Sunday, April 1, 2012

Envelope Detector


Envelope detector circuit uses a diode, a capacitor and a resistor and it is just like a half wave rectifier followed by a low-pass filter. It is a linear detector which takes high frequency RF signal as input and gives an output which is the envelope of the input signal. A diode detector is a type of envelope detector and is used for the detection of AM signal.
Circuit Operation
Here the input signal is rectified by the series diode D. The combination of capacitor C and resistor R behaves like a low-pass filter. The input signal contains both the original message and the carrier wave where the capacitor helps in filtering out the RF carrier waves. The capacitor gets charged during the rising edge and discharges through the resistor R in falling edge. Thus the capacitor helps in giving an envelope of the input as output.

Advantage
  • This technique is highly used in AM receivers as it is very simple and cheap. 
  • The circuit acts as a linear detector as the output varies as per the voltage of input and the distortion here is very less.
Disadvantage
  • The circuit does not have an amplification capability to amplify the rectified signal. Amplification should be done by separate devices.
  • Before giving input to the circuit the signal must be bandpass filtered otherwise the circuit will demodulate unnecessary signals.
Compared to advantages the disadvantages are very negligible and hence these circuits are extensively used for AM detection.

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