Re: Matching source and input impedances in power amplifiers

On Tue, 20 Jun 2006 20:48:12 -0500, "Tim Williams"
<tmoranwms@xxxxxxxxxxx> wrote:

Maximum power is always transferred when Zo = Zl.

That doesn't mean it's best.. most audio equipment has Zl >> Zo, and
speakers handle best when Zl >> Zo; amplifiers tend to go "poof" when Zl ~
Zo!.

RF equipment is (always?) designed to accommodate this matched condition,
primarily for two reasons (that I can think of): power efficiency, and
mismatched loads cause nasties on your line, namely reflections (manifest as
SWR).

I'll confess to not following this complete thread, so if I'm off-base
forgive me, but I have to jump in here. If you are speaking of the
amplifier output connected to a transmission line, then a mismatched
load will cause reflections on the line, but the amplifier output
impedance has -nothing- to do with it. The load produces the
mismatch, not the source.

More often than not, looking back into the output of an RF PA you will
-not- see an impedance match because the impedance matched condition
is not the condition for greatest efficiency.

If you are considering the input of the amplifier as the load for an
interstage transmission line, then that might well cause reflections
on the line but in narrow-band amplifiers this if often of little
consequence.


BTW, you would want Zin higher than Zs, otherwise your voltage disappears
(and most equipment is voltage-sensitive).

I have no idea what this means.


As for power delivered to the *load*, more power will be delivered IFF the
power amplifier is still linear. If it's saturated, more drive isn't going
to do anything.

But it goes non-linear long before "saturation". Driving beyond the
linear region does produce more power, even if it's contained in
distortion products. [g]

Wes
.

Relevant Pages

  • Rebuttal to Richard Clarks comments on my Chapter 19A
    ... RF power window at the input of the pi-network, ... plate, and the slope of the load line. ... the amplifier in this condition will later be shown to be 50 ohms. ...
    (rec.radio.amateur.antenna)
  • Re: Chapter 19A from "Reflections III" - Step 13 response
    ... It is thus evident that the amplifier has returned to delivering the ... was delivering 100 watts into the 50-ohm non-reactive load. ... reflected power, 30.6 watts, remains in the coax, adding to the 100 ...
    (rec.radio.amateur.antenna)
  • Chapter 19A from "Reflections III" - Step 13 response
    ... It is thus evident that the amplifier has returned to delivering the ... was delivering 100 watts into the 50-ohm non-reactive load. ... reflected power, 30.6 watts, remains in the coax, adding to the 100 ...
    (rec.radio.amateur.antenna)
  • Re: Simple variable AC supply?
    ... If your load is zero Ohms you will get maximum current through the load, ... so both of those cases will give you zero watts of output power. ... This is usually somewhere close to the lowest ohms speaker that the amplifier is rated to drive. ... The output impedance is very, very low - much much lower than the load impedance will ever be. ...
    (sci.electronics.design)
  • Re: Rebuttal to Richard Clarks comments on my Chapter 19A
    ... RF power window at the input of the pi-network, resistance RLP at the ... plate, and the slope of the load line. ... the amplifier in this condition will later be shown to be 50 ohms. ...
    (rec.radio.amateur.antenna)