On 5/17/2014 5:14 AM, Ian Jackson wrote:
In message , Rob
writes

It is a widely distributed misunderstanding that a linear that has
been designed to delever its output in a 50 ohm load always has a
50 ohm output impedance. In fact it almost never has.

My understanding of RF design doesn't include how to calculate the
output impedance of PA stages. However, it's obvious that of the output
impedance was 50 ohms, the PA efficiency would be only 50% at best - and
clearly this is not so (certainly for class-C operation, which is
notionally 66%-ish). I've always understood that, in practice, it's
usually resistively low-ish (around 25 ohms?) and quite capacitive. An
expert opinion is needed!

I may be jumping in where I don't understand all the issues as I know
little about RF... but...

The output impedance of an amplifier does not dictate the power
dissipation of that amplifier. In fact the one amp design I have done
used synthetic output impedance for the sole purpose of reducing the
power consumption of the amp circuit. As I mentioned in another post,
it used a small series resistor and positive feedback to synthesize a
larger output impedance. I only had 12 volts power supply available and
needed to swing over 8 Vpp at the output. This circuit worked well for
that.

Is this never done in an RF amp?

--

Rick