I have a degenerated common source amplifier. I want to know what's the maximum input signal I can apply before the output signal begins to clip.

What I currently do is to increase the input signal until I see in the scope that the output signal begins to clip. It's a trial an error method.

But I would like to know what's the general procedure to compute (by hand) the maximum input signal without clipping.


simulate this circuit – Schematic created using CircuitLab


Make all resistors 1Meg Ohm. Then go back and increase RG1 to 2meg Ohm. Define Vthreshold as zero volts. Define VDD as 10 volts. You will find Vdrain sitting at 2/3 of VDD or 6.6 volts. Your "linear swing", or swing with some small clipping, will be 6.6volts +- 3.3 volts. Approximately.

This works, approximately, for FETS and bipolars.

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That’s rather simple if you know your way around FET equations.

The absolute theoretical maximum swing will be:

  • on the top side the power supply when the FET is turned off by the input.
  • on the bottom side the voltage across the bottom resistor when the FET enters triode.

You have the conditions at the border of both regions Vgs = Vt, I=0 for one. Vds = Vgs-Vt, I = K*Vds^2 for the other.

Solve the drain and source mesh equations under each condition. This leads to a system of quadratic equations.

Bias the gate so that the drain, in the saturation region, is in the middle between those two extremes, and you have your maximum possible excursion.

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