I still need convincing.
Let's take a class A transistor to start with. Assume that the collector
volts can swing from 12 down to 1 and up to 23 because of the
transformer. That's 22 volts peak to peak, or 7.78 volts rms. Now assume
that the load that the transformer refers back to its primary is 10
ohms. The output (assuming the xfmr is lossless) is 7.78 squared
divided by 10, or 6.05 watts. The collector current is 2.2 amps peak,
and we'd bias the baby at 1.1Amps in theory, and about 1.25 Amps in
practice. DC wise, that puppy looks like 12/1.25 amps or 9.6 ohms, but
we're not interested in DC. The output impedance of the transistor,
especially if we're down at low frequency, will be high - typically 50
or a 100Kohms, depending on the early voltage. The transistor equivalent
circuit is a current generator of magnitude hfe times ib in parallel
with the output resistance, and maximum output appears when all the
current goes through the load, rather than the shunt output resistance -
which you can transform to a series out resistance and a voltage
generator if you like. So there's an optimum output load impedance
(resistance is a particular case of impedance) which is where the device
can provide the volts and amps. Another example is regulated PSU - it
can have an output impedance of 0.01 ohms, but can't deliver maximum
power into 0.01 ohms, because it current limits and stops looking like
.01 ohms. So we have the situation of an optimum load impedance for
maximum output power to be obtained, which is not the same as the source
impedance.
As far as silver plate is concerned, I do remember that not only can you
have problems with the wrong sort of plating dropping the Q, but even
the right sort can give lower Q if the polishing is done in certain
ways. A place where you'd certainly expect to want to minimise losses is
in tank circuits on 30KW HF SSB transmitters. We always used bare copper
tube. Some improvement (but not enough to worry about) could be obtained
with 99.999% pure copper - I believe that water pipe does have some
additives to modify the physical properties which pushes up the
resistance, but not enough to worry about.
But silver plate looks nice, and it makes the government think that
they're getting better value for money on their contracts!
73
Peter G3RZP
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|