Hi,
i'm studyin the transimpadance amplifier and i'm trying to stabilize it. I found this application report:
and the Figure 4 shows three different scenarios for the intersection of the closed-loop response curve with the open-loop gain curve.
Can anyone explain to me why stability degradation will occur when fP falls outside the open-loop gain curve?
Thanks.
Hello Marco,
This comes down to the fundamentals of modern control theory which applies to op amp stability. If the pole in the inverse feedback factor "1/Beta" falls outside of the open-loop gain (Aol) curve than the rate-of-closure between the Aol and 1/Beta curves will be >20dB. This indicates there will be at least 2 poles inside the bandwidth of the loop-gain (Aol*B) response which will degrade the phase margin to <45 degrees indicating stability issues. The first pole will take you from 180 degrees down to 90 degrees and if the 2nd pole falls before the frequency where the loop closes then it will cause >45 degrees of phase shift leaving you with less than the minimum required 45 degrees of phase margin. These concepts are covered in great detail in the TI Precision Lab Videos on op amp stability
I forgot, you should also take a look at these blog posts my colleague wrote. They're really easy to read through and cover the basics of designing these circuits starting with what op amp bandwidth is required. Here's a link to the final blog in the series which contains a .PDF file of the material: 
