Modifying AGC circuit

[CDRIVE]

My statements were specific to a midway biased single supply OpAmp. Nothing more and nothing less.

Chris

 

[Audioguru]

The AGC circuit will allow a high voltage signal to zap the ADC before it reduces the level. Because the little transistor takes time to discharge capacitor C4.

Also, the base-emitter diode of the transistor will charge capacitor C3 then the AGC will fail unless a diode is added to discharge C3 when the signal swings negative.

 

[AnalogKid]

C3-P1 is a time constant of 0.1 s and a corner freq of 1.6 Hz. On a positive peak, C3 will charge through the Q1 base-emitter junction. When the circuit turns down the signal and subsequent positive peaks are not greater than the voltage across C3, it will discharge through P1 until it starts tickling Q1 again.

ak

 

[Audioguru]

Without a reverse diode I think that C3 will keep charging until the AGC does not work anymore.

 

[CDRIVE]

I couldn't resist simulating this so here it is. I couldn't make out if R8 is 10K or 100K. 100K seemed a bit much so I used 10KΩ. The sim is swept 4 times starting at 40mVPP to 20VPP input.

Chris

 

[kruskal]

This looks great. Thanks! Which simulation tool is this?
The parts are on the way from digikey, so hopefully I will be breadboarding some time during the weekend.

 

[CDRIVE]

It was drawn and simulated in Tina11Classic Design Suit.

Chris

 

[kruskal]

Sorry for the late reply. Just wanted you to know that everything worked like a charm when breadboarding. For my purpose I could not find any difference using the LM358 or the OPA134, both worked excellent. Thanks for all helpful input!

 

[CDRIVE]

Sorry for the late reply. Just wanted you to know that everything worked like a charm when breadboarding. For my purpose I could not find any difference using the LM358 or the OPA134, both worked excellent. Thanks for all helpful input!

Happy to read that. When I simulate circuits I never know how accurate my results are until people like you tell me. Unfortunately there are more people in other category.

Chris

 

[Audioguru]

I am surprised that the LM358 worked well because it produces awful crossover distortion and hiss, and has poor high frequency response.

 

[CDRIVE]

My microcontroller ADC needs to tap the speaker wire output from the car head unit, and I was hoping that I could use this circuit to present a stable input to the ADC regardless of the head unit volume setting.

Good question. Maybe this is why?

Chris