The circuit shown was found in Intersil documentation from the 90s. The formula allows to calculate the voltage obtained as a function of the values of the resistors.
This circuit to increase the sensitivity of instruments to 100 uA (or close to it) was found in...
We found this chopper circuit in a 1967 publication. The transistors are of the time, and current...
This general purpose circuit was found in a Popular Electronics. It is intended for detecting the...