Question

explain the operation of an envelop detector circuit?
with a diode placed in between the input and output, connected to a resistor and capacitor in parallel from the output of the circuit to the ground.

Answer 1

The diode acts as a rectifier and the RC cell acts as a low-pass filter; usually this circuit is used in AM envelop detection. In this case you have a low frequency signal (information) superimposed on the high frequency signal (carrier). The role of this circuit is to extract the low frequency signal and to eliminate the high frequency signal. The diode acts so that when the modulated (AM) signal has a voltage higher than the capacitor voltage (more 0.7 V) then the diode is forward biased and lets the signal flow. The capacitor voltage grows up to the modulated signal maximum value. After the maximum we have that the signal goes down, while the capacitor retains voltage.
The diode becomes reverse biased and the capacitor discharges via the resistor through ground, with a time constant:\[\tau = RC\] The trick is to have a time constant big compared to the high frequency signal period but little compared to the low frequency signal period.