The regenerative circuit is a type of early design used in radio transmitters and receivers that is still in limited use today. At the most basic level, the concept involves feeding the output of a vacuum tube or a solid state component like a transistor back into itself. Combining this with a specially tuned circuit can greatly amplify a signal without the need for costly additional components. There were also some disadvantages to receivers that made use of regenerative circuits, such as a difficulty in tuning to new stations and the fact that they could also create interference by acting as transmitters. These circuits were widely used in radios until the 1940s, though their use in modern times has been relegated to radio frequency identification (RFID) readers and other simpler devices.
A basic regenerative circuit consists of only a few components. The main component is an active element, such as a vacuum tube or field effect transistor (FET), that acts as an amplifier. A signal is passed into the active element and then looped through a tuned circuit. The tuned circuit only allows a certain resonant frequency back into the active element, creating a feedback loop capable of greatly amplifying a signal. Since vacuum tubes were both expensive and cumbersome when regenerative circuits were introduced, this design allowed for the creation of radios that were less expensive and smaller.
One of the main drawbacks of the regenerative design is the way the feedback loop interacts with signal gain. Each time the station is changed on a radio with a regenerative circuit, the feedback level needs to be adjusted. Failure to do so can result in improper operation. Radios with these circuits also tended to suffer from noise pollution as a direct result of the simple regenerative amplification design, and could even generate powerful interference when used as transmitters.
Most radios stopped using the regenerative circuit in the 1940s, though it is still found in simpler devices. Garage door openers that send simple radio frequency (RF) signals typically make use of a regenerative circuit since they are so inexpensive and compact. These modern circuits typically use field effect transistors or other solid state components instead of vacuum tubes. Other modern applications of regenerative circuitry include radio frequency identification readers, keyless locks and the receivers in certain cellular phones. These are all applications where the compact nature of a regenerative circuit outweighs the various drawbacks of the design.