help! about this gain-boosting OTA

[spring1860]

pole zero doublet gain boosting

HI:
I designed a OTA with telescopic gain-boosting architecture,
the main OTA has a DC-gain of 50dB, and UGB 450MHz, the auxiliary amp are 61dB 308MHz, and 69dB 314MHz, respectively.
But, combine them together, the DC gain is only 70dB, but UGB 510MHz of the whole OTA.

I simulate them alone with the same OP operating point appeared when combined them gogether , they all works OK. BUT the performace of the whole CKT breakdown..
WHY! PLZ help !
Thanks .

 

[spring1860]

NO ONE HAVE EVER DESIGNED A GBOTA?

 

[present]

should can get at least 80dB DC gain

 

[electronrancher]

It could be that your ota is not strong enough to drive the input capacitance of your second stage.

What is the connection? You should upload a simplified schematic to allow others to debug your issue.

 

[john.dcheng]

I think you have to consider the output impedance and the input impedance.

Added after 32 seconds:

I think you have to consider the output impedance and the input impedance.

 

[spring1860]

HI
thanks to all your replies. the architecture i used as show below, and i find the problem now, that is the resistors used to detect the common mode voltage are too small, and it dominate the effective output impedance, make the gain-boosting dosn't work. when i change the common mode detect ckt, it works well now.

the DC gain is 120dB now ,and the GBW 550M. but it seems not stably (check the details use the following link ,please), the phase curve start from 180 deg ,fall to 90 deg and then it rise to 240deg , i don't know whether it is stably or not . Does the rise of the phase curve caused by the zeros?



plz give me some advice !
THANK YOU !!!

 

[yschuang]

Be careful about pole-zero doublet effect when desinging a gain boosting amplifier.
Please refer to following papers.
[1] K. Bult and G. Geelen. "A fast-settling CMOS op amp with 90-dB DC gain and 116 MHz unity-gain
frequency," ISSCC Dig. Tech. Papers, Feb. 1990. pp. 108-109.
[2] K. Bult and G. Geelen. "The CMOS gain-boosting technique," Analog Integrated Circuits and Signal
Processing, vol.1, (no.2), Oct. 1991. pp. 119-35.

 

[spring1860]

hi
guys
this is the transient response with a step signal input, and it is unstable obviously.
I think i should check the zeros (and pole-zero doublets) carefully .
thanks to all !!

 

[yschuang]

I want to reminod one thing about pole zero doulbet.
I remembered pole zero doublet effect may not be shown on your bode plot.
Please correct me if i made a mistake.

 

[spring1860]

I want to reminod one thing about pole zero doulbet.
I remembered pole zero doublet effect may not be shown on your bode plot.
Please correct me if i made a mistake.

yes , you are right ,pole-zero doublets doesn't shown in bode plot, but it slow down the settling.
But now i have a question: how can we know the doublet slow down the settling ?
thanks!

 

[zhaoshibin]

It is seem to a continuous CMFB in your OTA design.you should pay much attendtion of the poles introduced by the CMFB's difference pair.According to your step response curve,I don't think the fluctuate is attributed to the doulet,It look like that the CMFB's path have introduced a pole low than the dominant pole.Maybe a compensate of the CMFB is needed