Directly simulating an op amp in an open loop configuration is difficult because of the large gain.  The smallest offset cause the output to go to one of the rails.  The best way to simulate the op amp's open-loop characteristics is to use an ideal dc feedback that is active at dc and not at ac.  One this you can do is use huge resistors in the feedback path.  See the Baker and Li text for an example of this.

Really thanx to both of you.

Ken Kundert wrote on Jan 9^th, 2007, 4:10pm:


In Spectre, the method described in this article has been implemented in the stb analysis. So, if you are using Spectre, I would strongly recommend that you use the stb analysis for loop gain simulation. If you are using a different simulator, you might want to take a look at http://www.designers-guide.org/Forum/YaBB.pl?num=1155668476/12#12.

But still I am facing same problem,

If you consider CS amp,
load as pmos current mirror circuit (m2 and m3) and nmos (m1),...(m2 diode connected)
Here if you simulate this amp ckt,I am getting below mention result :

After plotting 'gain and frequency plot'

1. m1 in sat and m2 in sat --> 35 db gain.
2. m1 in linear and m2 in sat --> 12 db gain.
3. m1 in sat and m2 in linear --> 12 db gain.

As we have discussed earlier,it is very difficult to get both transistors in sat region without f/b.
But in two stg opamp when you are connecting this CS amp as a second stage,it has a input from
o/p of differetial amp circuit.
As this differential amp having large o/p range variation,it wont able to keep CS amp's both transistors in sat
region (it results in gain reduction).
Above dicussion regarding open loop configuration.
Then how we can get correct open loop gain?

Plz,help me out.
Thanks.
 

As Ken pointed out, you should not simulate the amplifier open loop - after all, you're not going to use it open loop are you? The stb analysis in spectre allows you to find the loop gain, phase margin and gain margin for a single-ended or differential feedback loop (as Frank pointed out).

Did you try those, or did you persevere with doing something that you were recommended not to?

I guess a picture showing what you're doing would have helped...

Regards,

Andrew.