I was reading this blog, and I’m referring to this schematic. The author calls this regenerative receiver a “minimalist” design, but I refuse to consider a circuit with 7 transistors minimalist. If one thinks of the design as constrained to using a supply voltage of only 1.2 volts, a single AA or AAA rechargeable NiMH cell, then it is a design compromise. There is no reason why a higher voltage – more batteries, or a V boost circuit can’t be used to supply a higher voltage to the circuit. In fact, the circuit uses a 9V battery to tune the voltage variable capacitor. This could be used and the 1.2V battery eliminated.
The author stated that the emitter follower (first audio) stage is to prevent the circuit from acting erratic. There are four stages of voltage amplification, apparently because the gain of each stage is too low, apparently because the supply voltage is very low. I guesstimate that the emitter follower stage and two of the four audio amp stages would not be needed if the supply voltage was higher. And with four transistors instead of 7, I would consider the circuit more minimalist than his original circuit.
To get a higher supply voltage, one of several methods could be used. The NiMH rechargeable could be replaced with a lithium rechargeable, which would give a supply voltage of 3.6 volts. That’s 3 times as much. Or the battery could be 2 or 3 NiMH rechargeable cells in series, giving 2.4 or 3.6 volts supply. Both of these completely eliminate 3 transistors and their associated parts, at least a dozen parts.
A voltage boost circuit could be used. But these typically operate at high frequencies that might interfere with radio reception. So it could take several parts to isolate and reduce the interference. The switched capacitor V boost chips are less interference than the circuit that uses an inductor.
I noticed that the last 3 audio amplifier stages are connected to the battery, without a decoupling capacitor and depend on the low impedance of the battery to prevent positive feedback through the DC supply. This is a violation of good design and could be the reason why the author found that it was oscillating – ‘motorboating’. There *must* be a 47uF or more capacitor in parallel with the battery.
More details to come…
