Re: Matching source and input impedances in power amplifiers
- From: rick H <rik_nntp@xxxxxxxxxxxxx>
- Date: Thu, 22 Jun 2006 07:51:00 GMT
theduder2005@xxxxxxxxx wrote:
So the biggest reason to match is to prevent reflections?No. Reflections are just fine; a quarter-wave transformer used
to match two different resistances, for example, *relies* on
reflections between the transmission line and the source and load
to perform the match.
Nor does the OP's question relate to a typical scenario (maximising
transducer power gain). Usually one looks at the load-pull data for
the amplifier in question, then you match your source and load such that
their impedances seen at the input and output ports of the amplifier
give you the gain, noise-figure, IP3, efficiency and stability that
you want.
It makes sense why you'd want to match Zl to Zo (assuming everything is
unilateral) to transfer the most power FROM the amplifier to the load.
But why would you want to transfer the most power TO the amplifier?
Aren't most amplifiers inherently voltage or current sensing?
Unless you perform some sort of match, which takes into account the
length of the transmission line feeding the source to the amplifier,
then you've potentially got no idea what the voltage will be at the
input terminal of the amplifier. Of course this isn't the case when
you've got a nice block diagram with 50 Ohm input/output impedances and
50 Ohm lines everywhere - but this isn't always the case in practice.
If the amplifier had say a common gate structure, won't there be more
output power (with a matched load) if you maximized the input voltage
(vgs) by making Zin >> Zs (given the frequency is low enough to
disregard reflections)?
If the frequency is "low enough to disregard reflections", then it ain't
RF, and you default back to the low-frequency case of the voltage and
current looking the same at all points along any given conductor.
Under those circumstances, a high input-impedance would indeed cause
the input-voltage to look like the unloaded generator voltage.
In the RF case, however, this circumstance would just mean that the
voltage *incident* on the amplifier's input will looks like the generator
voltage, but refelections (which depend on the amplifier's input impedance
loading the transmission line) would add to the incident voltage to
create the composite terminal voltage. The phase of the reflected wave
would be dependent on the capacitance Cgs and could produce a terminal
voltage anywhere between zilch (high Cgs providing a low-impedance shunt)
to nearly the same voltage as the unloaded generator.
.
rick H wrote:
theduder2005@xxxxxxxxx wrote:
Hi.
I have a question concerning impedance matching in RF power amplifiers.
Given a power amplifier with input impedance Zin and output impedance
Zo, a source with impedance Zs and a load Zl - does conjugate matching
Zin and Zs give the highest output power?
Yes, if the amplifier is unilateral (S12=0).
In the more general case of a bilateral network (S12 is not 0)
changing the output's load will change the impedance looking into
the input of the amplifier, and changing the input's load will change
the impedance looking into the output of the amplifier.
In terms of reflection coefficients and S-parameters, you end up
with a set of two simultaneous equations:
conjugate(Gamma_source) = S11 + (S12*S21*Gamma_load)/(1-S22*Gamma_load)
conjugate(Gamma_load) = S22 + (S12*S21*Gamma_source)/(1-S11*Gamma_source)
which can be solved to give Gamma_source and Gamma_load required for
maximum transducer power gain.
I know that matching Zo to Zl will transfer the most power to the load,
but shouldn't Zin be much greater than Zs so the amplifier "sees" the
whole input signal? (ignoring reflections and noise performance)
You can't ignore reflections. If you make Zin very high, you will indeed
maximise the voltage incident on the Zin terminal, but the magnitude of
the reflection coefficient between the transmission line and Zin will
be close to unity - i.e. almost all of the power incident on the PA's
input will be reflected back to the source. If little of the power
available from the source is absorbed by the amplifier, then whatever
the power-gain of the amplifier, little will be available at its output
for delivery to the load.
--
Rick
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- Matching source and input impedances in power amplifiers
- From: theduder2005
- Re: Matching source and input impedances in power amplifiers
- From: rick H
- Re: Matching source and input impedances in power amplifiers
- From: theduder2005
- Matching source and input impedances in power amplifiers
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