I’ve battled with the output power of my HB1A from the start and there is not much info on the Net. I’ve accumulated some info from various sources to help me understand this relatively simple but “technically sophisticated” RF power amplifier.
The PA is as simple as this!
During my tests I accidentally exceeded the drive to the transistor by removing R35 (I later put in a 1/4 W 47 ohm resistor in place of the tiny surface mounted resistor) and burnt out the 10uH inductor L6.. It needs to be replaced and I was thinking of winding a 10uH inductor on a T50-6 toroid and putting that in it’s place. The toroid winding info would thus be:
I wanted to understand the real purpose of RF Choke L6 and found this discription in an ebook “Crystal sets to SSB”.
That says a lot and I would then regard that component as being a nescessity especially with regards to the problem I have!
The upside of choosing correct inductor (L) is that you get higher efficiency of the transmitter and your transistor will run much cooler. The peak collector current will be larger too. Peak collector current is larger because while ON, the transistor builds extra current through the small inductor (L). This current is dumped into the pi-filter (during the OFF phase) and must be replenished during the next ON period. The result is that the filter sees a larger swing, which translates to slightly larger output RF power. So you can see that the extra voltage and current that the transistor encounters is not wasted.
Just before the PA circuitry we have the HB1A’s DDS (Oscillator) and buffer circuitry and this explanation says a lot.
So I would assume that the HB1A’s final PA is what is referred to as a Class C amplifier.
From the Net …. “Every transistor has what is referred to as a “threshold”. This is the voltage at which the transistor begins to conduct. By it’s nature, a radio signal consists of a train of waves. A wave consists of varying voltages and/or currents. A sine wave, as it pertains to radio, typically has a positive portion (half of the wave consists of positive voltages) and a negative portion (the other half consists of negative voltages). The simplest and most common transistors (BJTs or bi-polar junction transistors) have a threshold of .7V. This means that when a wave is input into a single BJT transistor in the simplest possible amplifier, the transistor only produces output when the input wave is at .7V or higher. Obviously this means that that the rest of the wave (the entire negative portion and some of the positive portion) is missing from the output signal. The amplifier is conducting less than 180 deg. which means most of the original signal is missing. This is a classic example of nonlinearity. We put in a sine wave and we get only part of it back. This chopped up output wave is said to be “distorted”. This describes “class C” amplifiers. Even though the output is distorted, class C amplifiers are still very useful ….. “.
From Wikibooks :
Class C amplifiers conduct less than 50% of the input signal, typically only conducting during input peaks. Distortion is high, but high efficiencies (up to 90%) are possible. Some applications can tolerate the distortion. A much more common application for Class C amplifiers is in RF transmitters, where the distortion can be vastly reduced by using tuned loads on the amplifier stage. The input signal is used to switch the amplifying device on during peaks, and the complete waveform is recreated by a tuned circuit.
The Class C amp has two modes of operation: tuned, and untuned. When the proper load (e.g., a pure LC filter) is used, two things happen. The first is that the output’s bias level is “clamped”, so that the output variation is centered at one-half of the supply voltage. This is why tuned operation is sometimes called a “clamper”. This action of elevating bias level allows the waveform to be restored to its proper shape, allowing a complete waveform to be re-established despite having only a one-polarity supply. This is directly related to the second phenomenon: the waveform on the center frequency becomes much less distorted. The distortion that is present is dependent upon the bandwidth of the tuned load, with the center frequency seeing very little distortion, but greater attenuation the farther from the tuned frequency that the signal gets.
The tuned circuit will only resonate at particular frequencies, and so the unwanted frequencies are dramatically suppressed, and the wanted full signal (sine wave) will be extracted by the tuned load. Provided the transmitter is not required to operate over a very wide band of frequencies, this arrangement works extremely well. Other residual harmonics can be removed using a filter.
There are a lot of similarities with this circuit and the actual HB1A’s transmitter circuit.
The HB1A’s output filter circuitry is as follows:
So back to the workbench. Building in a rugged 10uH RF Choke and maybe even replacing the RF transistor. Then all should be good to go!
As simple as that …………. or is it?
73 de Eddie ZS6BNE