This commercial earphone receiver was manufactured in the 1960s in Japan. It was powered by a 9 V battery and the transistors were made of germanium.
This circuit was found in a Motorola documentation from 1975. Modern equivalent transistors such...
This circuit is from the datasheet of STMicroelectronics, being indicated for the voltage...
This circuit was found in 1980's Radio Electronics. The circuit produces a 1500 Hz modulate 35 kHz....