Regenerative Circuit - Advantages and Disadvantages

Advantages and Disadvantages

Regenerative receivers require fewer components than other types of receiver circuit. The circuit's original attraction was that it got more amplification (gain) out of the expensive vacuum tubes of early receivers, thus requiring fewer stages of amplification. Early vacuum tubes had low gain at radio frequencies (RF). Therefore the TRF receivers used before regenerative receivers often required 5 or 6 tubes, each stage requiring tuned circuits that had to be tuned in tandem to bring in stations, making the receiver cumbersome, power hungry, and hard to adjust. Regenerative receivers, by contrast, could often get adequate gain with one tube. In the 1930s the regenerative receiver was replaced by the superheterodyne circuit in commercial receivers due to its superior performance and the falling cost of tubes. Transistors, either bipolar or JFETs are used in regenerative receivers today. 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, some cell phone receivers.

Regeneration can increase the gain of an amplifier by a factor of 15,000 or more. This is quite an improvement, especially for the low-gain vacuum tubes of the 1920s and early 1930s. The type 236 triode (US vacuum tube, obsolete since the mid-1930s) had a non-regenerative voltage gain of only 9.2 at 7.2 MHz, but in a regenerative detector, had voltage gain as high as 7900. In general, "... regenerative amplification as found to be nearly directly proportional to the non-regenerative detection gain." "... the regenerative amplification is limited by the stability of the circuit elements, tube characteristics and supply voltages which determine the maximum value of regeneration obtainable without self-oscilation." Intrinsically, there is little or no difference in the gain and stability available from vacuum tubes, JFET's, MOSFET's or bipolar junction transistors (BJT's).

A disadvantage of this receiver is that the regeneration (feedback) level must be adjusted when it is tuned to a new station. This is because the regenerative detector has less gain with stronger signals, and because the stronger signals cause the tube or transistor to operate on a different section of its amplification curve (i.e. grid V vs. plate V for tubes; gate V vs drain V for FET's, and base current vs. collector current for BJT's).

A drawback of early vacuum tube designs was that, when the circuit was adjusted to oscillate, it could operate as a transmitter, radiating an RF signal from its antenna at power levels as high as one watt. So it often caused interference to nearby receivers. Modern circuits using semiconductors, or high-gain vacuum tubes with plate voltage as low as 12V, typically operate at milliwatt levels—one thousand times lower. So interference is far less of a problem today. In any case, adding a preamp stage (RF stage) between the antenna and the regenerative detector is often used to further lower the interference.

Other shortcomings of regenerative receivers are the presence of a characteristic noise (“mush”) in their audio output, and 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 lack an RF amplifier between the antenna and the regenerative detectors, so any change with the antenna swaying in the wind, etc. can change the frequency of the detector.

A major improvement in stability and a small improvement in available gain is the use of a separate oscillator, which separates the oscillator and its frequency from the rest of the receiver, and also allows the regenerative detector to be set for maximum gain and selectivity - which is always in the non-oscillating condition. A separate oscillator, sometimes called a BFO (Beat Frequency Oscillator) was known from the early days of radio, but was rarely used to improve the regenerative detector. When the regenerative detector is used in the self-oscillating mode, i.e. without a separate oscillator, it is known as an "autodyne".