Regenerative circuit

A regenerative circuit is an amplifier circuit that employs positive feedback (also known as regeneration or reaction). Some of the output of the amplifying device is applied back to its input to add to the input signal, increasing the amplification. One example is the Schmitt trigger (which is also known as a regenerative comparator), but the most common use of the term is in RF amplifiers, and especially regenerative receivers, to greatly increase the gain of a single amplifier stage.

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.

The regenerative receiver was widely used from the mid-1910s through the 1920s, with use declining during the 1930s and becoming uncommon by the early 1940s. Its principal advantage was high sensitivity with little added hardware, achieved by applying positive feedback around an RF detector stage and operating the circuit below the onset of oscillation.

Armstrong’s key insight was that radio-frequency energy existed in the detector’s plate circuit and could be fed back to the input, contrary to the prevailing belief that only audio frequencies remained after detection. When carefully adjusted, this feedback greatly increased the effective gain of a single active device, though operating required skill.

Regeneration improves selectivity by increasing loop gain near resonance, sharpening the frequency response without altering the intrinsic Q of the tuned circuit itself. The effect is equivalent to compensating circuit losses through feedback, simulating a negative resistance. As noted by Terman, regenerative detectors suffer from excessive selectivity, frequency-dependent critical adjustment, and a tendency toward oscillation that can produce interference and audible whistles if regeneration is increased too far. With the development of radio-frequency amplifiers designed around better tubes, regenerative detectors found relatively little application after the early 1930s.

A receiver circuit that used larger amounts of regeneration in a more complicated way to achieve even higher amplification, the superregenerative receiver, was also invented by Armstrong in 1922. It was never widely used in general commercial receivers, but due to its small parts count it was used in specialized applications. One widespread use during WWII was IFF transceivers, where single tuned circuit completed the entire electronics system. It is still used in a few specialized low data rate applications, such as garage door openers, wireless networking devices, walkie-talkies and toys.