Peak detector and zero crossing detector using op-amp

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Opamp peak detector


Peak detector detects and holds the most positive value of attained by the input signal prior to the time when the switch is closed.

Working of Opamp peak detector 

The op amp peak detector is as shown below

Op amp Peak Detector

Op amp Peak Detector

The operation can be explained as follows assume the switch is open and  if

a)Vout < Vin   the op amp output V’ is positive so that the diode conducts and the capacitor charges to the input value at that instant as it forms a voltage follower circuit.

b) When Vout > Vin, op amp output V’ is negative and the diode becomes reverse biased.

Thus the capacitor charges to the most positive value of input.

Hence the operation of opamp peak detector can be summarized as follows 

  • Vout < Vin; D ON and C charges to peak value of input,  
  • Vout < Vin; D OFF and C holds the peak value of input.

Op amp zero crossing detector


In opamp zero crossing detectors the output responds almost discontinuously every time the input passes through zero. It consists of a comparator circuit followed by differentiator and diode arrangement.

Circuit operation

The circuit of zero crossing detector is shown in the figure below

zero crossing detector

zero crossing detector

In the figure shown above a sinusoidal signal is applied as input to opamp. Since the opamp is in open loop configuration Vo = Av*(Vi-0) the output of opamp i.e. Vo will be at Positive saturation voltage +Vcc when ever Vi > 0 V and is at negative saturation voltage -Vcc when Vi < 0 V. whenever the output of opamp transits from +Vcc to -Vcc the capacitor C charges to +Vcc if the output of opamp changes from -Vcc to +Vcc and it discharges through R to -Vcc if the output of opamp changes from -Vcc to +Vcc .The differentiator circuit(combination of capacitor and resistor) provides an output V’ = R*C*dVo/dt consisting of peaks at times where the square wave crosses zero voltage. The diode is kept to filter off the zero crossings where input voltage crosses zero voltage in rising fashion. 

Hence the operation of zero crossing detector can be summarized as follows 

  • Vin>0,Vo=+Vcc, V’ = R*C*dVo/dt positive spike, D ON and C charges through R and RL to +Vcc
  • Vin>0,Vo=-Vcc,V’ = R*C*dVo/dt negative spike, D OFF and C discharges through R to +Vcc

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