# [Amps] bipolar input and output impedance

Manfred Mornhinweg mmornhin at gmx.net
Thu Jun 4 18:08:36 PDT 2009

Hi Dan,

> What are the bipolar HF power transistor specs that influence or
> determine the input and output impedance?

The output impedance is a rather simple matter. If you like simplified
formulae, take the output impedance as the square of the supply voltage
divided by twice the power. And for the impedance between the collectors
of a push-pull pair, simply use the same formula, but doubling the
supply voltage.

An example, for a typical 100 watt push pull stage fed by 13.8V:

13.8 times 2 (for the push pull) is 27.6. Squared, it is about 762.
Divided by 200 (twice the power output), is 3.8. That would be the
approximate collector-to-collector impedance.

Now, do you want it more precisely? It's not all that difficult, and
it's no black magic. When you feed an amp with 13.8V, the peak RF
voltage the transistor can produce is a little less than 13.8V, because
of its saturation voltage. How much, depends on its characteristics, and
how you are using it. At higher frequency, the saturation voltage is
higher too. But let's take 1 Volt, which is typical. So the peak RF
voltage is 12.8, and between the collectors of a push-pull stage, it's
twice that, or 25.6V. And since the RMS voltage is 0.707 of the peak, we
have 18.1V RMS between collectors.

You want 100 watts. Since W = V * A, we need 100/18.1 = 5.52 amperes of
RF through the transformer primary. And by Ohm's law, 18.1 V divided by
5.52 A is 3.28 Ohm. That's the real output impedance of this stage. And
the discrepancy with the 3.8 Ohm given by the simplified formula is
simply because that formula assumes the transistors are perfect,
saturating at zero Volt! Plug in 12.8V instead of 13.8V in that simple
formula, and you will get the correct result, as long as your
transistors really saturate at 1V.

All this is for linear service. If you are willing to distort the
envelope of teh signal, then you will be driving the transistors into
saturation, resulting in the RMS voltage being more than 0.707 times the
peak. Up to 0.9 would be typical, at lower frequencies. This will make
the output impedance higher, for a given power and voltage. Or expressed
the other way around, a 100 watt, 13.8V amplifier like those used in
almost all our modern HF radios can deliver only those 100 watts in
linear service, but if overdriven will produce up to about 160 watts on
the lower bands, at the same impedance levels, but with severe
distortion (the splatter will make you famous if you do this).

Input impedance cannot be calculated as simply. A good practical
assumption is that it's about the same as the collector impedance! That
is, as long as the transistors don't have internal impedance matching.
VHF transistors often do, and then the input impedance is higher. And
the input impedance of non-ballasted transistors is lower, but
non-ballasted RF power transistors are pretty much an extinct species
today. They weren't fit for survival in the wild.

Anyway, this gives just the magnitude of the input impedance. Its phase
angle can easily be off by as much as 60 degrees, to either side. A
typical transistor will have inductive input impedance at some
frequencies, and capacitive at others. That's why most designers choose
to swamp the bases with low value resistors. That gives a more
predictable impedance into which to design the drive transformer and the
feedback network, and also adds a lot of stability, at the cost of some
gain.

By the way: Often it's a good technique to design the input transformer
to work basically into the reflected impedance of the feedback network,
and then add the transistors almost as an afterthought! If the feedback
network impedance is low enough, this works like a charm.

The output impedance also has reactive components, but they tend to
become really important only when the transistor is operated in the
higher part of its frequency range.

> I have an ENI circuit using
> TH430 transistors with 4:1 input and output transformers, the EB-63
> circuit based on MRF 454s uses 16:1 transformers.

If the two are of roughly the same power level and supply voltage, then
of course you got something wrong there. Perhaps one is talking about
turns ratio, and the other about impedance ratio? A turns ratio of 4:1
yields an impedance ratio of 16:1. And that's indeed the correct ratio
to use for a 100 watt push-pull stage running from 13.8 volt. It will
transform 50 ohms into 3.125 ohms, very close to the output impedance of
such a transistor pair, and the base-to-base impedance will also be
close enough to that.

> What transistor parameters influence transformer design?

All of them! ;-)

Kidding aside: For an HF broadband stage, you can pretty much do a
design based on the power level and supply voltage, and then pick some
transistors and plug them in. At VHF and higher, and generally for
resonant designs, it's important to pay closer attention to the specific
transistor's characteristics. The manufacturers usually give at least
some information about the impedances of their devices over the intended
operational range.

Manfred.

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