What is Field Effect Transistor (FET)? The FET is a terminal device that uses an electric field
to control the current flowing through the device. The three terminals of the FET are
known as Gate, Drain, and Source. It is a voltage controlled device, where the input voltage
controls the output current.
Application of FETs:
1) Amplifier
2) Oscillator
3) Analog Switch
4) Integrated Circuits
5) Buffer Amplifiers
Differences between the BJT and FET :
1) The FET is a unipolar device, while BJT is a bipolar device
2) The input impedance of FET is very high compared to BJT
3) The power consumption in FET is less than BJT
4) In size, the FETs are smaller than BJT
5) Thermally, FETs are more stable than BJT
Types of FET: There are many types of FETs. But mainly there are two types of FET:
1) JFET (Junction Field Effect Transistor)
1.1 n-channel JFET
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1.2 p-channel JFET
2) IG-FET (Insulated Gate Field Effect Transistor) MOSFET (Metal Oxide Semiconductor
Field Effect Transistor) is the most common type of IG-FET, where a layer of SiO2 is used
as an insulating layer to insulate the gate from the channel. The MOSFET can be further
classified into 2.1 Depletion-type MOSFET (n-channel and p-channel) 2.2 EnhancementType MOSFET (n-channel and p-channel).
Why drain current is constant in saturation region?
Because the pinch-off voltage remains more or less constant and Vds is subject to increase, the
field between them becomes stronger for increasing Vds. The electrons that are injected are
accelerated through the depletion region to the drain. Thus, (conventional+) current actually
flows from drain to source.
Difference between BJT and FET
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Bipolar junction transistors are bipolar devices, in this transistor there is a flow of both majority
& minority charge carriers.
Field effect transistors are unipolar devices, in this transistor there are only the majority charge
carriers flows.
Bipolar junction transistors are current controlled.
Field effect transistors are voltage controlled.
In many applications FETs are used than bipolar junction transistors.
Bipolar junction transistor consist of three terminals namely emitter, base and collector. These
terminals are denoted by E, B and C.
Field effect transistor consist of three terminals namely source, drain and gate. These terminals
are denoted by S, D and G.
The input impedance of field effect transistors has high compared with bipolar junction
transistors.
A BJT needs a small amount of current to switch on the transistor.The heat dissipated on bipolar
stops the total number of transistors that can be fabricated on the chip.
Whenever the ‘G’ terminal of the FET transistor has been charged, no more current is required to
keep the transistor ON.
The BJT is responsible for overheating due to a negative temperature co-efficient.
FET has a +Ve temperature coefficient for stopping over heating.
BJTs are applicable for low current applications.
FETS are applicable for low voltage applications.
FETs have low to medium gain.
BJTs have a higher max frequency and a higher cutoff frequency.
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Thus, this is all about comparison between BJT and FET, includes what is BJT and FET,
Construction of BJT, construction of FET, differences between BJT and FET. We hope that you
have got a better understanding of this concept. Furthermore, any queries regarding this concept
or electronics projects please comment in the comment section below. Here is a question for you,
what are the applications of BJT and FET?
5V Power Supply using 7805 Voltage Regulator with Design
In most of our electronic products or projects, we need a power supply for converting mains AC voltage
to a regulated DC voltage. For making a power supply designing of each component is essential. Here I
am going to discuss the designing of regulated 5V Power Supply.
Let us start with very basic things the choosing of components
Component List :
1.
2.
3.
4.
Step down transformer
Voltage regulator
Capacitors
Diodes
Let’s get into detail of rating of the devices :
Voltage regulator :
As we require a 5V we need LM7805 Voltage Regulator IC.
7805 IC Rating :
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Input voltage range 7V- 35V
Current rating Ic = 1A
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Output voltage range VMax=5.2V ,VMin=4.8V
Transformer :
Selecting a suitable transformer is of great importance. The current rating and the secondary
voltage of the transformer is a crucial factor.
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The current rating of the transformer depends upon the current required for the load to be
driven.
The input voltage to the 7805 IC should be at least 2V greater than the required 2V
output, therefore it requires an input voltage at least close to 7V.
So I chose a 6-0-6 transformer with current rating 500mA (Since 6*√2 = 8.4V).
NOTE : Any transformer which supplies secondary peak voltage up to 35V can be used but as
the voltage increases size of the transformer and power dissipation across regulator increases.
Rectifying circuit :
The best is using a full wave rectifier
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Its advantage is DC saturation is less as in both cycle diodes conduct.
Higher Transformer Utilization Factor (TUF).
1N4007 diodes are used as its is capable of withstanding a higher reverse voltage of
1000v whereas 1N4001 is 50V
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Capacitors :
Knowledge of Ripple factor is essential while designing the values of capacitors
It is given by
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Y=1/(4√3fRC) (as the capacitor filter is used)
1. f= frequency of AC ( 50 Hz)
2. R=resistance calculated
R= V/Ic
V= secondary voltage of transformer
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V=6√2=8. 4
R=8.45/500mA=16.9Ω standard 18Ω chosen
3. C= filtering capacitance
We have to determine this capacitance for filtering
Y=Vac-rms/Vdc
Vac-rms = Vr/2√3
Vdc= VMax-(Vr/2)
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Vr= VMax- VMin
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Vr = 5.2-4.8 =0. 4V
Vac-rms = .3464V
Vdc = 5V
Y=0 .06928
Hence the capacitor value is found out by substituting the ripple factor in Y=1/(4√3fRC)
Thus, C= 2314 µF and standard 2200µF is chosen
Datasheet of 7805 prescribes to use a 0.01μF capacitor at the output side to
avoid transient changes in the voltages due to changes in load and a 0.33μF at the input side of
regulator to avoid ripples if the filtering is far away from regulator.
Circuit Diagram
5V Power Supply Circuit using 7805 Voltage Regulator
Description
The 7809 is a fixed voltage linear regulator that can output 9V at up to 1A current with an input
voltage range of 11 – 35V.
PACKAGE INCLUDES:
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7809 Voltage Regulator
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KEY FEATURES OF 7809 VOLTAGE REGULATOR:
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Fixed voltage linear regulator
11-35V input voltage range
9V fixed output voltage
1A continuous current with 2.2A surge capability
TO-220 package
The 78xx series of fixed-voltage linear regulators are some of the most popular linear regulators
available and have been around for a very long time. They have built-in current limiting and
over temperature protection and tend to be fairly robust devices. The prefix may vary depending
on the mfr so you may see them listed as LM78xx, MC78xx, L78xx or just 78xx.
Basic Operation
The 7809 is a commonly used linear regulator. Input voltage can range from 11 – 35VDC and it
outputs a fixed 9V at over 1A of current and up to 2.2A of surge current.
For basic operation, no external components are required. Just hook up the input voltage and
ground and you have 9V available on the output.
If you are using it at a fair distance (> 10″) from the supply that is providing the input voltage,
then a .33uF or larger input filter capacitor is recommended. The part ideally should be a low
ESR part such as a Tantalum or Mylar capacitor, but small electrolytic caps generally work
fine. An output capacitor of 0.1uF or larger can also be added to improve output transient
response as shown below
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Power Dissipation
Linear regulators have less ripple on their outputs compared to DC-DC converters that can be
used for the same basic purpose, but the trade-off is that the linear regulators also tend to
dissipate more heat in the process. The reason is that the linear regulator uses a series pass
transistor on its output to drop the excess voltage.
The power dissipation of a linear regulator is dependent on the difference between the input
voltage (Vin) and the output voltage (Vout) along with the amount of current that is being drawn
from the regulator. The larger the voltage difference is between Vin and Vout, the higher the
power dissipation will be which limits how much current can be drawn from the device.
The power dissipation of the 7809 device is easily calculated as Power Dissipation = (Vin –
Vout) * Iout.
If the 7809 input is 12V and it is providing 1A of current, then Power Dissipation = (12V – 9V)
* 1A = 3W. The 7809 TO-220 package will need to dissipate 3W of power. Under typical
conditions, the device can dissipate about 1 – 1.25W before a heat sink becomes necessary, so in
our example here, the device would need a heat sink. Maximum output current without a heat
sink in this case would be limited to about 300 – 350mA and the device will be running in the
range of 85-95°C.
As a general rule, you always want to use as low of an input voltage as possible to minimize
power loss though the device and maximize the output current available.
Notes:
1. The tab of the 7809 is common with the Ground pin.
2. Under high current loads or with large input to output voltage differentials, the device can get
very hot, so use care when handling.
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Technical Specifications
Maximum Ratings
VIN
Maximum Input Voltage
35V
IO
Maximum Output Current
1A (Typical)
Peak Surge Current (typ)
2.2A
Output Voltage
9.0V +/- 4%
Drop-out Voltage
2.0V
IMAX
Operating Ratings
VO
VI – VO
Package
TO-220
Package Type
Plastic Tab, 3-lead, through hole
Manufacturer
ON Semiconductor
Datasheet
7809
Why do we use semiconductors instead of conductors in electronics fabrication?
Answer: In conductor, there is a full flow of current (zero resistance). Semiconductor can act
both an insulator as well as a conductor based on the voltage input. Hence, there is a possibility
to control the current flow in semiconductor.
A conductor conducts electricity, and this conduction is much better than that of a
semiconductor. However, we can change the conduction properties of a semiconductor. The
basic idea of electronics comes from this modification of conduction properties.
What is the difference between the voltage regulators and zener diodes?
They both used to sustain the voltage at a certain level.
Answer 1: Zener diode "shares" current with the load; the more is drawn by a load the less goes through
zener, and that is one reason for not very stable voltage regulation with zeners. On the other hand an IC
voltage regulator draws from power source a bias current which is almost constant regardless of the
load and input voltage and in most cases is negligible.
Second reason, and in the same time difference from IC voltage regulators, is poor temperature
response from zener diodes. Again, the IC voltage regulators are temperature compensated and their
output voltage is almost independent of temperature.
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Answer 2: Zener diode regulators are not so stable as it works like a series/shunt variable resistor
without any feedback for voltage or temperature changes and in sudden fluctuations in power supply.
However, IC voltage takes all this in to consideration while designing the ic with a feedback loop.
Answer 3: yes simply the main difference between zener and regulator that,
Zener: share current with load that makes load voltage unstable.
Regulator: isolate load from source, so whatever was consumed current by load voltage does not
effect till load current in the limit band current value of the regulator itself.
Answer 4: Zener diodes cannot provide voltage regulation for wide range of load current, and that is
why to need a voltage regulator.
Answer 5: Zener diodes are better to be used in very low power circuits as voltage references or simillar
functions. If you need constant voltage at high load currents, the zener as a shunt type regulator, should
consume a large amount of power. That is a waste in heat.
Also a voltage regulator (ICs like 78XX series etc.) give better voltage stability over a wider load variation.
Answer 6: using Zener or regulator is depend on your application. when you want a voltage around a six
voltage (for example you want 6.2 volt but can alter event to 6.8 ) and limited current Zener is good and
chip. However, regulator because of their feedback characteristic fix the voltage (the tolerance even
below .1%) and higher current they can support but more expensive than zener.
Construction and Working of PN Junction Diode
In PN Junction diode, potential barrier is approximately 0.7V for a silicon PN junction and 0.3V
for a germanium PN junction. If one side of a single crystal of pure semiconductor (Germanium
Ge or Silicon Si) is doped with acceptor impurity atoms and the other side is doped with donor
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impurity atoms, a PN junction is formed as shown in animation. P region has a high concentration
of holes and N region contains a large number of electrons. As soon as the junction is formed, free
electrons and holes cross through the junction by the process of diffusion. During this process, the
electrons crossing the junction from N-region into the P region, recombine with holes in the Pregion very close to the junction. Similarly holes crossing the junction from the P-region into the
N-region, recombine with electrons in the N-region very close to the junction. Thus a region is
formed, which does not have any mobile charges very close to the junction. This region is called
depletion region. In this region, on the left side of the junction, the acceptor atoms become negative
ions and on the right side of the junction, the donor atoms become positive ions. An electric field
is set up, between the donor and acceptor ions in the depletion region. The potential at the N-side
is higher than the potential at P-side. Therefore electrons in the N-side are prevented to go to the
lower potential of P-side. Similarly, holes in the P-side find themselves at a lower potential and
are prevented to cross to the N-side. Thus, there is a barrier at the junction which opposes the
movement of the majority charge carriers. The difference of potential from one side of the barrier
to the other side is called potential barrier. The distance from one side of the barrier to the other
side is called the width of the barrier, which depends upon the nature of the material.
P-N JUNCTION
A p-n junction is the basic building block of many semiconductor devices like diodes,
transistor,etc.A clear understanding of the junction behavior is important to analyse the working
of other semiconductor devices. We will now try to understand how a junction is formed and
how the junction behaves under the influence of external applied voltage (called bias).
P-N JUNCTION FORMATION
Consider a thin p-type silicon (p-Si) semiconductor wafer. By adding precisely a small quantity
of pentavelent impurity, part of the p-Si wafer can be converted into n-Si. There are several
processes by which a semiconductor can be formed. The wafer now contains p-region and nregion and a metallurgical junction between p-, and n- region. Two important processes occur
during the formation of a p-n junction: diffusion and drift. We know that in an n-type
semiconductor, the concentration of electrons (number of electrons per unit volume) is more
compared to the concentration of holes. Similarly, in a p-type semiconductor, the concentration
of holes is more than the concentration of electrons. During the formation of p-n junction, and
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due to the concentration gradient across p-, and n- sides, holes diffuse from p-side to n-side (p →
n) and electrons diffuse from n-side to p-side (n → p). This motion of charge carries gives rise to
diffusion current across the junction. When an electron diffuses from n → p, it leaves behind an
ionized donor on n-side. This ionised donor (positive charge) is immobile as it is bonded to the
surrounding atoms. As the electrons continue to diffuse from n → p, a layer of positive charge
(or positive space-charge region) on n-side of the junction is developed.
Similarly, when a hole diffuses from p → n due to the concentration gradient, it leaves behind an
ionised acceptor (negative charge) which is immobile. As the holes continue to diffuse, a layer of
negative charge (or negative space-charge region) on the p-side of the junction is developed.
This space-charge region on either side of the junction together is known as depletion region as
the electrons and holes taking part in the initial movement across the junction depleted the region
of its free charges (Fig. 1.1). The thickness of depletion region is of the order of one-tenth of a
micrometer. Due to the positive space-charge region on n-side of the junction and negative space
charge region on p-side of the junction, an electric field directed from positive charge towards
negative charge develops. Due to this field, an electron on p-side of the junction moves to n-side
and a hole on n side of the junction moves to p-side. The motion of charge carriers due to the
electric field is called drift. Thus a drift current, which is opposite in direction to the diffusion
current (Fig. 1.1) starts.
FIGURE 1.1 p-n junction formation process
Initially, diffusion current is large and drift current is small. As the diffusion process continues,
the space-charge regions on either side of the junction extend, thus increasing the electric field
strength and hence drift current. This process continues until the diffusion current equals the drift
current. Thus a p-n junction is formed. In a p-n junction under equilibrium there is no net current.
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The loss of electrons from the n-region and the gain of electron by the p-region causes a difference
of potential across the junction of the two regions. The polarity of this potential is such as to oppose
further flow of carriers so that a condition of equilibrium exists. Figure 1.2 shows the p-n junction
at equilibrium and the potential across the junction. The n-material has lost electrons, and p
material has acquired electrons. The n material is thus positive relative to the p material. Since this
potential tends to prevent the movement of electron from the n region into the p region, it is often
called a barrier potential.
FIGURE 1.2 (a) Diode under equilibrium (V = 0), (b) Barrier
potential under no bias
Semiconductor Diode (P-N Junction as a Diode)
Semiconductor diode can be constructed by doping one side of semiconductor material (say Si,
Ge etc.) with pentavalent impurities and the other side with trivalent impurities. It means like
joining the p-type material and n-type material. In the junction, the holes and electrons combine
each other resulting in a lack of carriers in the region near the junction. This region of uncovered
positive and negative ions is called the depletion region due to the depletion of carriers in the
region. The ions in the depletion region are called space charge.
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Figure: PN Junction Diode and Depletion Region
Barrier Potential:
The accumulation of electric charge on ions of opposite polarities in two separate regions cause
an electric field to be established between those regions. The direction of the field is from the
positive region to the negative region. This electric field opposes the flow of electrons from the
N-region into the P-region and the flow of holes from the P-region into the N-region. Hence this
electric potential is known as barrier potential. However, this potential enables the flow of
electrons from P-region into N-region and holes from N-region into P-region.
Energy Band Diagram of a P-N Junction
The valence and conduction bands in an N-type material are at slightly lower energy levels than
the valence and conduction bands in the P-type material. This is due to the difference in the
atomic characteristics of the pentavalent (N-type) and trivalent (P-type) impurity atoms. The
energy diagram for a P-N junction at the instant of formation is shown in the figure alongside.
The valence and conduction bands on the N-side are at lower energy levels than those on the Pside. However, the conduction bands on both sides and valence bands on both sides overlap to a
considerable extent. On P-side, the majority carriers i.e. holes occupy the valence band whereas
the thermally generated electrons (minority carriers) occupy the conduction band. Whereas on
the N-side, the majority carriers (electrons) are in the conduction band whereas the minority
carriers i.e. hole occupy the valence band.
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Diode
A pn junction is known as a semi-conductor or crystal diode.
The outstanding property of a crystal diode to conduct current in one direction only permits it to
be used as a rectifier. A crystal diode is usually represented by the schematic symbol shown in
Fig. 2. The arrow in the symbol indicates the direction of easier conventional current flow.
Fig 2. Diode symbol
A crystal diode has two terminals. When it is connected in a circuit, one thing to decide is
whether the diode is
forward or reverse biased. There is an easy rule to ascertain it. If the external circuit is trying to
push the conventional current in the direction of arrow, the diode is forward biased. On the other
hand, if the conventional current is trying to flow opposite to arrowhead, the diode is reverse
biased. Putting in simple words :
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i)
If arrowhead of diode symbol is positive w.r.t. bar of the symbol, the diode is forward
biased.
ii) If the arrowhead of diode symbol is negative w.r.t. bar, the diode is reverse biased.
Identification of crystal diode terminals. While using a crystal diode, it is often
necessary to know which end is arrowhead and which end is bar.
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