This Linear Technology circuit can measure currents from 100 pA to 100 uA. The circuit is from the 80's, using a component of the era. Transistors are general purpose types.
Details of the coil of this two-transistor generative receiver were not given. We found it in a...
This very high input impedance circuit is suggested by Texas Instruments. The circuit can be...
This circuit from an 80's publication also features low input capacitance. FET can be a modern...