On 06/15/2009 09:08 AM, Manfred Mornhinweg wrote:
> 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?
Yes, a family of collector voltage and collector current curves. The
step is the change in the base current.
>
>> 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.
The tracer is not able to drive that much current, however it is able to
drive one amp. At the this value the base emitter value is over .8 volt,
however the curve and the slope indicate it would go well over a volt.
I will mount the TH430 on a heat sink and test it.
Thanks, Dan
>
> 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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> http://ludens.cl
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