Позитивті кері байланыс қолданатын электрондық тізбектер

Electronic circuit using positive feedback

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. It was widely used between 1915 and World War II. Advantages of regenerative receivers include increased sensitivity with modest hardware requirements, and increased selectivity because the Q of the tuned circuit will be increased when the amplifying vacuum tube or transistor has its feedback loop around the tuned circuit (via a "tickler" winding or a tapping on the coil) because it introduces some negative resistance. Due partly to its tendency to radiate interference when oscillating, and is largely considered obsolete. Regeneration (now called positive feedback) is still widely used in other areas of electronics, such as in oscillators, active filters, and bootstrapped amplifiers. 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. wireless networking devices, In a regenerative receiver the output of the tube or transistor is connected back to its own input through a tuned circuit (LC circuit). The tuned circuit allows positive feedback only at its resonant frequency. In regenerative receivers using only one active device, the same tuned circuit is coupled to the antenna and also serves to select the radio frequency to be received, usually by means of variable capacitance. In the regenerative circuit discussed here, the active device also functions as a detector; this circuit is also known as a regenerative detector. Two important attributes of a radio receiver are sensitivity and selectivity. The regenerative detector provides sensitivity and selectivity due to voltage amplification and the characteristics of a resonant circuit consisting of inductance and capacitance. The regenerative voltage amplification is where is the non regenerative amplification and is the portion of the output signal fed back to the L2 C2 circuit. As becomes smaller the amplification increases. The of the tuned circuit (L2 C2) without regeneration is where is the reactance of the coil and represents the total dissipative loss of the tuned circuit. The positive feedback compensates the energy loss caused by , so it may be viewed as introducing a negative resistance to the tuned circuit. The of the tuned circuit with regeneration is Providing the oscillation separately from the detector allows the regenerative detector to be set for maximum gain and selectivity which is always in the non oscillating condition.

For the reception of CW radiotelegraphy (Morse code), the feedback is increased just to the point of oscillation. The tuned circuit is adjusted to provide typically 400 to 1000 Hertz difference between the receiver oscillation frequency and the desired transmitting station's signal frequency. The two frequencies beat in the nonlinear amplifier, generating heterodyne or beat frequencies. The difference frequency, typically 400 to 1000 Hertz, is in the audio range; so it is heard as a tone in the receiver's speaker whenever the station's signal is present. Demodulation of a signal in this manner, by use of a single amplifying device as oscillator and mixer simultaneously, is known as autodyne reception. The term autodyne predates multigrid tubes and is not applied to use of tubes specifically designed for frequency conversion.

For the reception of single sideband (SSB) signals, the circuit is also adjusted to oscillate as in CW reception. The tuning is adjusted until the demodulated voice is intelligible.

Regenerative receivers require fewer components than other types of receiver circuit, such as the TRF and superheterodyne. The circuit's advantage was that it got much more amplification (gain) out of the expensive vacuum tubes, thus reducing the number of tubes required and therefore the cost of a receiver. Early vacuum tubes had low gain and tended to oscillate at radio frequencies (RF). TRF receivers often required 5 or 6 tubes; each stage requiring tuning and neutralization, making the receiver cumbersome, power hungry, and hard to adjust. A regenerative receiver, by contrast, could often provide adequate reception with the use of only one tube. In the 1930s the regenerative receiver was replaced by the superheterodyne circuit in commercial receivers due to the superheterodyne's superior performance and the falling cost of tubes. Since the advent of the transistor in 1946, the low cost of active devices has removed most of the advantage of the circuit. However, in recent years the regenerative circuit has seen a modest comeback in receivers for low cost digital radio applications such as garage door openers, keyless locks, RFID readers and some cell phone receivers. A disadvantage of this receiver, especially in designs that couple the detector tuned circuit to the antenna, is that the regeneration (feedback) level must be adjusted when the receiver is tuned to a different frequency. The antenna impedance varies with frequency, changing the loading of the input tuned circuit by the antenna, requiring the regeneration to be adjusted. In addition, the Q of the detector tuned circuit components vary with frequency, requiring adjustment of the regeneration control. Another drawback is that when the circuit is adjusted to oscillate it can radiate a signal from its antenna, so it can cause interference to other nearby receivers. Adding an RF amplifier stage between the antenna and the regenerative detector can reduce unwanted radiation, but would add expense and complexity. Other shortcomings of regenerative receivers are the sensitive and unstable tuning. These problems have the same cause: a regenerative receiver's gain is greatest when it operates on the verge of oscillation, and in that condition, the circuit behaves chaotically. Simple regenerative receivers electrically couple the antenna to the detector tuned circuit, resulting in the electrical characteristics of the antenna influencing the resonant frequency of the detector tuned circuit. Any movement of the antenna or large objects near the antenna can change the tuning of the detector.

The inventor of FM radio, Edwin Armstrong, filed US patent 1113149 in 1913 about regenerative circuit while he was a junior in college. He patented the superregenerative circuit in 1922, and the superheterodyne receiver in 1918. Lee De Forest filed US patent 1170881 in 1914 that became the cause of a contentious lawsuit with Armstrong, whose patent for the regenerative circuit had been issued in 1914. The lawsuit lasted until 1934, winding its way through the appeals process and ending up at the Supreme Court. Armstrong won the first case, lost the second, stalemated at the third, and then lost the final round at the Supreme Court. At the time the regenerative receiver was introduced, vacuum tubes were expensive and consumed much power, with the added expense and encumbrance of heavy batteries. So this design, getting most gain out of one tube, filled the needs of the growing radio community and immediately thrived. Although the superheterodyne receiver is the most common receiver in use today, the regenerative radio made the most out of very few parts. In World War II the regenerative circuit was used in some military equipment. An example is the German field radio "Torn. E. b". Regenerative receivers needed far fewer tubes and less power consumption for nearly equivalent performance. A related circuit, the superregenerative detector, found several highly important military uses in World War II in Friend or Foe identification equipment and in the top secret proximity fuze. An example here is the miniature RK61 thyratron marketed in 1938, which was designed specifically to operate like a vacuum triode below its ignition voltage, allowing it to amplify analog signals as a self quenching superregenerative detector in radio control receivers, and was the major technical development which led to the wartime development of radio controlled weapons and the parallel development of radio controlled modelling as a hobby. In the 1930s, the superheterodyne design began to gradually supplant the regenerative receiver, as tubes became far less expensive. In Germany the design was still used in the millions of mass produced German "peoples receivers" (Volksempfänger) and "German small receivers" (DKE, Deutscher Kleinempfänger). Even after WWII, the regenerative design was still present in early after war German minimal designs along the lines of the "peoples receivers" and "small receivers", dictated by lack of materials. Frequently German military tubes like the "RV12P2000" were employed in such designs. There were even superheterodyne designs, which used the regenerative receiver as a combined IF and demodulator with fixed regeneration. The superregenerative design was also present in early FM broadcast receivers around 1950. Later it was almost completely phased out of mass production, remaining only in hobby kits, and some special applications, like gate openers.

The superregenerative receiver uses a second lower frequency oscillation (within the same stage or by using a second oscillator stage) to provide single device circuit gains of around one million. This second oscillation periodically interrupts or "quenches" the main RF oscillation. Ultrasonic quench rates between 30 and 100 kHz are typical. After each quenching, RF oscillation grows exponentially, starting from the tiny energy picked up by the antenna plus circuit noise. The amplitude reached at the end of the quench cycle (linear mode) or the time taken to reach limiting amplitude (log mode) depends on the strength of the received signal from which exponential growth started. A low pass filter in the audio amplifier filters the quench and RF frequencies from the output, leaving the AM modulation. This provides a crude but very effective automatic gain control (AGC).

Superregenerative detectors work well for AM and can also be used for wide band signals such as FM, where they perform "slope detection". Regenerative detectors work well for narrow band signals, especially for CW and SSB which need a heterodyne oscillator or BFO. A superregenerative detector does not have a usable heterodyne oscillator – even though the superregen always self oscillates, so CW (Morse code)and SSB (single side band) signals can't be received properly. Superregeneration is most valuable above 27 MHz, and for signals where broad tuning is desirable. The superregen uses many fewer components for nearly the same sensitivity as more complex designs. It is easily possible to build superregen receivers which operate at microwatt power levels, in the 30 to 6,000 MHz range. It removes the need for the operator to manually adjust regeneration level to just below the point of oscillation the circuit automatically is taken out of oscillation periodically, but with the disadvantage that small amounts of interference may be a problem for others. These are ideal for remote sensing applications or where long battery life is important. For many years, superregenerative circuits have been used for commercial products such as garage door openers, radar detectors, microwatt RF data links, and very low cost walkie talkies. Because the superregenerative detectors tend to receive the strongest signal and ignore other signals in the nearby spectrum, the superregen works best with bands that are relatively free of interfering signals. Due to Nyquist's theorem, its quenching frequency must be at least twice the signal bandwidth. But quenching with overtones acts further as a heterodyne receiver mixing additional unneeded signals from those bands into the working frequency. Thus the overall bandwidth of superregenerator cannot be less than 4 times that of the quench frequency, assuming the quenching oscillator produces an ideal sine wave.

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