Using a common crystal diode and a 741, this circuit can be used to detect microwaves. The circuit is from old documentation and the switch selects the gains.
With this circuit it is possible to obtain an input resistance of 100Gohms for a multimeter, with...
This amplifier, to detect biological signals in implanted transmitters, uses transistors such as...
This application is not modern since few computers have parallel ports. But you can use this...