This circuit was found in a 90s Popular Electronics. It also serves for the analysis of microcontroller signals. The power comes from the circuit under test.
This circuit was found in documentation from the 70s. We can try current JFET transistors....
This is the power supply circuit for the amplifier in the previous circuit. The source is...
This bridge circuit is designed for 8-ohm speakers delivering a few watts of power. The circuit is from...