This is a precision half-wave rectifier, suggested by Texas Instruments. Diodes are commonly used, and resistors can have signal values. The power supply must be symmetrical.
This simple transistor tester circuit was found in the February 1968 issue of Radio Electronics magazine....
This powerful inverter provides a high-quality sine wave signal. I found it in a Radio Electronics issue...
This squelch is activated by its own noise, acting automatically. It is used in communications...