This circuit was found in a 90s Electronic Engineering. The circuit has the tube's critical element. Operational amplifiers support equivalents.
This circuit is from the 70's by A. Fanzeres. We kept the original text. It has already been said...
Found in 1965s General Electric documentation, this circuit is intended for the control of servos...
This circuit is from the Linear Circuits Application of Texas Instruments, using an operational amplifier...