Designing an amplifier with low frequency gain > 100 dB?

[jbi00b]

Dear all,

I am a newbie in circuit design, and my project is about making an amplifier with a low frequency gain of larger than 100 dB. I have tried to make a two stage amplifier (folded cascode differential amplifier, then single input amplifer, and miller compensated) to obtain such requirement, but every time I only get 90 something in gain when I vary the gm and transistor size.

What would you do if you want to get a gain of > 100dB???

 

[elbadry]

Re: Designing an amplifier with low frequency gain > 100

One Possible solution is to use gm-boosting.

Another solution is to make the 2nd stage a cascode stage also ( add one extra NMOS and one extra PMOS ).

However, I think it is possible to get the gain you need with the topology you described. If GBW is irrelevant, then try to decrease the current in the 1st stage ( the folded cacscode ). this will give you better gain from the 1st stage. in general decreasing current increases gain

 

[tsb_nph]

Re: Designing an amplifier with low frequency gain > 100

I think you can use regulated cascode in the first stage and then CS amplifier in the second stage to achieve 100 dB of gain. Also, use lower current in the 1st stage (regulated cascode stage) as elbadry suggested. I would not use the second stage also as a cascode stage since compensation (60 deg phase margin) will become a big problem. Get as much gain as you can from the first stage.

 

[eldo]

I think tsb_nph is right. A 100 dB gain can be obtained by a three stage opamp or 2 stage opamp with gain boosting.

 

[johnsmith101]

Re: Designing an amplifier with low frequency gain > 100

The best way is to gain boost all stages. You can see some gain-boosting techniques in the Razavi CMOS text starting on pg. 310.

 

[eldo]

Gain boosted output stage will not give you +-1V output swing at 1.5V supply.

 

[pfd001]

Large transitor length will help you get high gain. Maybe simple fold coscade is enough for 100db gain

 

[jbi00b]

Thx guys I have got the params right!

 

[ambreesh]

Re: Designing an amplifier with low frequency gain > 100

you could use a folded cascode with gain boosting.
Have a high swing folded cascode design.