Hi Dan,
> 1. the trace is a collector voltage horizontal and collector current
> vertical. The tracer is a Tek 577, the collector sweep is set to 50
> volts and the step current to 20 ma. The parallel resistor is set to 50
> Ohm.
I'm not familiar with that curve tracer, so I can make only partial
sense of the above. Does it mean that the tracer produces a family of
collector current/voltage curves, for base currents stepped by 20mA? And
where does the resistor come in?
> 2. I do not know about transistor ballasting. Is there a standard way to
> measure it?
What I would do is applying a few volts to teh collector, and measure
the base voltage while increasing the base current. The idea is to get a
curve of base voltage versus emitter current (which is close to
collector current, so you can measure whatever is easier).
A non-ballasted transistor will show the flattest curve, basically
standard silicon diode curve. With increasing ballasting, the voltage
will rise more with the current, so it will behave like a diode in
series with a resistor.
> Are there standard values? What value would be sufficient to
> operate in class AB?
I really don't know. A lot depends on how well the transistor chip can
equalize the temperature across it, which depends largely on its size.
As the heat distribution gets harder, more heavy ballasting will be
required.
I would expect a typical linear service RF power transistor to have a
base-emitter drop of at least 1V, probably more like 1.3V, when the
emitter current is close to its maximum rating. But this is pure
guessing, don't rely on it! In comparison, a transistor that has no
ballasting at all would probably end up near 0.8V or so.
If someone out there happens to have a bunch of different RF power
transistors lying around, it would be interesting to test this!
Manfred.
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