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Home / Tutorials / FET
applications

FET applications
Last Updated on August 31, 2009
by jojo in Tutorials with 8
Comments

FET has a
very high input impedance (100 Mega ohm in case of JFETs and 104 to
109 Mega Ohm in case of MOSFETs), the major shortcomings of an
ordinary transistor i.e. low input impedance with consequent of loading
of signal source is eliminated in FET. Hence FET is an ideal device for use in
almost every application in which transistors can be used. FETs are widely used
as input amplifiers in oscilloscopes, electronic voltmeters and other measuring
and testing equipment because of their high input impedance.

As a FET chip occupies very small space as compared to BJT chip, FETs are
widely used in ICs.

FETs are used as voltage-variable resistors (WRs) in operational
amplifiers (op-amps) and tone controls etc, for mixer operation on FM and TV
receivers and in logic circuits.

FETs are generally used in digital switching circuits though their
operating speed is lower.

Applications of FET:

1. Low
Noise Amplifier. Noise is an undesirable
disturbance super-imposed on a useful signal. Noise interferes with the
information contained in the signal; the greater the noise, the less the
information. For instance, the noise in radio-receivers develops crackling
and hissing which sometimes completely masks the voice or music.
Similarly, the noise in TV receivers produces small white or black spots on
the picture; a severe
noise may wipe out the picture. Noise is independent of
the signal strength because it exists even when the signal is
off.

Every electronic device
produces certain amount of noise but FET is a device which causes very little
noise. This is especially important near the front-end of the receivers and
other electronic equipment because the subsequent stages amplify front-end noise
along with the signal. If FET is used at the front-end, we get less amplified
noise (disturbance) at the final output.

2.Buffer Amplifier.

A buffer
amplifier is a stage of amplification that isolates the preceding stage
from the following stage. Source follower (common drain) is. used as
a buffer amplifier. Because of the high input impedance and low output
impedance a FET acts an excellent buffer amplifier, as shown in figure.
Owing to high input impedance almost all the output voltage of the preceding
stage appears at the input of the buffer amplifier and owing to low output
impedance all the output voltage from the buffer amplifier reaches the input of
the following stage, even there may be a small load resistance.

3.Cascode Amplifier.
Circuit diagram for a cascode amplifier using
FET is shown in figure. A common source amplifier drives a common
gate amplifier in it.

Cascode amplifier circuit

The cascode amplifier has the same voltage gain as a common source
(CS) amplifier. The main advantage of cascode connection is its low input
capacitance which is considerably less than the input capacitance of a CS
amplifier. It has highinput resistance which is
also a desirable feature.

4.
Analog Switch. FET as an analog switch is
shown in figure. When no gate voltage is applied to the FET i.e.
VGS = 0, FET becomes saturated and it behaves like a small
resistance usually of the value of less than 100 ohmand, therefore,
output voltage becomes equal to

VOUT = {RDS/ (RD + RDS
(ON))}* Vin

JFET-analog-switch

Since
RD is very large in comparison to RDS 0N),
so Vout can be taken equal to zero.

Whena negative voltage
equal to VGS (OFF) is applied to thegate, the FET
operates in the cut-off region and it acts like a very high resistance usually
of some mega ohms. Hence output voltage becomes nearly equal to input
voltage.

5.
Chopper. A direct-coupled amplifier can be
built by leaving out the coupling and bypass capacitors and connecting the
output of each stage directly to the input of next stage. Thus direct current is
coupled, as well as alternating current. The major drawback of this method is
occurrence of drift, a slow shift in the final output voltage produced by
supply transistor, and temperature variations.

The drift
problem can be overcome by employing chopper amplifier as illustrated in
figure.

Chopper Amplifier

(a). Here
input dc voltage is chopped by a switching circuit. The output of chopper is a
square wave ac signal having peak value equal to that of input dc voltage,
VDC. This ac signal can be amplified by a conventional ac amplifier
without any problem of drift. Amplified output can then be ‘peak detected’ to
recover the amplified dc signal.

Square wave
is applied to the gate of a FET analog switch to make it operate like a chopper,
as illustrated in other figure. The gate square wave is negative-going
swing from 0 V to at least VGS (off)-
This alternately saturates and cuts-off the JFET. Thus output voltage is
a square wave varying from +VDC to zero volt alternately.

If the
input signal is a low-frequency ac signal, it gets chopped into the ac waveform
as shown in last figure (c). This chopped signal can now be amplified by an ac
amplifier that is drift free. The amplified signal can then be peak-detected to
recover the original input low frequency ac signal. Thus both dc and low
frequency ac signals can be amplified by using a chopper amplifier.

6. Multiplexer.

FET multiplexer

An
analog multiplexer, a circuit that steers one of the input
signals to the output line, is shown in figure. In this circuit each JFET
acts as a single-pole single-throw switch. When the control signals
(Vv V2 and V3) are more negative than
VGS(0FF) all input signals are blocked. By making any control voltage
equal to zero, one of the inputs can be transmitted to the output. For instance,
when Vx is zero, the signal obtained at the output will be
sinusoidal. Similarly when V2 is zero, the
signalobtained at the output will be triangular
and when V3 is zero, the output signal will be square-wave one.
Normally, only one of the control signals is zero.

7. Current Limiter.

JFET current Limiter

JFET current limiting circuit
is shown in figure.  Almost all the supply voltage therefore appears across
the load. When the load current tries to increase to an excessive level (may be
due to short-circuit or any other reason), the excessive load current forces the
JFET into active region, where it limits the current to 8 mA. The JFET now acts
as a current source and prevents excessive load current.

A
manufacturer can tie the gate to the source and package the JFET as a two
terminal device. This is how constant-current diodes are made. Such
diodes are also called current-regulator diodes.

8.
Phase Shift Oscillators.

FET-phase shift oscillator

JFET can
incorporate the amplifying action as well as feedback action. It,
therefore, acts well as a phase shift oscillator. The high input
impedance of FET is especially very valuable in phase-shift oscillators in order
to minimize the loading effect. A typical phase shift oscillator employing
N-channel JFET is shown in figure.

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Show Comments

8 thoughts on “FET applications”

Akshay N November 7, 2014 at 11:53
pm

What is the value of Resistance in JFET multiplexer?And What model of JFETs
do we need tp use in this experiment??

Reply
↓

susanne October 18, 2014 at 3:30
am

Very beneficial explanation. What I would like to add is one thing very
interesting. In this transistor, the constriction between the Source(S) and
(D) brings to mind the picture of squeezing a water hose to reduce the flow of
water!

Reply
↓

ankurpandey January 31, 2014 at 8:51
pm

by this i cleared my topic very easily thank u

Reply
↓

shashibhushan kumar December 29, 2012 at 2:22
am

good theory

Reply
↓

rehmani sadiq December 26, 2012 at 7:22
pm

sir i want to know fet transister use in replace of transister.i have lot
a fet .

Reply
↓

Eranda December 12, 2012 at 4:10
am

very valuable site…….thanx

Reply
↓

Sharath November 25, 2012 at 6:48
am

What is the value of Rg to be chosen for the analog switch
circuit?

Reply
↓

appu March 2, 2012 at 9:03
pm

use full

Reply
↓

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