) Voltage Follower Peak Detector
Circuit Operation
When Vi is greater than VC, the output of A1 is positive, D2 is forward-biased, and A1 behaves as a voltage follower, charging C1 to Vp.
When Vi falls below VP, VC remains at VP , and consequently, the inverting input terminal of A1 also remains at VP .
Therefore, the output of op-amp A1 goes negative, reversing D2 and forward-biasing D1 .
Negative feedback via D1 keeps A1 from going into saturation.
In the circuit of Fig.7-12, A2 should be a BIFET op-amp for input bias current, but A1 can be any op-amp type that is otherwise suitable.
Diode D2 should have a very low reverse leakage current.
However, with the use of a BIFET op-amp for A2, the reverse leakage of D2 (perhaps a maximum of 1A) is likely to be much larger than the input bias current of A2.
Therefore, the diode reverse leakage current IR(D2) is more effective in discharging than IB(max) of A2.
C1 should have a very high resistance dielectric.