A 'Reflex'-type AM Radio --
Speaker Volume with only 2 transistors!
Feb 2004 by Rick Andersen
A Reflex radio is similar to the Regenerative receiver design, in that both use a controlled amount of positive feedback of the amplified signal, to reinforce it and obtain extreme sensitivity. Just what is fed back is what sets them apart.
The Regen takes a fraction of the amplified RF signal and feeds it back in phase with the signal coming in from the antenna. As the amount of regeneration is increased, a point is reached where the detector breaks into self-oscillation at the signal frequency (actually, just slightly off from it, which allows a beat note to be formed for CW reception). For the AM broadcast radio listener, this means a loud squealing. Wouldn't it be nice if we could get tremendous gain without that annoying howl?
The Reflex design essentially feeds back just the demodulated audio to the RF front end; the input transistor is pressed into double-duty: it amplifies both the RF and the AF (audio frequencies) at the same time. This gives a large gain without a heterodyne squeal so common to regenerative receivers.
In the circuit below, an incoming station is tuned by the front-end LC circuit. A tap in the lower 3rd or 4th of the inductor feeds the base of Q1; this low-tapping is done for impedance matching; i.e., the L-C tuned circuit would be detuned and its Q lowered from the loading effect of having a 600 ohm transistor base hanging off it, if the transistor were not tapped down on the inductor.
The RF gets amplified by Q1, then fed into a diode-cap pair which basically acts like the detector in a crystal radio, but with RF preamplification. Since the capacitor filters the RF out by shorting it to ground, we find the bottom of the coil at ground for RF while still "hot" for audio frequencies. Now the demodulated (by the diode) and filtered (by the cap) audio is put back into Q1's base. The emitter bypass cap is of a value that gives voltage gain at audio frequencies. The pot controls how much emitter bypassing there is and therefore how much audio feedback occurs. So the transistor is amplifying both RF and audio at the same time! And now, when the "volume" (emitter bypassing) is turned up, we find that there is some audio distortion, but no heterodyne squeal.
Notice also that the diode doubles as a path for DC bias for Q1's base.
The 33 mH inductor acts as an RF choke / lowpass filter, along with the caps on either side of it. The 3.3k is Q1's collector load resistor, and you could put a crystal earphone from the bottom of the 3.3k resistor to ground, for a 1-transistor reflex with surprising audio volume (with a decent antenna).
By adding a second audio stage whose collector load is the primary of a small 1k : 8 ohm audio transformer, we find that we can run Q2 as a Class-A amp, and that this stage has enough power to drive a decent 4", 8 ohm speaker to a comfortable volume if nearby stations are strong. I found that the cheaper the speaker, the less adequately the receiver performed. Considering all the trouble radio designers have gone through in years past to come up with the slickest Class-AB power amp design, it's amazing that we can get good volume with one transistor that isn't even a power amp! (The 2N3904 is a small-signal NPN; it is not a power amp)
I've also noticed that the audio is very clean-sounding-- no crossover distortion in this little amp operating Class-A.
Like any simple receiver, this one has its "-isms" and annoyances. One is the lack of AGC (automatic gain control, or automatic volume control). It seems more pronounced in a Reflex than, say, in a Regen radio, that some stations are weak and some are LOUD.... we've grown accustomed to AGC in our superhets and don't realize how spoiled we've become until we listen to a receiver that doesn't have an AGC circuit. Also, it seems to be easily overloaded by strong nearby signals. Like the crystal radio, it seems to come alive at night, but may not pick up much of anything during the day (again-- AGC normally takes care of the gain adjustments between day and night reception).
But I still recommend you build a Reflex! It will amaze you that such a simple radio can work so well.