opamp design problem !!

[warlock_ajay]

I am trying to design an opamp and I am a newbie.
I don't get any bandwidth and there is some clipping in the signal.
If I try to increase the gain by increasing the size of M1-M4, the frequency response looks like a high pass.



I am totally lost, please help.

 

[windknows]

you have to change it to dB scale,
and in the schematic you are simulating the cm gain,
better to refer to the textbook..

 

[cesczhang]

the AC simulation setup circuit is not correct,there is no loop in your simulation

 

[yxo]

Use the real testbench for simulation,for example, non-inverting configuration. Feedback has to setup the bias points in an opamp.

 

[fcfusion]

You're making several mistakes in our circuit:

1) There is no DC voltage source to bias M1 and M2.

2) There is no feedback in your circuit. This means some of your transistor's will not have well defined DC operation points. Check the image I've attached. By using a 1T inductor and 1F capacitor you apply a DC feedback but in AC simulations the results will show the circuit's behaviour like if it was in open-loop because L and C are very large. This might seem odd but it works well. This is used only for ideal simuations.Once you have good results you should always test it with a non-inverting ampifier topology.

3) Your reference current is to high. You should use small current references and the use current mirrors with gain, by changing the W/L. That doesn't affect the circuit performance except for power efficiency. It's not very important right now but you should fix it.

 

[jennisjose]

The design of my Masters project operational amplifier is uploaded here. I have a gain of 71 db, PM of 45, and bandwidth of 700Mhz.

https://jennisjose.webs.com/operationalamplifiers.htm

 

[warlock_ajay]

Thank you guys.

I am trying to start it from the cratch now, from the CMOS differential pair to see that I do things right.
There seem to be a problem, pls see the snapshots. From -1 to 1 dc, the current curves are not looking good!
And so I get half the gain and half the bandwidth of what I started as specifications with.


fcfusion, jennisjose pls help.

Cheers