MOS source followers

• Similar to bipolar emitter followers
  
• Can be used as
  • buffers
  • dc-level shifters

Basic circuit:

 

 

Small-signal low-frequency equivalent circuit:

 

Current equation for the output node:

 

Substitution: v_gs1 = v_in - v_out

 

Voltage gain:

 

Output impedance:

Calculated by applying a test source v_x at the output

and considering v_in = 0, v_gs1 = -v_out

 

Since usually:  

 

 

R_out has relatively low value of the order of 1kW

Level shift provided by the stage can be controlled by the bias voltage