Degenerated diff pair

[exp]

Hi,

I have a simple diff pair setup where I add degeneration resistance Rs to the source:

In my opinion, the gain should become now gm/(gds*(1+gm*Rs)).

However, no matter which value of Rs I pick the output of the ac analysis always stays the same (roughly).

E.g., Rs=0: a0 = 80.2407, Rs=10k: a0 = 80.2374, Rs=1M: a0 = 79.9043 where a0 is taken from the output of the ac analysis.

The same happens if I set a constant input voltage and see the output voltage with .dc analysis.

This is crazy, absolutely impossible!

Is there an obvious mistake?

 

[FvM]

Did you check circuit operation points and node voltages?

 

[erikl]

However, no matter which value of Rs I pick the output of the ac analysis always stays the same (roughly).

I guess your circuit is over-determined by I2, I13, I14 and fixed DC input voltage. Suggest to try without the common-mode source I2 .