Simple Diode Peak Detector Analysis:
Another application of semiconductor diode. An envelope detector is a simple, highly effective, and low-cost solution for the demodulation of narrow band AM, where the percentage of modulation is less than 100%. It is popular in analog signal processing.
Diode Peak Detector Circuit For AM Signals:
An envelope detector circuit consists of a diode and capacitor. The purpose of the diode is to provide a unidirectional path for the current. Or simply it works as a half-wave rectifier. It allows the capacitor to charge only in one direction. The capacitor itself is a storage place. The voltage across the diode is proportional to the signal amplitude.
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| Figure 1 An Envelope Detector Using a Diode |
At the input of the circuit, we have a high-frequency amplitude-modulated signal. We get the envelope of the modulated signal at the output.
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| Figure 2 Input and output of peak detector |
Now let's understand the operation of the circuit.
Look at the input side, there is a single diode that acts as a half-wave rectifier.
The AM signal at the input, during the positive half-cycle diode, is forward-biased. (Consider diode drop which is 0.7V).
The capacitor charges up to peak value VP- 0.7V (consider practical diode).
Negative half-cycle clips. See the figure below.
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| Figure 3 Output observed after diode. Negative half-cycle clips |
See figure 4. This is diode equivalent, the diode has a small internal resistance rd. rd and capacitor constitute a low pass filter.
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| Figure 4 Replace diode with equivalent resistance rd |
During the forward-biased condition, this small internal resistance followed by a capacitor. It forms a low-pass filter. It allows only the low-frequency component of the rectified signal. It means high-frequency carrier blocks and only the envelope appears at the output.
The capacitor should have a fast charging time but a slow discharging time.
The peaks of the pulsed DC represent the modulating signal.
In the end, we get only the envelope of the modulated signal. See figure below
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| Figure 5 Final output |
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