This circuit is the receiver for the previous transmitter modulated by an audio signal. The transducer is a ceramic capsule, in which case the circuit becomes ideal for telegraphic transmissions.
The integrated circuit used in this small spy transmitter is not easy to obtain today. The coil is...
This AC operational amplifier has a power stage and a gain determined by R2. The maximum output...
This Hughes circuit is from 1968. Equivalent modern components can be used. The input impedance is...