Regenerative circuit

The regenerative circuit (or regen) allows an electronic signal to be amplified many times by the same active device. It consists of an amplifying vacuum tube or transistor with its output connected to its input through a feedback loop, providing positive feedback. This circuit was widely used in radio receivers, called regenerative receivers, between 1915 and World War II. The regenerative receiver was invented in 1912 and patented in 1914 by American electrical engineer Edwin Armstrong when he was an undergraduate at Columbia University. Due partly to its tendency to radiate interference, by the 1930s the regenerative receiver was superseded by other receiver designs, the TRF and superheterodyne receivers and became obsolete, but regeneration (now called positive feedback) is widely used in other areas of electronics, such as in oscillators and active filters.

A receiver circuit that used regeneration in a more complicated way to achieve even higher amplification, the superregenerative receiver, was invented by Armstrong in 1922. It was never widely used in general receivers, but due to its small parts count is used in a few specialized low data rate applications, such as garage door openers,wireless networking devices,walkie-talkies and toys.

The gain of any amplifying device, such as a vacuum tube, transistor, or op amp, can be increased by feeding some of the energy from its output circuit back into its input with positive feedback. This is called regeneration. Because of the large amplification possible with regeneration, regenerative receivers often use only a single amplifying element (tube or transistor). In a regenerative receiver the output of the tube or transistor is connected to its input through a feedback loop with a tuned circuit (LC circuit) as a filter in it. The tuned circuit allows positive feedback only at its resonant frequency. The tuned circuit is also connected to the antenna and serves to select the radio frequency to be received, and is adjustable to tune in different stations. The feedback loop also has a means of adjusting the amount of feedback (the loop gain). For AM signals, the tube may also function as a detector that recovers the audio modulation; when that is done, the circuit is also called a regenerative detector.

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