Ummmm... sorry, Richard.

I am very familiar with choppers, having designed and built quite
a few in my time. But no engineer I have ever met would refer to
a chopper as a "lossless resistor."

An (ideal) resistor is in the class of passive, linear, time-invariant
devices. A "lossless resistor" would be a 0 Ohm resistor.

A chopper is a time-variant device, and depending on the details,
could be modelled as linear or non-linear on top of that.

The two devices (resistor and chopper) have very little in common,
besides being usable in electronic circuits. One of many differences
(a BIG one for us radio guys) is that the resistor would generate
no radio noise (other than thermal); but the chopper would generate
horrific radio hash, from the chopper frequency to daylight.

As for analogies to power amplifiers, an ideal Class A amplifier
would normally be modelled as an active, linear, time-invariant
device; so it would have some commonality with a resistor.
But the difference between passive and active is huge, of course.

A Class C amplifier would normally be modelled as an active,
non-linear, time-invariant device. There is very little similarity
left to a resistor, except for the time-invariant part. But a sidewalk
is normally time-invariant, too. So we could just as well say a
sidewalk is like a "lossless resistor." Or maybe that would be a
sidewalk with ice on it. ;-)

But to each his own...

Regards, Ed

"Richard Harrison" wrote in message
...
Old Ed wrote:
"Couild you explain this concept, and/or provide some references?"

Suppose we adjust a variable d-c supply to full-scale indication on an
external meter. Next, install a chopper (lossless on-off circuit) driven
at a high frequency to produce a square wave interruption of the d-c
with a 50% duty cycle, and insert the chopper contacts in series with
the external meter.

The chopper connects the external meter 50% of the time and disconnercts
it 50% of the time. The meter reads 50% of full scale.

Another way to reduce the scale reading to 50% is to insert a resistor
in series with the meter. If it is an 0-1 ma meter and if it has an
internal resistance of 1000 ohms, insertion of a 1000-ohm resistor in
series with the meter will reduce the meter scale indication to 50%.

The chopper as part of the meter source eliminates current to the meter
50% of the time.

The resistor which has the same effect and produces the same scale
indication as the chopper on the effective output current exacts its
loss of 0.0005 amp x 0.5 volt or 0.25 milliwatt during 100% of the time.

The chopper eliminates the power-losing resistor by substituting
off-time in the power source. The source only supplies the power used by
the load. With a resistor limiting power to the load, the source
supplies its loss and the load power.

The power in the load, a meter in our example, is the same using either
the resistor or the chopper. The resistor is analogous to a Class-A
amplifier. The chopper is analogous to a Class-C amplifier.

The off-time has the same effective opposition to current to a load as a
dissipative resistance. As the time-limited currented opposition to load
current consumes no power, it is called a dissipationless resistance.

Best regards, Richard Harrison, KB5WZI