Bipolar OPAMP Noise - calculating Noise in a Bipolar OPAMP

[satheeshvelu]

bipolar op amp noise

Dear All

I'm using a bipolar OPAMP for my design.
I need to verify the OPAMP noise using the Simulation and Calculaion result.

Could somebody help me the procedure to calculate the Noise in a Bipolar OPAMP.

In the first stage, My OPAMP has a folded cascode followed by the differential pair.
(Similar to the one in Pg 447, Gray and Meyer.)

In the second stage it has a Darligton Pair.

I followed the same book Pg 788 to calculate the Noise.
But I could not match it with the simulation result.

Kindly help me to know the procedure for Noise Calculation.

Regards,
Satheesh

 

[FvM]

Bipolar OPAMP Noise

In a meaningful design, the input stage is almost determining the total noise. At least for a rough estimation, you should start with noise calculation of the input stage. You don't need to know much execpt the current.

 

[satheeshvelu]

Re: Bipolar OPAMP Noise

Hi

Thanks for your reply.

May I know why is it the input stage and not the other stages..??

Regards,
Satheesh

 

[diemilio]

Re: Bipolar OPAMP Noise

Hi

Thanks for your reply.

May I know why is it the input stage and not the other stages..??

Regards,
Satheesh

Because usually your input stage has a gain > than 1 so when you refer the noise of secondary stages to your input, that noise is going to be divided by that gain. Nevertheless you should be very careful with this because there are very specific situations in which, even if the gain of the first stage is large, the noise of your second stage is appreciable. I strongly recommend that you go through this book for more information:

Low-Noise Electronic System Design by Motchenbacher & Connelly.

Hope this helps,

diemilio

 

[satheeshvelu]

Bipolar OPAMP Noise

Hi

Thanks for your reply.
It was useful.

I have calculated the Noise of the OPAMP.

By hand calculation i'm getting 675.43n
By simulation I got 910.59n

Is it possible to get it more accurately..??

Regards,
Satheesh

 

[diemilio]

Re: Bipolar OPAMP Noise

Yes, it definitively should be much much closer. Perhaps if you could post your schematic and a brief description of what you're doing I could help you out.

Regards,

diemilio

 

[lladnar23]

Bipolar OPAMP Noise

Are you counting the extra noise from the current source in the folded cascode?