This modulator amplifier for transmitters was found in a 1947 American book. The circuit uses an output with 4 tubes connected in parallel and in push-pull.
This amplifier step with a transistor has an input impedance of 100k to 10 M. The transistor can be any...
This sensor circuit for currents of the order of nanoamperes was found in an American...
I found this circuit in a 1992 Electronics Experimenter Handbook. The circuit can be easily...