Envelope detector

An envelope detector (sometimes called a peak detector) is an electronic circuit that takes a (relatively) high-frequency signal as input and outputs the envelope of the original signal.

Its output approximates a voltage-shifted version of the input's upper envelope.

Between the circuit's input and output is a diode that performs half-wave rectification, allowing substantial current flow only when the input voltage is around a diode drop higher than the output terminal.

The capacitor is charged as the input voltage approaches its positive peaks.

[1] Envelope detectors can be used to demodulate an amplitude modulated (AM) signal.

To avoid negative peak clipping, the original signal that is modulated is usually limited to a maximum frequency

To minimize distortions from both ripple and negative peak clipping, the following inequality should be observed:[1]

Full-wave rectification traces both positive and negative peaks of the envelope.

Half-wave rectification ignores negative peaks, which may be acceptable based on the application, particularly if the input signal is symmetric about the horizontal axis.

The filtering for smoothing the final result is rarely perfect and some "ripple" is likely to remain on the output, particularly for low frequency inputs such from a bass instrument.

can be written in the following form In the case of AM, φ(t) (the phase component of the signal) is constant and can be ignored.

Hence an AM signal is given by the function with m(t) representing the original audio frequency message, C the carrier amplitude and R(t) equal to C + m(t).

An envelope detector can also be constructed using a precision rectifier feeding into a low-pass filter.

Envelope detectors are often a component of other circuits, such as a compressor or an auto-wah or envelope-followed filter.

Both expanders and compressors use the envelope's output voltage to control the gain of an amplifier.