Next consider the bias adjustment circuits. There are two identical circuits, each one consists of a
trimmer potentiometer powered by “BIAS+” which is a 5V regulator. There is a 0.1uF decoupling
capacitor at the wiper; then a 47-ohm resistor and 1uH inductor in series to the transistor gate.
Additionally the gate drive must be AC coupled to the phase splitting transformer by a DC-blocking
0.1uF capacitor. That’s 10 components in total (5 per transistor)!
As the 50W amplifier circuit development progressed and evolved, I found I was able to eliminate
9 of those 10 components! This is achieved by simply connecting the phase splitter transformer T1
output windings directly to the transistor gates, with no DC-blocking capacitor. Instead, the center-
tap of this input transformer is connected to the wiper of a single bias adjustment trimmer
potentiometer, R5. The inductance of the input transformer T1 windings replace the need for the
series 47-ohm and 1uH inductors. The DC bias voltage is supplied to the gates through this input
transformer.
A single trimmer potentiometer works because I don’t care much about linearity. I am assuming
that IRF510 transistors from the same batch will have approximately similar characteristics. I am
not going to adjust them for a precise bias current of 125mA each for linear operation. I’m going to
adjust them just for Class-C operation. Everything is a lot less critical! It turns out to be “good
enough” to make the single adjustment.
For whatever reason, omitting the decoupling at the trimmer potentiometer wiper was found to
slightly increase output power and efficiency. I’m not really sure why that is. But repeated
experiments showed it IS the case, and as it is another simplification, why not!
Reducing the number of components in this part of the circuit from 10 to 1 considerably lowers the
production cost and simplifies the board layout and assembly.
Here are some comparative measurements I made, of efficiency and power output vs supply
voltage. The test cases were:
•
Two bias trimmer potentiometers, with no inductors in series to the gate
•
Two bias trimmer potentiometers, with 47uH inductors in series to the gate
•
Single bias trimmer potentiometer at the input transformer center-tap
•
Single bias trimmer a the center-tap with 0.1uF capacitor at its wiper
Note that these are not final power out or efficiency figures, which changed again slightly due to
changes elsewhere in the circuit; they are just concentrating on a comparison of bias methods.
50W QCX PA kit assembly
1.00q
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