The circuit shown in the figure can be designed with more modern operational amplifiers, as it is from 1994 documentation by National Semiconductor. The power supply must be symmetrical.
A double port MOSFET is used in this circuit, which drives a 100 uA moving coil meter or close to it....
This is the receiver for the previous transmitter. The microphone is of the high impedance...
This 500 V line current monitor with the LTC6101 was obtained from Linear Technology's own 2005...