Page 101 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Chapter 7

A Low-Power Regenerative Radio

When I suggested writing a chapter on regenerative radios, I thought th is was
going to be a piece of cake. It had to be easy, right? Just add some positive
feedback to the system, and magically you get higher gain and selectivity. And after
all, I had already designed a vacuum-tube version without a problem back in 2007
that had been featured in EDN Magazine on the Web

(www.edn.com/blog/Designing_Ideas/41377-A_super_het_radio_runs_S_years_on_
a_ C_ cell_plus_a_pentode_rad io. ph p).
It turns out that my first regenerative radio was designed with a little luck. Yes,
luck sometimes plays a part in successful designs.
However, in trying to duplicate that success from a pentode vacuum tube to
transistors-well, that's a different story. The pentode tube design had just the

correct amount of transconductance, the antenna coil chosen received the
amplitude-modulated (AM) stations with good signal strength, and the number of
turns added to the antenna coil for regeneration resulted in a simple
high-performance radio that did not go into premature oscillation. One may say
that out of beginner's luck I hit the perfect triad in my first design of a vacuum-tube

regenerative radio.
For the transistorized version, the transconductance is much higher than in the
pentode, the antenna coil is less sensitive than the one in the pentode, and the
number of turns used for regeneration in the antenna coil also was different. So the
first few versions of the transistorized regenerative radio actually failed-yes, they
failed.

In the following sections of this chapter, some of best features of regenerative
radios will be stated, some of the problems encountered will be mentioned, and
finally, solutions to these problems will be presented. For now, let's see what a
regenerative radio ideally is supposed to do.

Improving Sensitivity by Regeneration
In Chapter 6 on reflex radios we saw that a single transistor can function as a radio

frequency (RF) and audio frequency (AF) amplifier at the same time. But the RF
amplification factor of this transistor is fixed by the biasing current of its collector
(see Figure 6-3). The direct-current (DC) bias voltage across resistor R2 determines
the transconductance of the transistor, and therefore, its gain is fixed.
For regenerative radiOS, we try to feed back part of the RF energy of the RF

amplifier to be reamplified without running into oscillation. This is very tricky
because one wants the highest gain possible but also a lack of oscillation within the