Richard Clark wrote:
On Tue, 26 Apr 2011 13:12:23 -0700, Jim Lux
wrote:

(If I had a very efficient op amp, I could simulate any arbitrary output
impedance, without dissipating any power in the source)

I can see why this is parenthetical, because it covers a lot of sins
of omission.

yep.. not possible to build such a thing, anymore than one can build a
zero ohm output impedance RF source with any signficant power.
Suggested more as an example that the power dissipation in the source
doesn't necessarily correlate with match, load Z, or anything else in
general.

(You can get pretty darn close at powers less than a watt and HF, though..)


However, by virtue of OP AMP characteristics we would benefit to
vastly better distortion figures, far less spurious content, and
virtually no need for either the conventional impedance transformer,
nor the bandwidth filter that follows the same power deck (provided,
of course, that the drive input is sinusoidal - which it never is,
unfortunately, for this scenario). This novel OP-AMP/Power-Deck
redesign would also confer considerable power supply rejection (that
voltage could sag or rise without appreciable effect) and noise
rejection (the internal noise from other circuitry would not migrate
into the signal output). ALL such benefits are strictly derived from
the amount of negative feedback (not to be confused, as are many
readers to this topic, with the rather ordinary compensation cap in
the last stage).

One can also do a lot of this with various clever schemes if the input
to your PA is coming out of some signal processing. Generically,
predistortion, but it can be so much more.
People have literally spent their lives working out ever more
sophisticated approaches



Why isn't this done as a service to the customer?

Cost.

Like race cars... how fast do you want to go..just bring money