DC analysis gain differs from AC analysis gain

[Amr Kassem]

When I design a simple common source amplifier to give a gain of 100
for example. The AC analysis indicates that the gain is actually 100.
But when I do DC analysis and plot the derivative of Vout against Vin
I see a gain of 30. Same happens when I use cascode amplifiers and
even aim for a gain of 5000. I see a gain of 40 from the DC analysis
and 5000 from the AC analysis.

I tried removing the load capacitor but nothing changes. Any idea what
the reason for that could be?

 

[pavan.s.pai@gmail.com]

On Jul 7, 5:02 pm, Amr Kassem <amrkasse...@gmail.com> wrote:

When I design a simple common source amplifier to give a gain of 100
for example. The AC analysis indicates that the gain is actually 100.
But when I do DC analysis and plot the derivative of Vout against Vin
I see a gain of 30. Same happens when I use cascode amplifiers and
even aim for a gain of 5000. I see a gain of 40 from the DC analysis
and 5000 from the AC analysis.

I tried removing the load capacitor but nothing changes. Any idea what
the reason for that could be?

Hi Amr,

Whatever you are seeing is right . It is bound to happen that way .
Because gain given by AC analysis is at the Qpoint or Bias Point
and provided signal is small compared to the bias point of ckt.

You can see the same Gain in DC as that of AC only if you
sweep you signal about the Q point in small range .

For example if your Qpoint is 0.9 volt (input common mode) then in
DC you sweep from about 0.9+/- 100uV.

In above case you will observe same gain in both.

With Regards
Pavan Pai

 

[Amr Kassem]

Does this mean that I should decrease the value of abstol and reltol
in cadence if I want to see the same gain in both analyses?

 

[Andrew Beckett]

Amr Kassem wrote, on 07/12/09 23:17:

Does this mean that I should decrease the value of abstol and reltol
in cadence if I want to see the same gain in both analyses?

No.