Gain control with 741 and JFET (CB25572E)

This circuit was found in the April 1993 Electronics Now magazine. With it, it is possible to control the gain of an operational circuit with a JFET.

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Variable zener diode (CB9969E)
In the figure we have the way to implement a 5 to 45 V zener diode with 50 mA of maximum current...
Wattmeter (CB23581E)
This circuit is from a Radio Electronics magazine from December 1958. The indicator instrument is analog...
12 or 24 W amplifier TDA1516Q or TDA1518Q (CB15980E)
These integrated ones consist of class B amplifiers with gain fixed 24 internally for better...