When we build an electronic project in 2016, the chances are that the active components will be integrated circuits containing an extremely large amount of functionality in a small space. Where once we might have used an op-amp or two, a 555 timer, or a logic gate, it’s ever more common to use a microcontroller or even an IC that though it presents an analog face to the world does all its internal work in the digital domain.
There was a time when active components such as tubes or transistors were likely to be significantly expensive, and integrated circuits, if they even existed, were out of the reach of most constructors. In those days people still used electronics to do a lot of the same jobs we do today, but they relied on extremely clever circuitry rather than the brute force of a do-anything super-component. It was not uncommon to see circuits with only a few transistors or tubes that exploited all the capabilities of the devices to deliver something well beyond that which you might expect.
One of the first electronic projects I worked on was just such a circuit. It came courtesy of a children’s book, one of the Ladybird series that will be familiar to British people of a Certain Age: [George Dobbs, G3RJV]’s Making A Transistor Radio. This book built the reader up through a series of steps to a fully-functional 3-transistor Medium Wave (AM) radio with a small loudspeaker.
Two of the transistors formed the project’s audio amplifier, leaving the radio part to just one device. How on earth could a single transistor form the heart of a radio receiver with enough sensitivity and selectivity to be useful, you ask? The answer lies in an extremely clever circuit: the regenerative detector. A small amount of positive feedback is applied to an amplifier that has a tuned circuit in its path, and the effect is to both increase its gain and narrow its bandwidth. It’s still not the highest performance receiver in the world, but it’s astoundingly simple and in the early years of the 20th century it offered a huge improvement over the much simpler tuned radio frequency (TRF) receivers that were the order of the day.
The basic regenerative receiver was patented in 1914 by the prolific inventor Edwin Armstrong, who you may also have heard of as the inventor of frequency modulation (FM). Armstrong’s original circuit applied its positive feedback through a small winding in series with the anode of this triode valve, coupled to the input tuned circuit. In use the coupling was adjusted until just before the point at which the circuit began to oscillate, at which point it was in its regenerative high gain and selectivity mode. A further refinement was the so-called super-regenerative receiver, in which the feedback was increased beyond the point of oscillation, but repeatedly “quenched” by an ultrasonic frequency turning on and off the regenerative detector.
The simplicity of a regenerative receiver did not come without problems though. The coupling adjustment became a small variable capacitor in later designs, and this could be found as a regeneration control on the front panel of a typical receiver. At every retune to a different station this would require readjustment for best performance, resulting in tuning a regenerative radio becoming something of a black art. In addition, if poorly adjusted they could sometimes oscillate and become transmitters in their own right. When the more complex but superior superhetrodyne receivers (another Armstrong invention) arrived around a decade later the popularity of regenerative receivers went into decline, and they had almost entirely disappeared by the end of the 1930s. Today they survive in niches such as amateur radio, toy walkie-talkies, toy electronics kits, and unexpectedly in very cheap UHF remote control modules.
It is this last application that points to one of the regenerative detector’s useful features. While most regenerative receivers are designed for AM broadcasts, the principle works at almost any frequency. It is possible to simply construct receivers using the principle that extend well into the UHF spectrum, and though they aren’t the best receivers on the block they can surprise you with their performance. [Roger Lapthorn, G3XBM] for example has published simple designs for a range of transceivers for the VHF bands with regenerative receivers, including the rather minimalist 2 metre (144MHz) “Fredbox”.
The regenerative receiver may not be the most advanced receiver ever conceived, and it certainly isn’t the most sensitive. But it’s one of those circuits that everyone should consider trying once, for its simplicity and ingenuity, and because it delivers results for relatively little effort. Go on, have one on your bench!
[Header image 1920s regenerative receiver, Charles William Taussig [Public domain], via Wikimedia Commons]