Class-AB, Push-Pull, Broadband RF Power Amplifiers for SSB OperationAuthor: R.J.Edwards G4FGQ © 8th July 1998
Transistors designed for this class of service are available with collector dissipation ratings up to 150 watts and up to 100 volts peak voltage ratings. There is a small internal resistance in series with the emitter to provide feedback to the base circuit. This makes the amplifier more linear and helps to stabilise collector current versus temperature. If required, this program allows an additional resistor, not bypassed for RF, to be used externally.
The program arbitrarily sets the collector operating angle to 184 degrees to minimise cross-over distortion. The quiescent, no-signal, DC collector current is then approximately 5 percent of DC current at full RF power output.
The ferrite output transformer core consists of two parallel cylinders, or may be formed from a single block with two holes. The collector primary winding is always centre tapped but since it is possible tap it at every half turn, the number of primary turns, collector-to-collector, may be either odd or even.
The number of turns on the secondary winding depends on RF power output, on DC supply volts and on the external load resistance. Because only a whole number of turns is allowed the designer must re-adjust one or more input values until this condition is satisfied. Input power is the most convenient value to adjust but occasionally it may be better to change the number of primary turns. The number of turns finally arrived at depends on the ratio of secondary reactance to the external load resistance at the lowest frequency of use. It is usual to make X/R => 4 which also depends on transformer core size and its permeability. This program computes the correct turns ratio. A separate calculation is needed to ensure the transformer core is consistent with reactance requirements.
It is not feasible to specify the required RF output power at the start because power efficiency is not known and it may not be possible to obtain that power with a whole number of secondary turns. Some experimentation will be needed.
The program estimates a typical value for the internal emitter resistance from an assumed collector power rating. An experimenter is prevented from inserting an impossibly high value of external emitter resistance - it is reduced and a a warning message is displayed. An external resistor will be used usually only on lower power amplifies. If no resistor, simply enter zero milli-ohms.
The large-signal current gain and the frequency at which current gain falls to unity can be found in device manufacturers' data books. For SSB transistors for use up to 30 MHz, unity gain will usually occur in the low VHF range. This program provides only a simple model. Some transistor characteristics may vary widely between nominally similar devices but performance is more consistent.
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