problem with gain boosted opamp.

[manissri]

hi ..
i am desinging the 10msps pipelined adc.
for this i am desinging the S/H amplifier which is of type fully differential folded cascode with gain boosted opamp.
when i simulated my fully differential folded cascode witout gain boosting ..
i found my gain is 48db. UGB=230Mhz, settling time is 18ns.
but to increase the gain of this OTA.
i used the gain boosted OTA .
by applying the Gain boosted ota with main ota i find my gain is increased to 102db and pm is 66 degree.
but my opamp is not going to settle.
i want to know why my opamp is not going to settle with gain boosted ota.
where as my ota without gain boosted is fairly settling
plz reply
manish

 

[willyboy19]

What is the GBW after you inserting the gainboost amplifier?

 

[manissri]

hi wll boy..
my UGB = witout gain boosted is 230mhz
and UGB=with gain boosted is 218mhz.
so the ugb is decresing slightly...
waiting for ur reply ..
manish

 

[scottieman]

hi ..
i am desinging the 10msps pipelined adc.
for this i am desinging the S/H amplifier which is of type fully differential folded cascode with gain boosted opamp.
when i simulated my fully differential folded cascode witout gain boosting ..
i found my gain is 48db. UGB=230Mhz, settling time is 18ns.
but to increase the gain of this OTA.
i used the gain boosted OTA .
by applying the Gain boosted ota with main ota i find my gain is increased to 102db and pm is 66 degree.
but my opamp is not going to settle.
i want to know why my opamp is not going to settle with gain boosted ota.
where as my ota without gain boosted is fairly settling
plz reply
manish

pls ensure the auxiliary amplifier used for gain-boosting is stable, since it is in a closed-loop feedback operation.

Scottie

 

[Shaber_Mezbah]

Hi ..
I think that you have a pole zero dublet near your unity gain frequency. that's why you r having a good phase margin and less good settleing behaviour.

there are one very important design choice:

additional gain stages need not be as fast as the main stage, and that the first order total roll-off frequency, (f3dB)total should be smaller than the additional gain roll-off frequency, (f3dB)add.
The realtion

(f3db)main < (fu)add < (f2ndpole)main

must be maintained for stable operation of the amp
structure.

You might have designed a very small band width boosting amplifire.
try to increas it's GBW.

good luck