Analog Switches:
The use of FETs as switches in simple circuits is an immediate consequence of their method of activity. This is on the grounds that when the entryway source voltage, VGS, is zero, the n-direct FET will work in the immersion district and act (nearly) like a short out. Hence the result voltage will be zero (Figure 1). Then again, in the event that a negative voltage is applied between the door and source terminals for example assuming VGS is negative, the FET works in its cut-off or squeeze off district. This intends that, for this situation, the FET goes about as an open circuit and the channel current, ID, will be zero. Along these lines, the voltage across the heap obstruction RD will become zero making VDD show up at V0.
Here each input signal (signal 1, signal 2, … signal n) is made to pass through a specific JFET (T1, T2, … Tn) before being connected to the output terminal, V0. Here only one of 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, the output signal will be signal 2.
Additionally, the switching property of insulated gate bipolar transistors (IGBTs) is used in internal combustion engine ignition coils that demand fast switching and voltage blocking capabilities.
Amplifiers:
Junction FETs (JFETs) are used in the amplification stage to isolate the previous stage from the next stage and thus act as a buffer amplifier (Figure 3). This is because JFETs have a very high input impedance which will cause the previous stage to be lightly loaded causing the entire output of stage 1 to appear at the input of the buffer.
Further, because of the low output impedance offered, the entire buffer output can be reflected to the input of stage 2 using JFETs in a common-drain configuration. This also means that buffer amplifiers are capable of driving heavy loads or small load resistances.
FETs are low noise devices compared to bipolar junction transistors (BJTs). This makes it a useful component for use as an amplifier at the front end of a receiver because the minimum amount of noise is required at the final output. It should also be noted that JFETs are voltage-controlled devices, which makes them ideal for use as radio frequency (RF) amplifiers. The reason behind this is that, one expects the RF amplifier to respond adequately even when the antennas at the receiver end receive weak signals (signals with a very low amount of current).
A FET amplifier in common-source (CS) configuration can be used to drive another FET amplifier in common-gate configuration, forming a cascode amplifier as shown in Figure 4. The input capacitance is significantly lower than that of a CS amplifier. Moreover, the cascade amplifier presents a very high impedance at its input.
Phase Shift Oscillator:
JFETs offer a higher impedance at their input terminals which reduces the loading effect. In addition, they can be used to fulfill both amplification as well as feedback functions. This nature of FETs makes them suitable for use in phase-shift oscillator circuits as shown in Figure 5.
Chopper:
A JFET acting as a switch can be used as a chopper (Figure 6) in which, when a DC voltage is applied to it, VDC is converted to an AC voltage with the same amplitude level, VAC. Is. This is due to the fact that the square voltage waveform applied as VGS causes the JFET to operate alternately in the cutoff and saturation regions. Such chopper circuits help overcome the amplification problem that exists in the case of direct-coupled amplifiers.
Current Limiter:
An n-channel JFET with its gate terminal shorted to the source terminal acts as a current limiter. This means that in this configuration, the FETs allow current to rise through them only up to a certain level after which it is maintained constant, regardless of fluctuations in the voltage level. They form an integral part of current limiting constant current or current regulator diodes.
In addition, FETs are widely used in integrated circuits (ICs) due to their compact size. They are used in mixer circuits of TV and FM receivers because of their low intermodulation distortion. Furthermore, FETs are also used as voltage variable resistors in OP-AMPS, tone control circuits and JFET voltmeter designs. JFETs can also be used to design timer circuits because they offer greater isolation between their gate and drain terminals. Additionally, JFETs also find their use in areas such as digital electronics and fiber optic systems.
