In article ,
Reg Edwards g4fgq,regp@ZZZbtinternet,com wrote:

===================================

Unfortunately, it makes a terrible mess of any modulation.

To avoid distortion and non-linearity, saturation can be permitted
only on the extreme peaks of the modulated driving waveform. Class-C
is ruled out. Class-B or Class-AB prevails.

My recollection is that Classes B and AB are used for AM and SSB,
while Class C works fine for CW and for FM.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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Yes, exactly. We just assume sinusoidal drive because that's the most common
case

Joe
W3JDR


"John - KD5YI" wrote in message
newszDtg.17708$Wh7.10655@trnddc07...
W3JDR wrote:

In order to get any semblence of efficiency, the drive waveform should
drive the stage from near-cut-off to near-saturation, somewhat like a
switch. However, depending on how hard the sinusoidal input signal drives
the "Clacc C" stage, the conduction-angle and thus the duty-cycle of the
output (before filtering) will increase or decrease, resulting in
variable output power in the load.

Joe
W3JDR


Who said the drive had to be sinusoidal? If the final can run almost
square wave, why can't the driver?

Unfortunately, it makes a terrible mess of any modulation
-------------------------

Reg,

Well not ANY modulation, just signals with amplitude modulation components.
But this is a well-known property of Class C amplifiers, so what's the
point?. If we want to talk about Class B or Class A, we should start a new
thread.

Joe
W3JDR


"Reg Edwards" wrote in message
...

The 'power dissipation' in a Cass C stage is primarily a function of
what
percentage of the cycle the stage is neither in saturation nor
cutoff. A
perfectly rectangular output switching waveform of any duty cycle ,
if it
could be achieved, would result in nearly 100% efficiency.

Joe
W3JDR
===================================

Unfortunately, it makes a terrible mess of any modulation.

To avoid distortion and non-linearity, saturation can be permitted
only on the extreme peaks of the modulated driving waveform. Class-C
is ruled out. Class-B or Class-AB prevails.
----
Reg.

"W3JDR" wrote
The 'power dissipation' in a Cass C stage is primarily a function of
what
percentage of the cycle the stage is neither in saturation nor
cutoff. A
perfectly rectangular output switching waveform of any duty cycle ,
if it
could be achieved, would result in nearly 100% efficiency.

Joe
W3JDR
=======================================
All power amplifiers have a tuned circuit in the plate. It is
essential to reduce output power contained in the harmonics.

In any case, power in the harmonics is wasted power.

With a tuned circuit in the plate it is impossible to achieve a
rectangular voltage output waveform. It is always a sinewaveform.

A rectangular plate current in conjunction with a tuned load always
causes harmonic power to be wasted at the plate.

So one might just as well use a sinusoidal driving waveform, Class-C
or not. It's easier. It also avoids generating and wasting harmonic
power in the driver.
----
Reg

Reg Edwards wrote:
=======================================
All power amplifiers have a tuned circuit in the plate. It is
essential to reduce output power contained in the harmonics.

In any case, power in the harmonics is wasted power.

With a tuned circuit in the plate it is impossible to achieve a
rectangular voltage output waveform. It is always a sinewaveform.

A rectangular plate current in conjunction with a tuned load always
causes harmonic power to be wasted at the plate.

So one might just as well use a sinusoidal driving waveform, Class-C
or not. It's easier. It also avoids generating and wasting harmonic
power in the driver.


Not true.

A network with an inductive input will allow a square waveform at the
device output but not waste significant energy in harmonics. I've done
that in designs.

In the 1950's RCA had an AM BC transmitter that drove a tube with a
near square wave, and had a near square wave. The RCA transmitter used
a low-mu triode that had parallel tuned circuits in the grid and anode
set at the third harmonic. It had conventional networks feeding the
grid and to the antenna from the plate resonantor. That transmitter
made over 95% anode efficiency.

73 Tom

) writes:
Reg Edwards wrote:
=======================================
All power amplifiers have a tuned circuit in the plate. It is
essential to reduce output power contained in the harmonics.

In any case, power in the harmonics is wasted power.

With a tuned circuit in the plate it is impossible to achieve a
rectangular voltage output waveform. It is always a sinewaveform.

A rectangular plate current in conjunction with a tuned load always
causes harmonic power to be wasted at the plate.

So one might just as well use a sinusoidal driving waveform, Class-C
or not. It's easier. It also avoids generating and wasting harmonic
power in the driver.


Not true.

A network with an inductive input will allow a square waveform at the
device output but not waste significant energy in harmonics. I've done
that in designs.

And the point is that the Class-C only conducts on a portion of
that sinewave anyway, requiring that tuned circuit in the output
as a "flywheel" in order for there to be a sinewave at the output.
The "harmonics" are a byproduct of that non-linear conduction
at the input. And of course, that's why an amplifier could easily
become a multiplier simply by tuning the output to a harmonic of
the input signal.

Michael VE2BVW

Michael Black wrote:
) writes:
Reg Edwards wrote:
=======================================
All power amplifiers have a tuned circuit in the plate. It is
essential to reduce output power contained in the harmonics.

In any case, power in the harmonics is wasted power.

With a tuned circuit in the plate it is impossible to achieve a
rectangular voltage output waveform. It is always a sinewaveform.

A rectangular plate current in conjunction with a tuned load always
causes harmonic power to be wasted at the plate.

So one might just as well use a sinusoidal driving waveform, Class-C
or not. It's easier. It also avoids generating and wasting harmonic
power in the driver.


Not true.

A network with an inductive input will allow a square waveform at the
device output but not waste significant energy in harmonics. I've done
that in designs.

And the point is that the Class-C only conducts on a portion of
that sinewave anyway, requiring that tuned circuit in the output
as a "flywheel" in order for there to be a sinewave at the output.


The point is Reg said a square wave would decrease efficiency and a
sine wave was just as good, but that really isn't true at all.

73 Tom

This is what is meant by "Class F." A Class F amplifier has traps
tuned to various harmonics to allow the use of a longer conduction
angle and/or square wave drive with high efficiency and excellent
modulation characteristics.

There is also Class E-with a SERIES tank that is an open circuit
instead of a short to harmonics. These are run somewhat detuned so as
to cause the circuit voltage to fall to zero at switchon and switchoff
with square wave drive. 90% efficiency not uncommon with both E and F.
I

There are still other such tunings such as "inverse F," where EVEN
harmonics are blocked but odd shorted. This can be as simple as a
push-pull circuit with no center tap in the output tank if the
interelectrode capacitance is low enough at the lower harmonics, as it
often is for an AM broadcast or a 160M ham amplifier. For higher
frequencies this design requires parallel resonating the interelectrode
capacitance at one or more even harmonics abd becomes pretty much a
fixed-frequency affair as a result.

Inverse F(odd) is again capable of 90% plus efficiency.

All of these amps work by blocking the high harmonic current that flows
when a square wave pulse of 180 degrees angle is fed to a normal tank
circuit, which is essentially a short to harmonics. this allows the
active device to turn on and off with essentially no extra
resistance(from the device) beyond full-on in series with the load, for
all of the conduction angle.

The resulting tunings effectively square off either voltage or current
waveforms depending on the partucular tuning used. In other words,
current or voltage in the tank is NOT a sine wave, but the portion that
is trasferred to the load must be a sine wave or else filtered. In
fact, Class F may have been invented when someone put a harmonic trap
in a plate circuit to stop a nettlesome harmonic current from flowing
and then being coupled out. That is in some old RAH editions as a
desperate measure for stoppign harmonic TVI, but also allows more amp
efficiency if part of the design.

Straight Class C is in fact also capable of 90% efficiency, but only
with a very narrow conduction angle so the plate current pluse flows
only when tank voltage is already very low. This requires high back
bias, high drive-and a larger than normal tube or transistor. The
"power density" sucks, and in fact Class B is capable of more power for
a given maximum current and voltage, assuming the heat can be
tolerated. Class E and F amps can match Class B power densities with
early(narrow-angle) Class C efficiency.

For a tube low power density means higher filament power, giving
back some of that efficiency. Going to flourescent lights in the
station may save as much electricity cheaper in that case.



A network with an inductive input will allow a square waveform at the
device output but not waste significant energy in harmonics. I've done
that in designs.

In the 1950's RCA had an AM BC transmitter that drove a tube with a
near square wave, and had a near square wave. The RCA transmitter used
a low-mu triode that had parallel tuned circuits in the grid and anode
set at the third harmonic. It had conventional networks feeding the
grid and to the antenna from the plate resonantor. That transmitter
made over 95% anode efficiency.

73 Tom

WSQT wrote:
This is what is meant by "Class F." A Class F amplifier has traps
tuned to various harmonics to allow the use of a longer conduction
angle and/or square wave drive with high efficiency and excellent
modulation characteristics.

Class F, class X, class Y.

They are all recent "names" hung on what are really just class C
amplifiers.

I can do the exactly the same thing without a so called "trap" so long
as the output device sees a high impedance at the third harmonic or
better yet at all off harmonics.

That isn't a trap anyway, it is a resonator. The function is to provide
a high impedance at the third harmonic so the waveform slope is steep.
It allows the output device to go through the transitional state where
it is dissipating maximum power very fast. The goal is NOT to allow a
longer conduction angle. The goal is to have the output device either
on or off and spend less time in the high dissipation area where device
resistance is neither very low or very high.

The ideal duty cycle is 50%, with half time totally off and half time
in saturation.

This is why a very low mu trode with tons of negative bias worked so
well for RCA. The grid and anode 3rd harmonic resonators allowed the
grid and anode to see a very high impedance at the third harmonic, and
this sped the transition time up.

73 Tom

Tom says,

The ideal duty cycle is 50%, with half time totally off and half
time
in saturation.

=====================================
But 50% off and 50% on corresponds to Class-B bias conditions.

This is inconsistent with "tons of negative bias".

All that's required is a plate load which allows a square-wave plate
voltage waveform during the "on" period of the operating cycle. (A
very wide bandpass filter circuit configuration sounds appropriate).

Subject to this condition, a less than 50-50% operating cycle probably
does provide, somwhere, maximum efficiency.

I like your lumping together of Class-C, F, X, Y amplifiers.
----
Reg.