Envelope Detector is using for analog communications and quite popular method for demodulate convectional AM signals. As we know we have many ways to transmit the message signal.
Demodulation of Conventional DSB AM Signals: The major advantage of conventional AM signal transmission is the ease with which the signal can be demodulated. There is no need for a synchronous demodulator. Since the message signal m(t) satisfies the condition |m(t)| < 1, the envelope (amplitude) 1+m(t) > 0. If we rectify the received signal, we eliminate the negative values without affecting the message signal as shown in Figure 3.7. The rectified signal is equal to u(t) when u(t) > 0 and zero when u(t) < 0. The message signal is recovered by passing the rectified signal through a lowpass filter whose bandwidth matches that of the message signal. The combination of the rectifier and the lowpass filter is called an envelope detector.
The simplicity of the demodulator has made conventional DSB AM a practical choice for AM radio broadcasting. Since there are literally billions of radio receivers, an inexpensive implementation of the demodulator is extremely important. The power inefficiency of conventional AM is justified by the fact that there are few broadcast transmitters relative to the number of receivers. Consequently, it is cost effective to construct powerful transmitters and sacrifice power efficiency in order to simplify the signal demodulation at the receivers.