Open Loop Fast Peak Detector

In this peak detector circuit, D1 is the detector diode and D2 is a level shifting or compensating diode. A load resistor RL is connected to – 5V and an identical bias resistor, RB, is used to bias the compensating diode.  This  equal value resistors RL and RB make sure that the diode drops are equal.

High values of RB and RL provide good low frequency accuracy but cause the amplifier to slew rate limit, resulting in poor high frequency accuracy.  Low values of RB and RL (1k to 10k) yield in fast response, at the expense of poor low frequency accuracy.  Adding a feedback capacitor  CFB, which improve the negative slew rate on the (–) input, is the solution to improve high frequency accuracy.  [Linear Technology Application Notes]