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op amp AGC

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salviador:
Hi,
I want to create an AGC with op-amp and an input dynamic that varies from min 40mVpp to max 700mVpp with a constant voltage output.

This circuit work with MC33078P



the problem arises when I replace op amp with another type lm358, why ??

David Hess:
The difference in input bias current may be preventing the logarithmic stage from working properly.  It needs a better way to handle positive inputs.

Zero999:
What do you mean by not working?

The LM358 has lots of crossover distortion, so if the output looks bad, that would explain it.

David Hess:

--- Quote from: Zero999 on May 22, 2020, 08:33:15 pm ---What do you mean by not working?
--- End quote ---

I mean the logarithmic amplifier behaves poorly when the AC input goes positive; it has no way to sink a positive input.


--- Quote ---The LM358 has lots of crossover distortion, so if the output looks bad, that would explain it.
--- End quote ---

You could force the LM358 into class-a mode by loading the outputs to either supply rail with load resistors.

salviador:
thank you all for your answers, they helped me a lot!

I made a terrible evaluation error, I work with a frequency of 50Khz and I noticed that MC33078P has a sufficient bandwidth while LM358 was not working due to its more limited bandwidth.
making the AGC circuit work at 1Khz also LM358 works properly

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