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Design >> Analog Design >> question on diff amp
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Message started by bharat on Mar 19^th, 2006, 9:22am

Title: question on diff amp
Post by bharat on Mar 19^th, 2006, 9:22am

I am having conventional differential amplifier with pmos current mirror as load, nmos diff pair and nmos tail current. My Vcc in worst case goes down to 0.945 V (1.05 V typical). As my Vt drop is close to the same my Vcc (3xVt approx equal to 0.945) plus Vod (overdrive voltage), my current mirror, diff pair or nmos tail has to go out of saturation.
My question is, which one I can bear to be out of saturation (I understand, ideally none) and still I can get close to ideal diff amp performance, i.e. minimum offset, better gain, bandwidth & less propagation delay.

thanks
-Bharat

Title: Re: question on diff amp
Post by raul on Mar 20^th, 2006, 4:56pm

I think you should use a folded cascode in order to avoid having your nmos input pair going into triode. I don't know how you are going to get this circuit to operate over temperature though, since Vt generally varies between 1 and 2 mV per degree celsius. If you have low-vt devices or natural nmos devices you should use them.

Title: Re: question on diff amp
Post by icarus on Mar 23^rd, 2006, 11:34pm

Hello Bharat,

I would think if you have to do a tradeoff, I would compromise on the tail current source. As long as you are not worried too much about high frequency CMRR, it should be okay. Simulate and check.
All teh best.

Title: Re: question on diff amp
Post by Paul on Mar 24^th, 2006, 5:12am

Hi Bharat,

if your diff pair or the load go out of saturation, this will directly affect your gain due to the increase of output conductance in the triode region. In that sense I can only confirm Icarus' statement to accept operating the tail current source in linear region. However in this case, your bias current will be supply, temperature and input common-mode dependent, which affects the gain-bandwidth product through the variation of transconductance and is not ideal either.

You may want to modify your bias current mirror so that it can work in linear regime without being subject to channel length modulation effects. For this, you need to monitor the current source's drain voltage and apply the same to the drain of the "diode-connected" mirror transistor through a cascode device, as shown in the following paper:
A high compliance CMOS current source for low voltage applications
Quarantelli, M.; Poles, M.; Pasotti, M.; Rolandi, P.;
ISCAS '03

Paul

Title: Re: question on diff amp
Post by Rajesh on Mar 26^th, 2006, 8:17pm

Bharath,
try a pmos input pair since u can avoid body effect.
Having tail current in linear region may not be a good idea and will be decided by ur input common mode voltage. If PMOS input pairs alone is not doing the job then I will go for a folded cascode with top most current mirror txtrs(where u connect ur input pair's drain) biased in linear .
All the best.