FET applications

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FET applications

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 high input 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 FETi.e. V_GS = 0, FET becomes saturated and it behaves like a small resistance usually of the value of less than 100 ohm and, therefore, output voltage becomes equal to

V_OUT = {R_DS/ (R_D + R_{DS (ON)})}* V_in

JFET-analog-switch

Since R_D is very large in comparison to R_DS 0N), so V_out can be taken equal to zero.

When a negative voltage equal to V_{GS (OFF)} is applied to the gate, 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, V_DC. 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 V_GS (off)- This alternately saturates and cuts-off the JFET. Thus output voltage is a square wave varying from +V_DC 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 (V_v V₂ and V₃) are more negative than V_GS(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 V_x is zero, the signal obtained at the output will be sinusoidal. Similarly when V₂ is zero, the signal obtained at the output will be triangular and when V₃is 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 howconstant-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.