How to find poles in a open loop gain 100db with phase margin 55 and bandwidth 10Mhz?

[kwanmajun]

Hi, recently i got a interview question about the poles"

we have a open loop gain 100db, with phase margin 55, bandwidth 10MHz, then where are the two poles?"

Please give me the details answers, thanks, waiting for your answers. thanks~

 

[WimRFP]

How to find poles?

Hello,

100 dB = gain of 100k. So first -3 dB point is around 10M/100k = 100 Hz. I asume that the 10 MHz bandwidth is based on the slope of the first pole.

So around 1kHz to 1 MHz you have a 90 degrees phase shift.

55 degrees phase margin (at unity gain I guess) means that the second pole introduces a phase shift of 180-90-55 = 35 degrees at 10 MHz.

As the last pole does less then 45 degrees, it is above 10 MHz. I did a fast phasor diagram calculation, the second pole must be around 14 MHz.

The gain/phase versus frequency plot in the datasheet should show you the -20dB/dec slope, and the further decreasing slope above 10 MHz.

 

[kwanmajun]

How to find poles?

Got it, thanks man~