Differential input, output amplifier simulation

[Henry-cool]

differential input simulation

Dear all~!

Thanks in advance!

I simulate differential input, output amplifier without CMFB.

Because output impedance is high and there is not CMFB, output dc voltage goes

near VDD.

And, the AC simulation result is wrong.

How do you simulate Fully differential Amplifier?

Please let me know how I set the test bench.

Thank you!!

 

[raptor1981]

Please refer grey , mayer (last chapter).

 

[PaloAlto]

OK, I think that you already have the answer:

Because output impedance is high and there is not CMFB, output dc voltage goes near VDD. And, the AC simulation result is wrong.

You need CMFB! If you don't have it implemented yet or it is not continuous time, then you need an ideal one. You have to create one block that fixes the cm output voltage.

What you have to do is just use some ideal sources:

1. Take to vcvs in series each one having one of the output voltages as inputs, in that way you get vop+vom at the output
2. Now, put that voltage through another vcvs with a gain of 0.5, then you have vcm
3. Put that voltage at one input of a vccs, at the other input put the value that you want to have for vcm
4. Feed the output current of that vccs to the gates of the current sources in the intermediate stage of your diff amp, in that way, the feedback is closed.

Now you can do ac sim