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Converting small AC signals (46mV to 200mV) to DC

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--- Quote from: mawyatt on April 12, 2024, 05:19:10 pm ---Maybe consider a Log-Amp?


--- End quote ---

I was thinking that too. Maybe this one but it also contains a VCA.



--- Quote from: Sacodepatatas on April 13, 2024, 04:32:46 am ---But, when I tried a circuit like this (without fb) in a breadboard, I found that a BJT such as the SS8050 (Hfe=250, rHfe=57) behaved much better at the corner case than a 2N2222 (rHfe=25) or 2N3904 (rHfe=7), although every transistor did a pretty decent job rectifying over 100mV peak. SS8050 did a sharp transition from inverting to non-inverting even below 20mVpeak, while 2N3904 "rounded" the bottom corner. I guess the rHfe has something to do here eventhough this BJT isn't working in reverse.

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I'd expect that more an artifact of low VCEsat, than reverse HFE operation.
Muting Transistors tend to have very high reverse HFE, and they are spec'd sub ~1ohm  RBC on


--- Quote from: kushal4269 on April 12, 2024, 04:24:19 pm ---Is there any analog circuit which can convert small AC signals (46mV to 200mV) to DC, I know diode based half and full bridge won't works due to forward threshold voltage.

Need for this conversion is to use the out put of this DC volt as control voltage for Voltage controlled amplifier which amplifies prior mentioned AC signal.

--- End quote ---

For such AGC system you may rather look at a peak detector. (as in #4) and also see ADI's notes here
For Audio you can relax the Opamp specs.

To answer the original question.

What gain?
Full wave or half-wave?

Here's a full-wave precision rectifier with a gain of 10.
R1 = R2
AV = 1+R3/R4

U1 and U2 need to be a single supply op-amp i.e. must work with the inputs at the negative rail. The LM358 will do for most applications,.

Terry Bites:
Barrie Gilbert lives on.......


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