Hi Bob,
> I have been told that modern solid state HF transceivers have been designed
> to deliver rated power into a stated load resistance-almost always around 50
> ohms-but that their effective internal impedance looking into the output
> port while it is active and delivering rated power is nowhere near 50 ohms
> but rather closer to a few ohms if you can define such an impedance at all.
> Is this more or less accurate?
Yes, indeed the internal impedance can be quite far removed from 50 Ohm.
The heavier the negative feedback is in the amplifier, the lower its
output impedance becomes. In many solid state amplifiers the feedback
varies with frequency, so that the impedance also varies. And to
complicate all this, the low pass filter can transform this impedance
too, depending on its design!
So, yes, if you measure the impedance of a solid state transmitter
output, very likely you will not find 50 Ohms, nor anything close to it.
Peter,
> For maximum power transfer the output impedance and the load must be the
> same.
That's correct, but misses the point! A solid state amplifier is not
designed to be loaded to highest power output, but to its correct design
output!
A transistor operating at a given base current will try to draw a
specific collector current, regardless of collector voltage. In other
words, the transistor behaves like a current source, whith a very high
internal impedance. But then, designers usually employ negative
feedback, sensing the collector VOLTAGE, deriving a current from it, and
feeding it back into the base in reverse phase. This results in the
amplifier behaving more like a voltage source, having a very low
internal impedance. Depending on how strong the negative feedback is,
the internal output impedance of a solid state amplifier can be from
much higher to much lower than the intended load impedance!
To best illustrate this, let's take a typical audio amplifier. It will
be designed to work into a load impedance of typically 8 Ohm, sometimes
4 Ohm, and occasionally something different. But one of the design
requirements is good speaker damping, and that's achieved through low
output impedance. It's very common to see audio power amps having an
output impedance of 0.01 Ohm and even much lower, while they are
intended to drive an 8 Ohm speaker!
Of course, if you lower the speaker impedance, it will get more power
from the amp, theoretically peaking when the speaker impedance equals
the amplifier impedance - but in a practical situation, either the
protection will kick in, or the amplifier will burn up, much before that
point is reached!
Back to the RF world, the situation is less dramatic, with negative
feedback being much less dominant than with audio amps. Still, a typical
transistor might have 12dB gain at 10 meters, but 36dB gain at 160
meters. When it is then used to provide a constant 10dB gain over the
whole range, maybe it will have a pad attenujating almost zero at 10
meters and 12dB at 160 meters, but the other 12dB difference are
implemented in the feedback network, having 2dB negative feedback at 10
meters but 14dB at 160 meters. As a result, this amplifier might have an
output impedance above 50 Ohm at 10 meters, and far below 50 Ohm at 160
meters. Still, it should be loaded with 50 Ohm on all bands, to deliver
its rated power and work properly!
> The internal low impedance of a solid state amp is converted to 50 Ohms by a
> wideband step-up transformer
I prefer to view this the other way around: The standard 50 Ohm load
impedance is converted down by the transformer to the optimal load
impedance for the amplifier, which depends mostly on the supply voltage
and output power, and to a lesser degree on the saturation voltage of
the transistors and the class of operation. Whether the amplifier has no
feedback and thus very high internal impedance, or intense feedback and
thus very low output impedance, does not matter!
Manfred.
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|