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

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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.

Attached block diagram.

For low frequencies (e.g.  < 10 kHz) one can use active rectifiers with OP-amps or similar. For higher frequencies there are square law power detector circuit - similar to one way rectifiers with a germanium or schottky diode and maybe prebiased. Another option is to first amplifiy the signal and than rectify.

One-chip solutions are readily available:


Maybe consider a Log-Amp?


Going from the block diagram, it appears as though you want an automatic gain control circuit, in which case, take the voltage from the output, not input signal and feed it into the control input.

Here's a microphone pre-amplifier circuit I designed awhile ago, for a project which was cancelled.

Q1 is a common emitter amplifier.

D1., D2, C3, C4 and R4 convert the AC part of the output to DC, which biases M1 on, once it exceeds its gate threshold voltage and it starts to short the microphone to 0V, thereby limiting the gain.

I forgot to post a link to the thread. The above circuit is designed to be two wire. The output terminal is connected to a power supply via a resistor and the output sits at a couple of VDC, with the amplified AC waveform superimposed on it.


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