Which schematic to chose for my Op-amp

[a0034162]

Hi All,

I have a need to design a Op-Amp with follow specifications, which schematic is better for following requirement?

UGB 100Mhz
Phase Margin 55 deg
Open Loop Gain 10000/80dB
Power consumed below 5mW
Power supply 3V
Output swing 2V
Input swing 1V
Closed loop gain -2/active low pass filter

My first version folded cascode only gives a 74dB gain and 40deg phase margin.
Is two stage schematic easier for this requrement?

Best Regards,
Wu Nan

 

[a0034162]

I have come out a Op-amp, but phase margin is not good. Any suggestion to improve the phase margin?

 

[Dominik Przyborowski]

When You designing two stage ota, the first of all You need to add a compensation network (e.g. Miller cap with nulling resistor or used a cascode compensation). Your amplifier is unbalanced - output transistor should be matched with first stage current mirror load. In addition the phase margin and GBW depends to feedback type, reference voltage, etc.

I think that should be possible to meet your requierements with single stage RFC - telescopic doesn't provide enough swing with nominal power supply, two stage will probably be too slow for 100MHz of GBW and resonable power consumption (but I don't know which technology You are using - 180nm? but which?).

 

[a0034162]

When You designing two stage ota, the first of all You need to add a compensation network (e.g. Miller cap with nulling resistor or used a cascode compensation). Your amplifier is unbalanced - output transistor should be matched with first stage current mirror load. In addition the phase margin and GBW depends to feedback type, reference voltage, etc.

I think that should be possible to meet your requierements with single stage RFC - telescopic doesn't provide enough swing with nominal power supply, two stage will probably be too slow for 100MHz of GBW and resonable power consumption (but I don't know which technology You are using - 180nm? but which?).

Thank you very much, I am using 0.18u technology. RFC means folded cascoded?

 

[Dominik Przyborowski]

RFC means Recycled Folded Cascode - folded cascode with small positive feedback which boosting transconductance increasing gain and GBW, but with cost of stability. Since 2009 it's quite a lot of papers analysing this architecture (and some talking about improvements).

BTW. I checked in 0.35um a simple miller ota and I obtained for 1pF load a 102MHz GBW with 75dB open loop gain and 60degrees of PM with 1mA current consumption from 3.3V, so for the first try You can checked a simple miller OTA. Of course in 0.18um simple two stage (without cascodes) OTA hasn't enough gain, but first of all it should be stable and fast enough. In next step for increase a gain You should add a cascodes in first stage (should be enough). If You have low threshold device a self cascode first stage will be simpler to design (you don't need special biasing for self-cascode).

 

[a0034162]

Dear Dominik,

Thank you very much. I have come out a two stage OP-AMP: first stage telescopic second stage common source.

Open Loop Gain 96dB
UGB 110MHz Phase Margin 60deg
all parameter meet requirement but speed. it require a 99.9% settling with 30ns but my design only within 35ns, when working as -2 closed loop gain, when input a 0.5V step voltage.
According what you said, I need faster the first stage? or the second stage?

Best Regards,
Wu Nan

 

[Dominik Przyborowski]

0.5V step is in fact large signal so if your tail current is 20uA and Cc 50fF You should have slew rate around 400mV/ns which should be enough.
But maybe for the first stage the response is still small signal like so I think You should increase W/L ratio of input pair for increase a gm.