Applications of Field Effect Transistor

A field-effect transistor (FET) uses a gate voltage to control current. Common uses are analog switches, buffers, RF amplifiers, oscillators, choppers and current limiters.

Analog Switches

The application of FETs as analog switches follows from the channel. When V_GS is zero an n-channel JFET is ON (low channel resistance, often drawn as a short) so the output in Figure 1 is low. When V_GS is more negative than pinch-off the FET is OFF. Drain current I_D is then near zero, the drop on R_D is near zero and V_DD appears at V₀.
That ON/OFF action lets a JFET bank form the analog multiplexer in Figure 2.

Each input goes through its own JFET (T₁, T₂, … T_n) to V₀. Only the device whose gate is ON passes its signal.

If V_GS2 is past pinch-off and the other V_GS pins are at 0 V, Signal 2 is the one that reaches the output.
IGBTs are a related voltage-controlled switch used in ignition coils, where they need fast turn-off and a high blocking voltage.

Amplifiers

JFETs are used as buffers between amplifier stages. High input impedance loads the previous stage only lightly, so almost all of Stage 1 appears at the buffer input.

Using JFETs in a Common Drain Configuration, the entire buffer output can be sent to Stage 2. The low output impedance allows buffer amplifiers to drive heavy loads or small load resistances.

FETs are low noise devices when compared to Bipolar Junction Transistors (BJTs). This makes it a useful component to be used as an amplifier at the receiver front-end as one needs the minimal amount of noise at the final output. Further it is to be noted that JFETs are voltage controlled devices which makes them ideal to be used as Radio Frequency (RF) amplifiers. The reason behind this is, one expects the RF amplifier to respond appropriately even when the antennas at the receiver-end receive the weak signals (signals with a very low amount of current).
A FET amplifier in a common source (CS) configuration can be used to drive another FET amplifier in common gate configuration, forming a Cascode Amplifier as shown by Figure 4. Although the gain of the cascade amplifier is same to that of CS amplifier, its input capacitance is significantly low when compared to that of CS amplifier. Moreover the Cascade amplifier offers a very high resistance at its input.

Phase Shift Oscillator

JFETs offer high impedance at their input terminals which reduces the loading effect. Further, they can be suitably used to accomplish both amplification as well as feedback functions. This nature of FETs makes them suitable to be used in phase shift oscillator circuits as shown by Figure 5.

Chopper

JFET acting as a switch can be used as a chopper (Figure 6) wherein the DC voltage applied to it, VDC is converted into AC voltage with the same amplitude level, VAC. This is due to the fact that the square voltage waveform applied as the VGS causes the JFET to operate in cut-off and saturation regions, alternately. Such chopper circuits aid to overcome the problem of drift which exists in the case of direct-coupled amplifiers.

Current Limiter

A n-channel JFET whose Gate terminal is shorted with the Source terminal acts as a current limiter. This means that in this arrangement, FETs allow the current through them to rise to only a particular level after which it is maintained constant, irrespective of the fluctuations in the level of voltage. These current limiters form an integral part of constant-current or current-regulator diodes.
Apart from these, FETs are extensively used in Integrated Circuits (ICs) due to their compact size. They are used in mixer circuits of TV and FM receivers due to low intermodulation distortions. Moreover FETs are also used as voltage-variable resistors in OP-AMPS, tone control circuits and JFET voltmeter design. JFETs can also be used to design the timer circuits as they offer high isolation between their gate and drain terminals. Further, JFETs also find their usage in the fields like digital electronics and fibre optic systems.