what is the loop gain at each pole

[circuitking]

Hello, I connected a circuit like shown below. How do I find out what should be the loop gain at each pole frequency? is there a quick way to find this out?

 

[deep_sea]

For stability, the two poles should not be close to each others. If the first pole is at 600 MHz and the second is only double it, you will not have a sufficient phase margin. You should split them. A simple way to move the first pole inward (to lower frequency) so that you have a "real" dominant pole. Let us say fp1 is the dominant pole, this means unity gain frequency (UGF) is around DC_gain*fp1. fp2 should be around 4 times UGF to give you around 75 degree phase margin.

 

[FvM]

Are you asking how to calculate magnitude (and possibly also phase) of a first order low pass response?

 

[circuitking]

Let us say fp1 is the dominant pole, this means unity gain frequency (UGF) is around DC_gain*fp1. fp2 should be around 4 times UGF to give you around 75 degree phase margin.

Thanks. I made fp1 = 300 MHz. Loop gain = 100/5 = 20. UGF=300 MHz*20 = 6 GHz. fp2 = 24 GHz.

I got the values close to what is expected but not exact. For ex. UGF is 5.96 GHz. All the components are ideal, shouldn't I see exact values?

 

[FvM]

UGF is also affected by second pole. 6 GHz is only valid for first order system.

 

[deep_sea]

UGF is also affected by second pole. 6 GHz is only valid for first order system.

Exactly. There is an effect nevertheless from the second pole on UGF.