CALIFORNIA INSTITUTE OF TECHNOLOGY
PHYSICS MATHEMATICS AND ASTRONOMY DIVISION
Sophomore Physics Laboratory (PH005/105)
Analog Electronics
Final Project, Some Useful Circuits
c
CopyrightVirgínio
de Oliveira Sannibale, 2003
(Revision December 2012)
Chapter 9
Final Project, Some Useful
Circuits
This appendix is a collection of practical consideration, suggestions, and
useful simple circuits to be used with transducers that can help you building your final project.
9.1 Solderless Breadboard
The figure below shows the circuit breadboard contacts topology and suggested power supply connections.
W
X
A
B
C
D
E
TRENCH
10
15
20
25
30
35
40
45
50
F
G
H
I
J
Z
Y
60
−15V
GND
+15V
DR
IC
55
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5
Here some rules and guidance to follow when assembling a circuit using a solderless breadboard
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CHAPTER 9. FINAL PROJECT, SOME USEFUL CIRCUITS
• Wires gauge AWG 24 must be used.
• Do not force thick leads into the board holes connections. Leads fit
must be somehow loose to allow the spring loaded contact to work
properly.
• Green jacket wires should be used to connect components to ground
(GND), red jacket wires for +15 V, and black jacket wires for −15 V.
A different color should be used for feedback connections.
• Component’s leads should be cut short to place the components as
close as possible to the board.
• ICs must be placed on and along the trench otherwise opposite on
side IC’s pins will be short circuited.
• Always double check power supply connections to avoid damaging
or more likely destroying ICs components.
This type of pragmatism will minimize the number of surprises you will
have building your circuit and also will help you debug your circuit.
9.2 Decoupling/Bypassing Capacitor
Decoupling or bypassing capacitors are capacitor placed between the ICs
power supply inputs and ground to filter out (decouple) noise coming
from the power supply lines.
We can somehow distinguish two type of noise:
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• transient noise due to fluctuation of the voltages generated by sudden increase of current demanded by the ICs,
• broad band or high frequency noise either random or constant with
a given frequency spectrum.
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Large capacitors acting as a current reservoir are needed to compensate
for voltage transients, and small capacitor are required for high frequency
noise filtering.
A typical value for high frequency noise decoupling capacitors is 100 nF.
These capacitors should be placed very close to the ICs power supply inputs to avoid the inductive effects of long electric connections. The faster
9.3. VARIABLE RESISTORS
179
the IC circuitry is the more important the decoupling becomes especially
for high frequency noise. For transient noise, much larger capacitor with
values, between 1 µF to 10 µF, are necessary to provide some kind of "current reservoir". Apart from special cases, they can be placed somewhere
in the circuit board. Usually, decent off the shelf power supplies already
include those capacitors.
A very extensive source of information about decoupling capacitor is
found in the bibliography.
9.3 Variable Resistors
It is good practice to place potentiometer to set amplifier gains, match resistor pairs, or in circuits to null voltage offset or set the proper voltage
divider outputs. Variable resistors come in so many flavors that it is time
consuming to choose the right components. For the project purpose, it is
convenient to use at least 10 turn trimmers potentiometers with up right
wiper screw, which are compact and fit reasonably well onto the solderless
breadboard. As shown in the figure below, trimmers should be placed in
series with a resistor every time one needs to limit the current to a maximum . The figure also shows where usually the wiper (B) is located.
Vi
Rx
Vo
Wiper
Wiper
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9.4 Surface Mount Devices
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R
Due the absence of leads, Surface Mount Devices (SMD) cannot be easily used with solderless prototyping boards. Adapters, which permits to
CHAPTER 9. FINAL PROJECT, SOME USEFUL CIRCUITS
180
solder SMDs onto a small printed circuit boards with legs, can be used to
properly wire these type components on the solderless board.
There is a plethora of SMD standards such as Small Outline Integrated
Circuit (SOIC), mini-SOIC, Small Outline Package (SOP), Shrink SOP (SSOP)
Thin SSOP (TSSOP), et. cetera. Each standard differs by ICs dimensions
(especially the footprint) and distance between leads. Some of them have
so small packages that it is practically impossible to solder ICs without
expressely made tooling.
Soldering SOIC components on PCBs is somehow a quite simple operation once done two or three times. SOIC leads are about 1.3 mm apart
and therefore small an narrow solder iron tips allow to solder each lead
at a time. Smaller packaging are quite difficult to solder and because of
their small size, it is easy to damage the ICs if overheated. Consult the
data-sheet before soldering SMDs on adapters to check how long the component can withstand the maximum temperature. SOIC adapters with 8
pin connection are available in the laboratory.
9.5 Switches
Switching circuits to drive high power load are shown in the figure below
Vcc
Load
R
D0
BJT
1k−3.3kΩ
Load
R
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D0
D1
D1
Vcc
N−MOSFET
15kΩ
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The diode D0 at the input is necessary in case the switch input can go
lower than zero volts. The diode in parallel with the load is necessary if the
load is inductive to damp over-currents generated when the load switch
status. Typical inductive load are relays whose large inductance values
come from the relay’s solenoid.
9.6. VOICE COIL LOUD-SPEAKERS DRIVER
181
9.6 Voice Coil Loud-speakers Driver
Voice coil loud-speakers are transducers with a quite low input impedance,
usually of 4 Ω or 8 Ω and few to several watts of power consumption. They
therefore need quite some current to be properly driven. For our purposes
(unless the project is to make a very powerful speaker) a 50 mW to 100 mW
speaker for the mid-range acoustic frequencies is more than sufficient.
Usually, because Op-Amps can provide no more than few milliamperes,
they cannot drive a speaker directly. A current booster such as the pushpull amplifier with BJTs able to dissipate about 0.5 W, which can increase
the current by a factor 10 to 100 easily, can drive a small speaker in the
mid-range acoustic frequencies. It is fundamental to limit the current to
ensure that the transistor power rating are not exceeded. The circuit below includes the resistor necessary to limit the current flowing through
the BJTs which needs to be calculated based on the power supply voltage
VCC and the rms dissipation Prms the transistors can sustain. Resistors R
power dissipation must be also computed .
Vcc
vi
R
R0
R1
v+
Q1
−
G
vo
vA
+
Q1
R
RL
DR
−Vcc
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v−
Heat-sinks should be placed on the BJTs to ensure that the amplifier
works reliably.
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CHAPTER 9. FINAL PROJECT, SOME USEFUL CIRCUITS
9.6.1 Off the shelf Audio Amplifiers
ICs power audio amplifier suitable to power small speakers (~1 Wspeakers)
are for example the LM380 and the LM386. Heat sink should be used with
those off the shelf audio anplifiers.
9.7 Microphones
A simple condenser (capacitor) microphone can be used as a transducer
to convert audio waves into a current. A condenser microphone exploits
the change in capacitance caused by sound waves modulating the distance
of the capacitor plates. This change corresponds to a charge variation of
the capacitor which generates a current proportional to the sound wave
intensity. A voltage bias is required to charge the capacitor otherwise there
would not be a capacitance to be modulated by the sound waves. Usually
one capacitorŽs plate is a stiff conductor and the other is a light small
conducting membrane under tension which can absorb the sound wave
energy and vibrate.
Electret microphones are condenser microphones with a so called electret material as dielectric which has an intrinsic electric field analogously
to permanent magnet and its magnetic field. The electric field generates
a capacitance without the need of a bias voltage as in a conventional condenser microphone. A n-channel JFET connected as show in the figure
below "hide" the capacitive load of the electret microphone.
VDD
R
+
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VDD
R
Vo
Vo
+
Mic.
Q0
Mic.
−
+
−
C
DR
V0
−
9.7. MICROPHONES
183
The following basic preamplifier circuit can be used to amplify the
weak microphone signal to further conditioning.
Cf
15 V
VCC
100pF
Rf
R1
Mic.
AOM−4544P−2
+
6.8kΩ
C
100kΩ
+
−
−
G
10µ F
+
R2
3.3kΩ
H ( jω ) = −
R f jωRC
1
,
R 1 + jωRC 1 + jωR f C f
R=
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The voltage divider resistors R1 and R2 set the positive bias voltage for
the condenser microphone to the required level specified by the transducer
characteristics. It also sets the maximum current that the transducer will
drain. The capacitor C bypass the DC component which is too large to
be amplified. The capacitor value should be such that the high pass cutoff frequency is about 20 Hz. Electrolytic capacitor can be used because
of the microphone positive bias and therefore even large values such as
10 µF can be easily used. The capacitor C f adds a pole at high frequency
to filter out high frequency oscillations and noise. The low-pass cut-off
frequency should be at about 15 kHz. Between the poles, the inverting
amplifier gain is set as usual by the ratio R f /R and should probably be
quite large, between 50 to 100 depending on the microphone used. The
amplifier transfer function is
R1 R2
.
R1 + R2
• Part #: AOM-4544P-2-R
• Directivity: Omnidirectional
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The electret condenser microphone available in the lab has following
characteristics
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CHAPTER 9. FINAL PROJECT, SOME USEFUL CIRCUITS
• Sensitivity: -44dB
• Frequency Response: from 20 Hz to 20 kHz,
• Max supply voltage Max: 10 V
• Max drain current : 500 µA
• Impedance: R (source impedance of microphone and bias, see schematic)
• Pin-out:
(+)
(−)
9.8 Phototransistors
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It is worthwhile to notice that a speaker can be used as a microphone
as well. In fact, vibrations induced by the sound waves on the speaker
membrane will move the solenoid respect to the permanent magnet. The
relative velocity between solenoid and magnet will induce a current in the
solenoid proportional to the sound wave intensity. Dynamic microphones
have essentially the same topology of a speaker and do not require a bias
voltage as condenser microphones do.
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A common type of phototransistor is the photo-BJT whose base region is
enlarged compared to standard BJT and has the base exposed such that
light can be absorbed to generate a photocurrent. Photo-BJTs can either
have two leads , Collector and Emitter leads or all the three BJT leads.
The circuit below is the simplest configuration that can be used to have
a photo-BJT working as switch or an amplifier driven by a light source.
The operation mode depends on the value of the load R L that sets the
current ICC . For the circuit to work as a switch or as an amplifier, we need
9.9. PHOTODIODES
185
to either keep the transistor working on the saturated or the active mode
when light is impinging on the transistor, and therefore
VCC = R L IC
Switch
⇒
(max )
⇒
VCC > R L IC
Amplifier
VCC
RC
Vo
RL
Usually, because of their faster response and lower noise, photodiodes
work better as transducer to convert light power into voltage. With modulated light, photodiodes should be therefore preferred.
9.9 Photodiodes
Rs
A
Io
ID
Vo
Iph
VD
D
A
Io
Cj
Rp
DR
Pi
K
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Photodiodes are essentially semiconductor junctions which generate a current (photocurrent) when exposed to light. They are therefore used to measure light intensity. The equivalent photodiode circuit is show below.
Vo
K
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CHAPTER 9. FINAL PROJECT, SOME USEFUL CIRCUITS
Note the opposite direction of the photocurrent I ph and the forward
biased diode current ID .
• I ph : photocurrent proportional to the amount of incident light of
power P on the photodiode active surface, i.e.
I ph = −α (λ) P ,
where α depends on the light wavelength λ.
• ID : diode PN junction current
− qVD /K B T
ID = Is e
−1 .
The saturation current is usually called dark current, i.e. current with
no incident light.
• R p : shunt resistance (in parallel.)
• Rs : series resistance.
• Cj : capacitance due to the PN junction. The smaller the N doped
active region surface, the smaller is the capacitance.
The I-V characteristic of the photodiode for different values of incident
power is qualitatively shown in the figure below. Note how the input dynamic range where the response is linear increases when the photodiode
is reverse biased.
0
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T
ID
VD
Pi
2P
i
3P
i
Vo
DR
Vo
9.9. PHOTODIODES
187
Fast and low noise photodiodes are hard to build and it is even harder
in case of high power application. Ultra fast photodiodes can have an
active area of about less than 100 µm2 and bandwidth of 25 GHz. Typical
semiconductor substrate for visible light are Silicon (Si) and for infrared
light Indium-Gallium-Arsenide (InGaAs).
9.9.1 Photovoltaic Mode
In this mode, the photodiode works thanks to the photovoltaic effect. Some
of the electron-hole pairs generated by incident photons do not give up
their energy as in a metal or are complectely extracted from the solid in
the photoelectric effect. They just stay inside the semiconductor with energy in the conduction band of the semiconductor. These pairs then build
up generating a charge distribution and therefore an electric field that finally produces the photocurrent.
As shown in the figure below, the photocurrent is measured as a voltage drop across a load or is transformed into a voltage with a transimpedance
amplifier.
Rf
Vo
−
D
+
Major advantages of this configuration:
• Smaller dark current ⇒ low current noise.
• linear output because smaller dark current.
• accurate voltage response.
Major disadvantage:
Vo
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R >> R p
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• slow response. The Cj in parallel to the current generator low pass
filter the signal.
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CHAPTER 9. FINAL PROJECT, SOME USEFUL CIRCUITS
9.9.2 Photocurrent Mode
In this mode, the generated photocurrent is collected by the revers bias
voltage applied across the photodiode. As shown in the figure below,
apart from having the photodiode reverse biased with −VDD the circuits
are the same as the photovoltaic mode circuits.
Cf
−VDD
Rf
Vo
−
R
D
+
−VDD
Major advantages of this configuration:
• Fast response. The reverse voltage bias −VDD reduces the junction
depletion region and therefore the junction capacitance.
Major disadvantages:
• larger dark current than the unbiased photodiode.
Consult [3, 4] for more detailed information.
9.10 Ultrasonic Transceivers
TBD
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9.11 Velocity Sensors
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• small OpAmp current bias amplified by the transimpedance.
TBD
9.12. POSITION SENSORS
189
9.12 Position Sensors
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TBD
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CHAPTER 9. FINAL PROJECT, SOME USEFUL CIRCUITS
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190
Bibliography
[1] Semiconductor Packaging Information by Type, Texas Instruments,
http://www.ti.com/sc/docs/psheets/type/type.html
[2] Decoupling Techniques,
MT-101 Tutorial,
Analog Devices,
www.analog.com/static/imported-files/tutorials/MT-101.pdf
[3] Photodiode Technical Information - Hamamatsu Photonics,
http://sales.hamamatsu.com/assets/applications/SSD/photodiode_technical_information.pd
191
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[4] Characteristics and use of infrared detectors, Hamamatsu Photonics,
http://sales.hamamatsu.com/assets/applications/SSD/infrared_kird9001e04.pdf
BIBLIOGRAPHY
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