[Amps] Amp question

[Karl-Arne Markström]

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 at engineer.com>
To: <amps at 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 has 
>  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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