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Ultra Low Power Opamp Issues.

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|IAngel|

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Hey all - (Hopefully) Quick question for you all.

I designed a very low power 2-stage opamp with the following specs:
Vsupply+ = 500mV
Vsupply- = 0V

When I test the open loop gain with fully differential small signal inputs (1uV), I get a gain of about 60dB max and 40dB min at the extremes of the CMR. I tested teh CMR by connecting the output to the negative input (UG buffer) and found that from about 80mV - 450mV the output voltage followed the input, aka CMR is from 80-450mV. So far so good. Now to test the stability, I biased the input AC signal at 250mV and swept the AC from 1Hz-1GHz. I found the Phase Margin to be about 45Degrees (where it should be) with the compesation capacitor I used, so the amp should be stable. Still so far so good.

Here is the problem then:

I want to use this amp with closed loop feedback as an amplified difference amp. I want to take a small DC signal that varies from about 260mV - 240mV in a linear fashion, remove the offset (somewhat) and amplify that 20mV difference to be very large (aka a large linear signal - maybe 200mV - gain of 100). Now, that gain I want is only 40dB and my amplifier can do much more, so I'm not worried about that - here's my problem:

Before I even bothered simulating my linear signal, I applied my same 1uV sine wave (same as I tested my open loop gain, phase margin, etc with) and I hooked up the amp as an inverting amplifier with feedback (a resistor from my signal to -ve input on amp, resistor from amp output to -ve input on amp) - the standard way as mentioned. To test its functionality, I hooked up a 1k resistor at the front of the input, and a 2k resistor in the feedback loop (Vout/Vin = 2K/1K = gain of 2). When I put my AC signal in at the same bias point as before (250mV), I get a severly compressed signal. This in no way works as an inverting amplifier. I tried at 0V bias as well and that didn't work either.

I then tried a voltage summer to see if that would work properly. Grounded +ve input, put 2 equal resistors between the input signals and -ve input, put a feedback resistor from output to -ve input. Didn't work at all either. With a 0V bias I get almost unity gain (1uV) otherwise, I get SERIOUS compression of the signals.

So: If my amp is working in open loop differential mode (with stability), it works as a UG buffer, I can pull all "working specs" from the buffer config and unity gain config, but it won't work in any other closed feedback situations...

Any ideas? I thought maybe my gain wasn't high enough, but I've read that 20dB should be enough for these applications (with a little error - error should decrease with higher gain). Then I thought that maybe my CMR was too low, so that the signals would only work in a VERY limited DC bias band, but unity gain buffer CMR showed my CMR to be almost rail-to-rail.
What could I be missing? I am running in the subthreshold region, but since you can make opamps out of BJTs, that shouldn't do it...not to mention the amp works in the ways mentioned... (plus I tried a 1.2V version with all the FETS deep in saturation...same problem)

Any ideas?

- btw: Spectre simulator, Cadence Analog Artist schematics, IBM 0.13u toolkit.

Thanks!
 

bageduke

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How much is your output resistance?
 

|IAngel|

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The load right now hasn't been added... I wanted to test open loop gain and see how the closed loop inverting amplifier did - which basically was "not work" :p

Output resistance... it needs to be extremely low, right? If it's too high the output stage will be placed in parallel with the feeback resistor, right? I'll check that. Thanks! That's something I haven't checked yet.
 

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