Problem with a gain boosting opa design as it oscillates

[umount]

hi everyone,

i am doing a gain boosting opa recently. the GBW of the additional opa meets the requirement of BULT's paper. but the opa still ocillates. i've no idea how this happen, and how to modify. can any body help me?

thanks

 

[hylas]

Re: gain boosting opa

Can you provide more info? E.g. Schematic and waveforms?

 

[umount]

Re: gain boosting opa

Can you provide more info? E.g. Schematic and waveforms?

Thanks for you reply.

i can describe it in details. the main amplifier is a folded cascode. the additional opamp is fully differential folded cascode. i did ac simulation, and get a GWB of about 300M. the second pole located at 1G. PM is about 80.

the GWB of additional opamp is about 500M. so i suppose slow settling contributes little in this case, and AC sim told me PM is enougn. but still, the opa ocillates.

Why?

 

[hylas]

Re: gain boosting opa

From AC simulation, you can only know small-signal characteristic of the OPA at one specific operation point. It is possible that in large signal transient, your OPA and additional OPA deviate from high-gain region.

Another possible reason may due to the instability of the gain-boosting active circuit. Although you know that the GBW of the additional OPA is 500MHz, when you attach the OPA to the cascode transistors for gain boosting, its freq. response may changed. Stability of the gain-boosting loop may be hidden from AC simulation of the whole OPA.

It will be good if you could provide the "Oscillate" waveform.

 

[umount]

Re: gain boosting opa

hi hylas,

thanks for your kindness.
here is the ocillate waveform. please have a look.

 

[umount]

Re: gain boosting opa

can anybody help me?

 

[hylas]

Re: gain boosting opa

I suspect that the gain-boosting modules are oscillating. Hope you could provide circuit schematic for further investigation. Gain-boosting OPA involves many local feedback loops, the OPA cannot work if one or more local loops are not stable.

Usually the source of the instability can't be located directly from a single bode plot. Please check the additional opa with the gain boosting stage (not the additional opa solely) could work properly or not.

 

[sayuri]

Re: gain boosting opa

I have the same problems here.

I know the discussion was ended in 2007 but I hope there's someone know the solution about this problems.

In my case, when I try to simulate it, even without the boosting amplifier, the op-amp still oscillate. Did anyone have an idea?

 

[checkmate]

Re: gain boosting opa

Do a DC extraction at the transient point in which it oscillates and run an AC analysis there. Good if you can provide your phase margin measurements.