P-T,
If one uses the voltage divider example, yes the maximum efficiency is 50%,
But the output of a PA stage is not a resistor, and the collector load
"resistance" is set by the designer to produce the output power desired.
The 'resistor' assumption is a simplification that can be used when
constructing equivalent circuits to analyze the voltages and currents in
the network that follows the perfect generator.
The theory of large signal amplifiers is too great for detailed
discussion on this reflector, so we usually have to settle for
simplifications. An equivalent circuit is not a full representation of
the workings of an actual PA stage. I believe that is where the
misunderstanding comes from.
73,
Don W3FPR
On 7/31/2014 6:57 PM, Per-Tore Aasestrand wrote:
Would not this imply a maximum efficiency of 50% ?
P-T
On 1 August 2014 00:40, Matt VK2RQ <matt.vk...@gmail.com> wrote:
I think we have hit the posting limit for this topic, so I'll just finish
with this remark: the maximum power transfer between a power amplifier and
its load will occur when the source and load impedances are complex
conjugates (this can be seen using the voltage divider equation and some
differential calculus). In the case of a purely resistive load, this
reduces down to the source and load resistances being equal.
73,
Matt VK2RQ
On 1 Aug 2014, at 8:24 am, Per-Tore Aasestrand <p...@ieee.org> wrote:
Hi list,
Isn't an RF transmitter or PA actually more like a current generator? Its
internal resistance is kept low in order to reduce internally dissipated
power, with consequent reduction in efficiency. It is usually designed to
work efficiently into 50 ohms.
It is important to know the difference between these two statements:
(A) The transmitter has a 50 ohm output impedance.
(B) The transmitter is designed to work into a 50 ohm load
A transmitter will be able to put its rated power into a 50 ohm load, but
this says nothing about its output impedance. That is why I don't think
conjugate matching will be appropriate.
(Hope this is not getting too much OT now)
Per-Tore / LA7NO
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