GATE Electrical Engineering (EE) Test: FET Amplifier - 1 Free Online Test

Similar to the transistor emitter follower, the source follower configuration itself provides a high level of buffering and a high input impedance. The actual input resistance of the FET itself is very high as it is a field effect device. This means that the source follower circuit is able to provide excellent performance as a buffer. The voltage gain is unity, although current gain is high. The input and output signals are in phase.

For a Common gate amplifier, Current gain is about unity, input resistance is low, output resistance is high a CG stage is a current “buffer”. It takes a current at the input that may have a relatively small Norton equivalent resistance and replicates it at the output port, which is a good current source due to the high output resistance.

In common drain amplifier
Writing KCL at the source node ;
G_m(v_in – v_out) – g_mbs v_out – g_ds v_out = 0
v_out v_in = G_m / G_m + G_mbs + g_ds
Therefore gain is less than one.

Q-point” needs to be found for the correct biasing of the JFET amplifier circuit with single amplifier configurations of Common-source, Common-drain or Source-follower and the Common-gate available for most FET devices.

A common collector amplifier (also known as an emitter follower) is one of three basic single-stage bipolar junction transistor (BJT) amplifier, typically used as a voltage buffer.
In this circuit the base terminal of the transistor serves as the input, the emitter is the output, and the collector is common to both. The analogous field-effect transistor circuit is the common drain amplifier and the analogous tube circuit is the cathode follower.

The transconductance, g_m is defined as
g_m =  ΔI_D / ΔV_GS
so g_m = Voltage gain / R_D
Therefore, voltage gain = g_m  x  R_D
= 4  x 1.5
= 6.

The transconductance, g_m is defined as
g_m = ΔI_D / ΔV_GS
so g_m = Voltage gain / R_D
Therefore, voltage gain = g_m x  R_D
= 2500 x 10^-6 x 10 x 10³
= 25.

Common gate amplifier just like common base amplifier had a very large voltage gain but input impedance is very low. Also, it gives unity current gain. Hence, the power of the amplified signal will be less.

In a common source amplifier the source resistance is connected to the ground(i.e. is grounded) because of which its source resistance is kept zero.

A common-drain amplifier is also called a source-follower. Self-biasing is used in this particular circuit. The input signal is applied to the gate through a coupling capacitor, and the output signal is coupled to the load resistor through the other capacitor.