Common mode gain of a diff. Stage with non- ideal current source

[vijaysunil]

For the diff stage with non ideal current course with current source resistance as Rss.(ckt diagram attached)
The differential gain is given by -gmRD.

I have biased and simulated the same with Iss = 100uA and Vin(common mode) = 4V.

When i did hand calculations, the differential gain comes out to be -8.98, whereas when i simulated it, the gain comes out to be -10.7 which is more then the calculated one.

My understanding is that since we have neglected gds( or Ro) in parallel with RD we should get a smaller gain when the circuit is simulated; however, the simulated value(-10.7) of the gain is more instead of being less than calculated value(-8.98)

Can somebody please explain this.

Thanks lot in advance.
Vijay

 

[mosys]

hi,
In differential mode tail node is almost like ac short so it doesn't matter how bad your current source but it matters for common mode gain. Many times i have observed problem with simulation setup, So please check or explain to us how did you measured differential gain.