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Peak Detector

A Peak Detector is any circuit that functions to detect the peak voltage of an incoming waveform. Basically the circuit is used as an Amplitude Modulation [AM] detector, Pulse Amplitude Modulation [PAM] detector or Envelope detector. The particular circuit covered in this topic is the diode detector, in its basic form.

 The most basic form of a peak detector uses a single diode to limit current flow in only one direction and a capacitor which charges up to the highest voltage applied to it [the peak voltage]. A resistor is than added to provide a discharge path for the capacitor. Peak Detector

Of course the circuit is using the minimum number of parts. However the peak detected voltage will always be one diode drop below the actual incoming voltage peak. The capacitor may leak, and not stored, or hold the maximum voltage applied to it, and the discharge time of the capacitor needs to occur before the next incoming peak. That is the RC time constant needs to be selected to match the incoming signal, so the capacitor will charge through the diode [Dr x C] and discharge through the resistor [R x C].

However your not likely to see this circuit used, other than in a lab experiment, but you should still find it in text books. The waveforms to the right are just an example of an amplitude modulated signal, which is detected by the diode, and than Envelope detected by the capacitor. The next circuit is an upgrade, of the same approach, using a few more components of course.

This next circuit is an example of a peak detector used as a pulse amplitude demodulator. The same waveforms to the right still apply, although PAM is normally just a single polarity. Diode CR1 performs the same function as before and allows capacitor C1 to charge while a positive pulse is applied to the circuit. When the incoming pulse falls back to zero volts, CR1 becomes reverse biased and the detector is isolated from the input. Diodes CR2 & CR3 are reversed biased, and non-conducting, due to the positive voltage applied to Ecc.

Peak Detector Circuit

Capacitor C1 charges through diode CR1 [the ON resistance of the conducting diode], so the time constant is CR1r x C1. The capacitor discharges through R1, so that time constant is R1 x C1. Read more on the Definition of Time Constant. The values of the resistor and capacitor, which produce the RC time constant are selected so that the time constant is 10 times the period between pulses.

So the capacitor is not allowed to fully discharge in between pulses, and so maintains a voltage. Just before the next incoming pulse the capacitor is allowed to discharge through CR2 and CR3 by the application of the negative going discharge pulse. The capacitor is allowed to quickly discharge through the forward conducting CR2 diode. Diode CR3 insures that the output voltage is just one diode drop off ground. The third waveform to the right would be the approximation of the output voltage after some additional filtering, which is not shown in the circuit diagram.

An additional upgrade to the basic peak detector, or envelope detector would be to use transistors instead of diodes. This next circuit example shows a Transistor Envelope Detector.