We found this circuit in a documentation for radio amateurs from the 70s. The transistors can be the BC548. With it, small speakers of 4 or 8 ohms can be used as microphones.
This circuit uses an operational that is not currently common. We found it in an old American...
This circuit uses two transistors to obtain low frequency sinusoidal signals. We found it in a...
This amplifier can be powered with voltages between 2 and 5 V with power up to 100 mW. The minimum...