MOSFET source follower 

In this article we are going to explain about MOSFET source follower, it is a type of MOSFET circuit configuration like the common emitter (emitter follower) at the transistor, here at MOSFET the source follower is a common drain configuration circuit.

The MOSFET source follower is also known as the voltage buffer, this is due to its characteristics shows as a circuit, in this post we try to explain some information’s about this circuit.

MOSFET source follower design 

MOSFET source follower design
MOSFET source follower design

The figure shows the source follower circuit that having a common drain configuration. We can see the input signal is applied between the gate terminal and the ground, the output signal is taken between the source and the ground terminal.

The drain terminal is connected to the ground, this is why the drain terminal will act as common for both output and input of the circuit.

  • The Ci capacitor is used to couple DC signal at the input and the C2 capacitor is also used to coupled AC and block DC element at the output signal.
  • R1 and R2 resistor voltage divider networks will be used to provide biasing and hold the gate the ground potential.
  • RS resistor is called the source resistor will be used to determine the channel current that is required for the circuit.


MOSFET source follower biasing 

Typically MOSFET circuit will be biased concerning gate and source, in this circuit, the source terminal will be connected to the ground but the output will be taken from the source terminal so,

VBIAS = VGS + Vout


Input impedance 

Zin = rG||rGS ~ rG

The input impedance of the source follower circuit will be VERY HIGH 


Output impedance 

Zout = Rs || 1/gm

The output impedance of the MOSFET source follower circuit is LOW


Voltage gain 

V= Av = gm Rs/gm Rs+1


The voltage gain of the source follower circuit will be UNITY or less than 1


Current gain 

Ai = ioutput / iinput

The current gain of the source follower circuit will be UNITY 


Power gain 

The power gain of the source follower circuit will be MEDIUM 

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