This circuit is from an American 80's documentation. The circuit indicates when the voltage of a 9 V battery drops below 7 V. The transistors can be BC548.
Found in ancient American documentation, this circuit uses some components from the time, such as...
A JFET is used in this circuit to maintain constant gain. The circuit is from an Electronics Now...
This very low power amplifier is designed by Amperex for hearing aids. It is based on TAA370 which is...