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Tips for making Op-amp with high Output Resistance

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chikaofili

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Hello.
I am trying to design an op-amp (using Mosfets) that has an output resistance > 1M

I was wondering if there was any tips in designing that


Specs:
PM >45db
Out R >1M
Width of transistors <30u
Length of transistors <500n

Thank you
 

With your maximum L this resistance may not be achieved by just cascoding. To achive 1MegaOhm I think you should look at some gain boost topologies. They are called regulated cascodes and examined in Razavi's book. If you do not want to boost the output resistance with loops try triple cascoding your output stage if the output range doesn't matter for you.
The regulated cascode solution shouldn't change your PM if your loop is excess stable. Also you will get a smoother 20 dB/dec slope. You might want to keep your W's low but not to low to cause errors arising from their mismatch.
Considering your motive, I have a question; Are you designing a gm-C filter?
 

I do not see why you want to constrain the L a priori,
when it is the easiest way to get decent Zout in a current
mirror.

With high Zout your gain will depend hugely on Zload.
 

Hello,
Thanks for the reply, I am trying to make a transimpedance amplifier for an OTA. I cant use a regular output stage because of noise. But when I add the feedback resistor for the Transresistance amplifer, I need it to be at least 10M for it to work properly. I was told to increase the output resistance of the transresistance amplifier in other to reduce the feedback resistor needed.



---------- Post added at 13:44 ---------- Previous post was at 13:42 ----------

The output range has to be between 64mV and 1.5V. In the transimpedance stage, I also using a Rail-to-rail opamp with gain of 72dB and output resistance of 8K
 

As your output range specifications dictates 64mV to 1.5V output range; I believe you are not cascoding your NMOS's at the output stage. This is a very hard problem to get 1M resistance without cascoding. I've got no clue. I do not want to sound like pessimistic but I'm not even sure if that is possible with your transistor Width and Length.

If you can cascode them and use regulated cascode configuration you may get Megaohms of output resistance.

I am pretty unexperienced in IC design but if this is a very critical specification for your sub system you may consider a switched capacitor alternative for this circuit.
 

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