Hi all,

Why after adding external compensation to an op-amp, do the internal lag networks stop contributing to attenuation? I understand that the compensating network's attenuation is designed to level off near the first internal lag network's critcal frequency, such that the -20dB/dec will then be handled by the latter, but why when surpassing the second lag networks critical frequency does the roll-off rate not just become -40dB/dec since the compensating network is no longer affecting the gain?

Also, if the compensating network really just pushes the bode plot to the left, we're not gaining anything are we? (Other than reducing bandwidth?)

One more quick question. Can I assume that all the bode plots of op-amp frequency response are drawn with straight lines for educational purposes only? That is, the gain is not at 100% until (fc1) at which point it takes a sharp angle and starts rolling off at -20dB/dec, and then again at (fc2) at -40dB/dec, etc... right?

I could not find one iota of information related to these topics online or elsewhere, so any help would be greatly appreciated!

 

Hello,

What kind of opamp are you talking about ?
Many opamps do not have the possibility.

Greetings,
Bertus

 

HI gain amplifier will oscillate at hi gain unless it is gently roll -off when crossing 0 DB. by ADDING COMPENSATION IT BECOMES FLTTER AT HOSE POINTS.

 

Opamps have used internal compensation for 40 years or more. What year are you talking about? Which very old opamp?

An old uncompensated opamp will oscillate when negative feedback makes its gain to be low, not high.

 

I think the key to understanding op amp compensation is to look at open loop gain and phase response. Have a look at the Wikipedia entry about phase margin.

 

(As a note: ) Op-amps are available with or without internal compensation. The reason why op-amps are still available without comp is so that the user can add just enough external compensation to achieve the gain desired without sacrificing anymore bandwidth than neccesary.

I greatly appreciate your responses, however I'm still open to more suggestions.

 

(As a note: ) Op-amps are available with or without internal compensation. The reason why op-amps are still available without comp is so that the user can add just enough external compensation to achieve the gain desired without sacrificing anymore bandwidth than neccesary.

I greatly appreciate your responses, however I'm still open to more suggestions.

After reading about phase margin, what are you stuck on?
You are correct about the straight lines in Bode plots. It makes them easier to draw, and easier to see where the actual corner frequencies are.