2 stage cascode opamp phase margin

[impulse123]

Hi all,

I am designing a 2 stage cascode opamp.
My specs are 100 MHz 3-db Bandwidth, phase margin of >= 60 degree.
Also, I have to meet linearity specs.
H
I have attached my schematic and gain and phase plots.
However, I am not able t0 understand the phase plot. Can anybody let me know if its correct? Or am i looking at the wrong thing?

P.S. : I am very new to analog domain
Thanks

 

[jgk2004]

Correct me if I am wrong but you are looking at the closed loop response. If your wondering about the amplifiers phase margin you will need to do an open loop response. Also you say you want a 100MHz 3db BW but with what feedback factor? If its open loop your 3dB BW will be small, but if your close loop gain is small say by looking at your picture 10dB, then you can very easily get your 100MHz 3db BW.

You can get alittle info on your phase margin form you closed loop response by looking at how it is peaking. I would say you should have around 60 to 70ish PM, but that is just guessing.
Hope this helps
Jgk

 

[impulse123]

Hi jgk2004,

Thanks for your reply. Yes you are correct. I am looking at closed loop response. I was just wondering if closed loop response gives me correct picture of phase margin or not? Also, my closed loop gain varies from -12db to 6db.
So I would have a look at the open loop response now and hope to get 100MHz 3db BW and anrd 70 degree phase margin.
Also, my circuit looks stable when i do a transient step simulation. Does that imply that my circuit has enough phase margin ?

 

[jgk2004]

The transient response will show the real stability. You can tell by how your step response looks and get a idea of your PM. You will not get exact values thou. Can you post your step response. If it has no overshoot or an RC rising, you have ~90 or higher PM, you have a two stage with no lead compensation so you should be anywhere from 90 down. If you have alittle overshoot and settle you will have between 80 to 70. If you have an overshoot and one to two rings you have like 50 to 60. If you have more then two rings you will have 50 to 30ish.....If it rings forever you have 30 to 0... Just note this is just basic estimating here....

Jgk

 

[LvW]

Hi impulse123,

in all of your postings you speak about "closed-loop response" without mentioning HOW you are going to close the loop - that means: What closed-loop gain? This is an important point as far as the PM is concerned. As you probably know, the most critical case is unity feedback (gain=0 dB).

 

[impulse123]

Hi jgk2004,

I am attaching my closed loop and open loop transient response. My closed loop transient response look fine. but open loop has a ring, and my phase margin is also too less.
I am not able to understand where are things getting wrong?
My design has resistive feedback in fully differential mode.

 

[jgk2004]

Well first of all, if your using the amplifier in closed loop, don't do an open loop step response. As for the closed loop step response you need to zoom in on the transition to see the detail. Also put in a square wave so you can see the rising and falling edges.
As for your open loop response it does not look correct. You would want to have your two stage amplifier after compensation have a first order role off till the unity BW. Your amplifier is looking stable for your feedback factor 20 (10dB) because you are shifting everything down in your AC response, thus you have good margin. If you connect your amplifier as a follower it will not be stable anymore.

Also in your amplifier you are doing something funny with your output, unless this is some new kind of amplifier. You have your cascodes upside down. The output of the first stage should to the bottom NMOS transistors, then the NMOS transistors on the top should be connected to the bias from the left. This could be your problem right there!

Jgk