for components such as diodes, as well as Lissajous patterns. Lissajous
figures are an example of how an oscilloscope can be used to track phase differences between
multiple input signals. This is very frequently used in broadcast engineering to plot the left and
right stereophonic channels, to ensure that the stereo generator is calibrated properly.
Historically, stable Lissajous figures were used to show that two sine waves had a relatively
simple frequency relationship, a numerically-small ratio. They also indicated phase difference
between two sine waves of the same frequency.
Complete loss of signal in an X-Y display means that the CRT's beam strikes a small spot, which
risks burning the phosphor. Older phosphors burned more easily. Some dedicated X-Y displays
reduce beam current greatly, or blank the display entirely, if there are no inputs present.
Bandwidth
Bandwidth is a measure of the range of frequencies that can be displayed; it refers primarily to
the vertical amplifier, although the horizontal deflection amplifier has to be fast enough to handle
the fastest sweeps. The bandwidth of the oscilloscope is limited by the vertical amplifiers and the
CRT (in analog instruments) or by the sampling rate of the analog to digital converter in digital
instruments. The bandwidth is defined as the frequency at which the sensitivity is 0.707 of the
sensitivity at lower frequency (a drop of 3 dB). The rise time of the fastest pulse that can be
resolved by the scope is related to its bandwidth approximately:
Bandwidth in Hz x rise time in seconds = 0.35 [8]
For example, a oscilloscope intended to resolve pulses with a rise time of 1 nanosecond would
have a bandwidth of 350 MHz.
For a digital oscilloscope, a rule of thumb is that the continuous sampling rate should be ten
times the highest frequency desired to resolve; for example a 20 megasample/second rate would
be applicable for measuring signals up to about 2 megahertz.
Lissajous figures on an oscilloscope, with 90 degrees phase difference between x and y inputs.
One of the most frequent uses of scopes is troubleshooting malfunctioning electronic equipment.
One of the advantages of a scope is that it can graphically show signals: where a voltmeter may
show a totally unexpected voltage, a scope may reveal that the circuit is oscillating. In other
cases the precise shape or timing of a pulse is important.
In a piece of electronic equipment, for example, the connections between stages (e.g. electronic
mixers, electronic oscillators, amplifiers) may be 'probed' for the expected signal, using the scope
as a simple signal tracer. If the expected signal is absent or incorrect, some preceding stage of the
electronics is not operating correctly. Since most failures occur because of a single faulty
component, each measurement can prove that half of the stages of a complex piece of equipment
either work, or probably did not cause the fault.
Once the faulty stage is found, further probing can usually tell a skilled technician exactly which
component has failed. Once the component is replaced, the unit can be restored to service, or at
least the next fault can be isolated. This sort of troubleshooting is typical of radio and TV
receivers, as well as audio amplifiers, but can apply to quite-different devices such as electronic
motor drives.
Another use is to check newly designed circuitry. Very often a newly designed circuit will
misbehave because of design errors, bad voltage levels, electrical noise etc. Digital electronics
usually operate from a clock, so a dual-trace scope which shows both the clock signal and a test
signal dependent upon the clock is useful. Storage scopes are helpful for "capturing" rare
electronic events that cause defective operation.
Heterodyne

AC hum on sound.

Sum of a low-frequency and a high-frequency signal.

Bad filter on sine.

Dual trace, showing different time bases on each trace.
2. Explain types of DVM and its type
3. Explain about Humidity Measurement?
Humidity measurement finds wide applications in different process industries. Moisture in the
atmosphere must be controlled below a certain level in many manufacturing processes, e.g.,
semiconductor devices, optical fibres etc. Humidity inside an incubator must be controlled at a
very precision level. Textiles, papers and cereals must be dried to a standard storage condition in
order to prevent the quality deterioration. The humidity can be expressed in different ways: (a)
absolute humidity, (b) relative humidity and (c) dew point.
Humidity can be measured in different ways. Some of the techniques are explained below.
Hygrometer
s
A Hygrometer (UK: /haɪˈɡrɒmɪtə/) is an instrument used for measuring the moisture content in
the environmental air, or humidity. Humidity is difficult to measure accurately. Most
measurement devices usually rely on measurements of some other quantity such as temperature,
pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. From
calculations based on physical principles, or especially by calibration with a reference standard,
these measured quantities can lead to a measurement of humidity. Modern electronic devices use
temperature of condensation, changes in electrical resistance, and changes in electrical
capacitance to measure humidity changes.
Many hygroscopic materials, such as wood, hair, paper, etc. are sensitive to humidity. Their
dimensions change with humidity. The change in dimension can be measured and calibrated in
terms of humidity.
From Wikipedia, the free encyclopedia
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Not to be confused with hydrometer.
A Hygrometer (UK: /haɪˈɡrɒmɪtə/) is an instrument used for measuring the moisture content in
the environmental air, or humidity. Humidity is difficult to measure accurately. Most
measurement devices usually rely on measurements of some other quantity such as temperature,
pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. From
calculations based on physical principles, or especially by calibration with a reference standard,
these measured quantities can lead to a measurement of humidity. Modern electronic devices use
temperature of condensation, changes in electrical resistance, and changes in electrical
capacitance to measure humidity changes.
Dial Hygrometer, in this case a hair tension style, note nonlinear scale
Types
Metal/Pulp Coil type
The familiar metal/paper coil hygrometer is useful for giving a dial indication of humidity
changes, but its accuracy is limited. Humidity is absorbed by a salt-impregnated paper strip
attached to a metal coil, causing it to change shape. These changes in length (analogous to those
in a bimetallic thermometer) cause an indication on a dial.
Hair tension hygrometers
These devices use a human or animal hair under tension. The length of the hair changes with
humidity and the length change may be magnified by a mechanism and/or indicated on a dial or
scale. The traditional folk art device known as a "weather house" works on this principle.
Electronic hygrometers
Electronic hygrometer
Dewpoint is the temperature at which a sample of moist air (or any other water vapor) at constant
pressure reaches water vapor saturation. At this saturation temperature, further cooling results in
condensation of water. Chilled mirror dewpoint hygrometers are one of the most precise
instruments commonly available. These use a chilled mirror and optoelectronic mechanism to
detect condensation on the mirror surface. The temperature of the mirror is controlled by
electronic feedback to maintain a dynamic equilibrium between evaporation and condensation on
the mirror, thus closely measuring the dewpoint temperature. An accuracy of 0.2 degrees C is
attainable with these devices, which correlates at typical office environments to a relative
humidity accuracy of about +/-0.5%. These devices need frequent cleaning, a skilled operator
and periodic calibration to attain these levels of accuracy.
For applications where cost, space, or fragility are relevant, other types of electronic sensors are
used, at the price of a lower accuracy. In capacitive humidity sensors, the effect of humidity on
the dielectric constant of a polymer or metal oxide material is measured. With calibration, these
sensors have an accuracy of +/-2% RH in the range 5–95% RH. Without calibration, the
accuracy is 2 to 3 times worse. Capacitive sensors are robust against effects such as condensation
and temporary high temperatures.[1] Capacitive sensors are subject to contamination, drift and
aging effects, but are suitable for many applications.
In resistive humidity sensors, the change in electrical resistance of a material due to humidity is
measured.[1] Typical materials are salts and conductive polymers. Resistive sensors are less
sensitive than capacitive sensors - the change in material properties is less, so they require more
complex circuitry. The material properties also tend to depend both on humidity and
temperature, which means in practice that the sensor must be combined with a temperature
sensor. The accuracy and robustness against condensation vary depending on the chosen resistive
material. Robust, condensation-resistant sensors exist with an accuracy of up to +/-3% RH.
In thermal conductivity humidity sensors, the change in thermal conductivity of air due to
humidity is measured. These sensors measure absolute humidity rather than relative humidity.[1]
Psychrometer
Psychrometric method for measurement of relative humidity is a popular method. Two bulbs are
used- dry bulb and wet bulb. The wet bulb is soaked in saturated water vapour and the dry bulb is
kept in the ambient condition. The temperature difference between the dry bulb and wet bulb is
used to obtain the relative humidity through a psychrometric chart. The whole process can also
be automated.
Dew point measurement
If a gas is cooled at constant pressure to the dew point, condensation of vapour will start. The
dew point can be measured by placing a clean glass mirror in the atmosphere. The temperature of
the mirror surface is controlled and reduced slowly; vapour starts condensation over the mirror.
Optical method is used to detect the condensation phenomena, and the temperature of the mirror
surface is measured.
Conductance/Capacitance method of measurement
Many solids absorb moisture and their values of the conductance or capacitance change with the
degree of moisture absorption. Moisture content in granules changes the capacitance between
two electrodes placed inside. By measuring the capacitance variation, the moisture content in the
granules can be measured. Similarly, moisture content in paper and textiles change their
resistance. A schematic arrangement for measurement of moisture content in paper or textiles
using Resistance Bridge is shown in Fig. 6. Infrared Technique
Water molecule present in any material absorb infrared wave at wavelengths 1.94μm, 2.95 μm
and 6.2μm. The degree of absorption of infrared light at any of these wavelengths may provide a
measure of moisture content in the material.
4. Explain about Measurement of pH ?
pH is a measure of hydrogen ion concentration in aqueous solution. It is an important parameter
to determine the quality of water. The pH value is expressed as:
101log=pHC
+
+
Where C is the concentration of H ions in a solution. In pure water, the concentration of H ions
-7
o
is 10 gm/ltr at 25 C. So the pH value is
71017log10pH−==.
The advantage of using pH scale is that the activities of all strong acids and bases can be brought
down to the scale of 0-14. The pH value of acidic solutions is in the range 0-7 and alkaline
solutions in the range 7-14.
The pH value of a solution is measured by using pH electrode. It essentially consists of a pair of
electrodes: measuring and reference electrode, both dipped in the solution of unknown pH.
These two electrodes essentially form two half-cells; the total potential developed is the
difference between the individual electric potential developed in each half cell. While the
potential developed in the reference cell is constant, the measuring cell potential is dependent on
the hydrogen ion concentration of the solution and is governed by Nernst’s equation:
0ln()RTEEaCnF=+
Where: E= e.m.f of the half cell
E0= e.m.f of the half cell under saturated condition
0
R= Gas constant (8.314 J/ C)
T= Absolute temperature (K)
N= valance of the ion
F= Faraday Constant = 96493 C
a= Activity co-efficient ; for a very dilute solution, (01)a≤≤1a→
C= molar concentration of ions.
Measuring Electrode
The measuring electrode is made of thin sodium ion selective glass. A potential is developed
across the two surfaces of this glass bulb, when dipped in aqueous solution. This potential is
+
0
sensitive to the H ion concentration, having a sensitivity of 59.2 mv/pH at 25 C. Fig. 7 shows
the basic schematic of a measuring probe. The buffer solution inside the glass bulb has a constant
+
H ion concentration and provides electrical connection to the lead wire.
Reference Electrode
The basic purpose of a reference electrode is to provide continuity to the electrical circuit, since
the potential across a single half cell cannot be measured. With both the measuring and reference
cells dipped in the same solution, the potential is measured across the two lead wires. A
reference electrode should satisfy the following basic requirements:
+
(i) The potential developed should be independent of H ion concentration.
(ii) The potential developed should be independent of temperature
(iii) The potential developed should not change with time.
Considering all these requirements, two types of reference electrodes are commonly used: (i)
Calomel (Mercury-Mercurous Chloride) and (ii) Silver-Silver Chloride. The construction of a
Calomel reference electrode is shown in Fig. 8. The electrical connection is maintained through
the salt bridge.
Sometimes the reference and measuring electrodes are housed together, as shown in Fig. 9. This type of
electrode is known as Combination Electrode. The reference electrode used in this case is Silver-Silver
Chloride. The combination is dipped in the solution whose pH is to be measured and the output voltage
is the difference between the e.m.f.s generated by the measuring glass electrode and the reference
electrode.
Measuring scheme
0
The sensitivity of pH probe is around 59.2mv/pH at 25 C. This sensitivity should be sufficient
for measurement of voltage using ordinary electronic voltmeters. But, that is not the case; special
measuring circuits are required for measurement of pH voltage. This is because of the fact that
8
9
the internal resistance of the pH probe as a voltage source is very high, in the order of 10 -10 Ω.
This is because of the fact; the electrical path between the two lead wires is completed through
the glass membrane. As a result, the input resistance for of the measuring device must be at least
ten times electrode resistance of the electrode. FET-input amplifier circuits are normally used for
amplifying the voltage from the pH probe. Not only that, the insulation resistance between the
leads must also be very high. They are normally provided with moisture resistance insulation
coating.
The voltage in the pH probe is temperature dependent, as evident from Nernst equation. As a result
suitable temperature compensation scheme should also be provided in the measuring scheme.