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Re: [Amps] Transceiver Output Impedance

Subject: Re: [Amps] Transceiver Output Impedance
From: Manfred Mornhinweg <>
Date: Sun, 09 Mar 2008 15:53:27 +0000
List-post: <">>
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.


> 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 

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!


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