folded cascode OTA cmfb prob. help pleeeease.

[arunkumar446]

folded cascode ota

i have designed a folded cascode OTA with SC-CMFB and when i look at its transient response its output is oscillation can any one help please, it's urgent, Thanks in advance.

 

[yschuang]

folded cascode

Hello,

I would replace your cmfb with an ideal cmfb for simplicity.
Hope that is helpful.

 

[sutapanaki]

can folded cascode oscillate?

But are the oscillations common-mode or differential-mode?

 

[arunkumar446]

foldcascode ota

Thanks for replying, i just gave input dc and observed the transient response so vo1 and vo2 both are oscillating identically. so i thought it is because of the SC-CMFB unstability.

 

[sutapanaki]

ota cmfb

Yes, in this case it is a common-mode instability. Now, I guess, you'll have to post some more information about the circuit itself. Meanwhile, break the common-mode loop and see it's phase margin.

 

[arunkumar446]

cmfb folded cascode

sutapanaki, thanks for the reply, can you tell me in time domain how does the phase margin affect the stability, and thanks once again for your reply, mean while i will check my circuit and post more data.

 

[sutapanaki]

folded-cascode ota

Well, if you're asking how insufficient phase margin displays itself in time domain, you already have it - it oscillates. But in a more stable situation, if you inject a common-mode disturbance in the loop and observe how the loop settles (corrects the disturbance) you can generally judge about the phase margin by the amount of overshoot you see and also ringing.
If you post your circuit may be we can discus more specifics.

 

[arunkumar446]

common mode feedback for folded cascode

hi sutapanaki, i have tried with a simple diff amp with pmos as load and used a SC-CMFB and it didn't work it was oscillating. if you don't mind can you please specify full circuit and parameters W/L and cap values i just want to see a fully functioning circuit, i am designing in 0.18u technology. Thanks in advance.

 

[sutapanaki]

folded cascode ota phase margin

Well, actually, I also have my things to do during the day and I'm not sure I will have time to really specify a complete circuit with W/L etc. We can discuss your stuff if we have visual material.

 

[shobhit]

folded cascode common mode feedback

The gain of the common mode feedback circuit shoud be comparable to gain of the amplifier
So seperate out the CMFB circuit and check out its gain. If it is not the same thne try to tweak W/L of cmfb transistors.

 

[arunkumar446]

folded cascode with sc-cmfb

thanks guys for your replies, i found out by simulations that if the common mode gain is not high but reasonable then the output settles to the desired level in one clock phase and in the other clock phase the capacitance at output nodes is different so feedback factor changes and as said if the common mode gain is small then output settles to near by reference, if we look at this over time in transient response it seems oscillating.

Added after 1 minutes:

Well, actually, I also have my things to do during the day and I'm not sure I will have time to really specify a complete circuit with W/L etc. We can discuss your stuff if we have visual material.

Thanks for your concern, im really thankful to you, it's appreciated.