Regeneration of Radio Signal

The regeneration technique involves feeding back the same RF signal into the same transistor at least three times in order to amplify it. In the early days of valve radios, these techniques helped to increase the gain of a circuit whilst keeping the component count low. The great thing about a regenerative receiver is that it not only amplifies the signal, but also demodulates simultaneously. This means that you do not need to use a germanium diode.

A wideband regeneration circuit such as that shown above is capable of receiving all the stations within the MW and SW bands, and you can hear them all at the same time. A disadvantage of such a circuit is that it requires a very high antenna and good earth connection. If you construct a ferrite coil as shown in Project Ultra, 8-band crystal radio, then a circuit such as this will work very well with it, providing you use a good antenna and ground with it.

Demodulation

Most people will remember building a circuit like this as their first transistor amplifier circuit for their crystal radio. I remember building it when I was a whippersnapper and being surprised that it was demodulating without a germanium diode! This circuit works because of the non-linear input characteristic of the transistor. If you can imagine the RF signal as a sine wave, then the negative half of the signal does not amplify to the same extent as the positive half, and this is what non-linear characteristic means. This means that there is always a residual amount of current left at the collector, because both the positive half, and negative half, of the signal do not cancel each other completely. In formulas, we express this residual amount as an average. However, it is precisely this, which we are listening to as the demodulated signal.

Regeneration Principle

In this circuit, the 1 MΩ resistor feeds some of the amplified signal back to the base of the transistor. However, this signal will also consist of audio signals (AF) as well as RF; hence, a 1 µF electrolytic capacitor is used to short the audio signals to ground through the ferrite coil. This ensures that the base junction will always have a RF signal present, and nothing else.

On the other side of the transistor, there is a 10 nF capacitor which shorts RF signals to ground, because, there, we want only audio signals in the audio frequency range to be present.

This Article Continues...