Designing an Envelope Detector Circuit for AM Demodulation

 Amplitude Modulation (AM) is a method of transmitting information by varying the amplitude of a carrier wave in proportion to the message signal. However, in order to recover the original information from the received AM signal, we need to demodulate it. One of the most common methods of demodulating an AM signal is through the use of an single diode based envelope detector circuit.

 An envelope detector circuit is a type of demodulator that extracts the original information from the AM signal by detecting the envelope of the modulated wave. The envelope is the peak amplitude of the modulated wave and contains the original information. The circuit typically consists of a diode and a low-pass filter. The diode rectifies the incoming signal, meaning it only allows current to flow in one direction. This results in a pulsating DC signal that follows the envelope of the original modulated wave. The low-pass filter then removes any high-frequency noise and smoothens the output signal to recover the original information.

Circuit Diagram and Operation

 The following shows circuit diagram of envelope detector circuit diagram for AM demodulation.

This circuit is simple and yet highly efficient for demodulation of a narrow-band AM signal. A narrow-band AM signal is one which has carrier frequency that is large compared to the message bandwidth. The above envelope detector circuit consist of a diode(D1) and low pass filter(C1 and R1). The resistance Rs represents the internal impedance of the AM signal source. The working principle of this circuit is as follows. In the positive cycle of the AM signal, the diode becomes forward biased(the amplitude of the signal must be greater than the forward diode voltage) and the capacitor C1 gets charged upto the peak value of the input signal. Then when the input signal starts the negative cycle and lower than the diode forward voltage, the diode becomes reverse biased and so no signal passes through the diode. At the same time, during the negative cycle of the input signal, the capacitor C1 starts to discharge and the current flows from the capacitor to the resistor R1. This discharging process continues until the next positive cycle. This cycle of charging and discharging repeats.

The component values of the RC filter depends upon the modulating signal frequency. For  audio signal the maximum audio frequency can be taken to be 22KHz. Considering the diode to be ideal with zero impedance when forward biased and infinite impedance when reversed biased we have two criteria for the component values selection for the capacitor C and the resistor RLwhich is as follows.

The first condition is that the charging time \(R_SC\) should be short compared to the carrier signal period, that is,

\[R_SC<<\frac{1}{f_c}\] 

where, \(f_c\) is the carrier signal frequency

 This second condition is that the discharging time constant \(R_LC\) must be long enough so that the capacitor discharges slowly through the resistor RL in time interval between each positive peak of the carrier signal while at the same, the discharge time constant should not be so long that the capacitor is not discharged at a rate lower than the modulating signal. This condition can be expressed as follows.

\[\frac{1}{f_c}<<R_LC<<\frac{1}{B.W}\] 

where, \(B.W\) is the bandwidth of the message signal.

Envelope Detector LPF calculator

Here is a calculator that can be used to determine the value of the resistor conditioned on the above two criteria.

 The above calculator helps to calculate the source voltage internal impedance Rs and the resistor RL for the low pass filter, provided the carrier frequency, the signal bandwidth and the capacitor values are given.

Video Demonstration

See the following video which demonstrates demodulation of AM signal using the envelope detector circuit.

In conclusion, designing an envelope detector circuit for AM detector demodulation is not a complex task, but it's important to choose the correct type of circuit based on the specific requirements of the application. The peak detector circuit is the simplest, the balanced modulator circuit has a faster response time, and the synchronous detector circuit has the fastest response time and can be used for high-frequency signals. 

If you are interested in learning more about AM modulator circuits, AM demodulator circuits then see the following tutorials.

- Standard AM with AD633 Analog Multiplier IC

- DSB-SC AM Generation with AD633 Analog Multiplier IC 

- Arduino AM radio receiver

- Amplitude Modulation with Emitter Modulator