The reason why this works is that the output combining network in most
solid-state PA's are
"zero-degree hybrids", which means that they expect the voltages (or currents)
driving them to be
in-phase to provide a combined output. Output transformers in p-p stages can on
the other hand
be classified as "180-degree hybrids", which require opposite-phase driving
voltages to operate properly.
As the phase shifts in the two amplifier halves can be assumed to be equal,
it remains to drive the halves with in-phase voltages.
The RC-network on the input side just acts as a resistive power dividing
network with its outputs in-phase,
and as the amplifier power gains decrease with frequency, capactive shunt
elements
can compensate for this decrease, as the insertion loss of the network also
will decrease with frequency.
Finally, the reason for using 200 ohms for the balancing resistor is simply
that the designer has chosen a 100 ohm
input design impedance for the combining network, which requires the amplifier
outputs to be 100 instead of 50 ohms.
This is "perfectly legal" in an environment where you are at liberty to choose
the design impedances freely, and do not have to take long runs of
interconnecting RF cables into account. But the stray and winding capacitances
present can be more difficult to handle in a system with a higher design
impedance.
In professional circles this is not very common, as most solid-state
professional PA's are built from modules
having a design impedance of 50 ohms.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "Will Matney" <craxd@engineer.com>
To: <amps@contesting.com>
Sent: Tuesday, July 26, 2005 6:57 AM
Subject: [Amps] Amp question
> All,
>
> Ok, most know how splitter/combiner transformers work in solid state amps.
> Here's the question, how can an amp work using two sets of push-pull amps,
> having a conventional combiner, but not have a splitter in the front end?
> What this amp has is a divider network made up of a RC network using several
> resistors in paralell with a capacitor on either side of the RF input. In
> other words, when the RF comes into this RC divider, it applies RF equally to
> both sets of push-pull amps at one time. There is no splitter transformer
> there. How can this work by having both push-pull sets on where they should
> be 180 degrees out of phase, and one being off while the other is on? To use
> a combiner transformer, the RF from one set makes 1/2 wave and so from the
> other set. The combiner then adds these two halves together making a full
> sine wave. So, how can this work having RF going to both sets at once instead
> of using a splitter which would put them 180 Deg out of phase? This
> absolutely ha
s
> me stumped. By the way, it's for an Ameritron ALS-500M.
>
> One last question, most combiners/splitters use a 100 ohm resistor in
> paralell with it's inputs for a 50 ohm load. How can this work if the
> resistance is 200 ohms? That would be for a 100 ohm load wouldn't it?
>
> Best,
>
> Will
>
> --
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