A simple half-wave peak detector using a single transistor is very compact and low cost, but if the application require precise detection for signal under 100mV then that circuit become useless. Here you can use more precise half-wave peak detector circuit using only a single op-amp with few passive components. Look at the circuit’s schematic diagram of the peak detector circuit below.

precision half wave peak detector circuit schematic

This peak detector circuit can be implemented both for average peak detector or a fast peak detector by selecting R2 and R3 values as shown in the table. Note that the output impedance is set by R3, so make sure that the output of this peak detector circuit will be connected to a higher impedance input stage. [Circuit’s schematic diagram source: National Semiconductor Application Note]