Field Effect Transistors (FETs) are used as
The application of FETs as the switches in analog circuits is a direct consequence of their mode of working. This is because when the Gate-Source voltage, VGS is zero, n-channel FET will operate in saturation region and will act like (almost) a short circuit. Thus the output voltage will be zero (Figure 1). On the other hand, if a negative voltage is applied between the gate and source terminals i.e. if VGS is negative, then the FET operates in its cut-off or pinch-off region. This means that, in this case, the FET acts like an open circuit and the drain current, ID will be equal to zero. Due to this, the voltage across the load resistance RD will be zero which further causes the VDD to appear at V0.
This property of JFET to behave like a switch can be exploited in order to design an Analog Multiplexer as shown by Figure 2.
Here each of the input signal (Signal 1, Signal 2, … Signal n) is made to pass through a dedicated JFET (T1, T2, … Tn) before being connected to the output terminal, V0. Here only one signal among the multiple input signals will appear at the output terminal depending on the voltages VGS at the gate terminals of the FETs.
For example, if VGS2 is negative while all other VGS supplies are zero, then the output signal will be Signal 2.
Moreover, the switching property of the Insulated Gate Bipolar Transistors (IGBTs) is exploited in internal combustion engine ignition coils which demand fast switching and voltage blocking capabilities.
Junction FETs (JFETs) are used in the stage of amplification which isolates the previous stage from the next stage and thus acts as buffer amplifiers (Figure 3). This is because JFETs have very high input impedance due to which the preceding stage will be lightly loaded causing the entire output of Stage 1 to appear at the input of the buffer.
Further, the entire buffer output can be made to appear at the input of Stage 2 using JFETs in Common Drain Configuration, due to the low output impedance offered. This even means that the buffer amplifiers are capable of driving 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.
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.
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.