Author Topic: op amp AGC  (Read 1465 times)

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Offline salviadorTopic starter

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op amp AGC
« on: May 22, 2020, 12:13:11 pm »
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

995814-0

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

Offline David Hess

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Re: op amp AGC
« Reply #1 on: May 22, 2020, 07:53:12 pm »
The difference in input bias current may be preventing the logarithmic stage from working properly.  It needs a better way to handle positive inputs.
 

Offline Zero999

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Re: op amp AGC
« Reply #2 on: May 22, 2020, 08:33:15 pm »
What do you mean by not working?

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

Offline David Hess

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Re: op amp AGC
« Reply #3 on: May 22, 2020, 08:45:52 pm »
What do you mean by not working?

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.

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

Offline salviadorTopic starter

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Re: op amp AGC
« Reply #4 on: May 23, 2020, 10:07:06 am »
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
 

Offline Benta

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Re: op amp AGC
« Reply #5 on: May 23, 2020, 04:53:14 pm »
Why not just use an ON Semi NE570?
Much simpler (and probably better performing).

 

Online SiliconWizard

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Re: op amp AGC
« Reply #6 on: May 23, 2020, 04:57:52 pm »
Why not just use an ON Semi NE570?

Not bad.
 

Offline salviadorTopic starter

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Re: op amp AGC
« Reply #7 on: May 24, 2020, 09:42:21 am »
thanks for the tip I did not know this integrated NE570
 


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