Hello everybody,

This is my first post in the forum. I hope I can learn as much as possible from all of you, and help you when you need it.

I'm starting right now on IC design, I would like to desing a Two Stages OpAmp. With a differential pair in the first stage.

The differential pair I'm using as reference has two PMOS as input and two NMOS as active load. I'm using AMS 0.35um technology.

My problem is that the output of the differential pair is saturated, and I really don't know how to solve it. Any idea or advice??

Thank you very much in advance.

Sorry. Here you can see the shematic. I didn't know that in open loop it get saturated. In the next post I show the schematic used to simulated it.

hello,
By doing operating point simulations, have you convinced that every transister biased in proper region? If so there is no mistake with the circuit. May be it's having high gain and thats why it is getting saturating. Do one thing, connect in unity gain configeration and ramp the input and see wether it is tracking or not at the o/p.

Thanks.

hello,
it is very difficult to control gain, you wil get some which is strongly depends on the process, at the end of the day you need to put your opamp in -ve feedback, so probably you might have some gain spe based on steady state error. So first fully simulate this block and see how much gain you have, and then try to design 2nd stage with common Source. You can check razavi for bias control (make sure that 2nd stage bias current equal to half of the tail current in the first stage)

I feel his is more than enough for normal two stage opamp.

THnaks.

Take a look at Chapter 5 of "Analog Integrated Circuit Design" by Johns and Martin. There is a section in there on designing the 2-stage opamp to have a low systematic offset. A good place for you to start. You might be able to find some of it on Google Books if you don't have a copy of the book yourself.