This circuit is from a 1968 Motorola power circuit manual. The circuit uses transistors of the time. The passing band is 45 Hz to 100 kHz.
This circuit using an operational amplifier was found in old American documentation. It can be...
If the circuit oscillates, emitting sound, it is because the transistor is good. This simple...
Like the previous circuit, this is used in the analog part of the telephone lines. The same...