wrote:
Back in July I wrote:

I'm reading David Rutledge's excellent "The Electronics of Radio."
In Chapter 10 -- Power Amplifiers, he discusses Class C amps and says,
"In addition, if we drive the transistor clear to saturation, using the

transistor as a switch, the dissipated power can be greatly reduced,
because the saturation voltage is low. This is Class C
amplification..."
I'd always throught that in Class C, while you'd operate the device so
that it was cutoff during most of the cycle, but not saturated.
Is this just a different definition of Class C?

Look at it this way: If you had to drive the device into saturation
before it was class C, then there could have been no class C tube
amplifiers -- yet I see schematics for them all over my older amateur
radio literature.

The definition that _I_ was taught was all about conduction angle.

I checked back with SSDRA and EMRFD, and didn't see anything about
driving Class C amps into saturation?
What says the group? Do we saturate in Class C or not?
--------------------------------------------------------
I've been thinking about this some more. The 1980 ARRL handbook points
out that "Solid State power amplifiers should be operated just below
their saturation points for best efficiency and stability." Also, the
formula that we use to determine load resistance (Rl=Vcc^2/2Po) implies
that we are looking for a combination of Vcc, Load resistance and power
out that will prevent saturation.
And wouldn't we end up with far lower harmonic content if we only clip
one side of the wave form (at cutoff) instead of both sides (cutoff and
saturation)?

I know there are more exotic modes beyond C, but for plain old ordinary
ham radio applications, don't we normally avoid saturation in Class C
amps?

The "saturation" in bipolar transistor terminology means "current
saturation", but it could just as well mean "carrier saturation". When
the transistor is saturated the base region is stuffed full of carriers
(holes, for an NPN transistor). It takes a while for those carriers to
go away, during which the transistor stays on. This is a very nonlinear
effect, and can be very slow. The old 74Sxx series logic put a
schottkey diode from collector to base on the transistors to keep them
out of saturation, and sped them up considerably.

So yes, with an otherwise ordinary bipolar transistor you probably want
to avoid saturation.

Also, what about this business of having the efficiency improve through
saturation "because the saturation voltage is low" Could that be
right? If you put a voltage across a conductor and generate a large
current, you can't sit back and say "Great! Power consumption across
the conductor is low because the voltage drop across it is now
minimal!"

That's the whole point of the class D amplifier (AKA switching amp, or
switching supply) at baseband, and the class E amplifier at RF -- you
arrange the circuit so the transistor is either on with low voltage
across it, or off with no current, with as little time in between as you
can manage.

But it's not class C, or at least it isn't _just_ class C.

I don't think you could manage a class E amplifier with bipolar
transistors at RF frequencies, although I'm willing to be surprised.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tim Wescott wrote:
. . .
The "saturation" in bipolar transistor terminology means "current
saturation", but it could just as well mean "carrier saturation". When
the transistor is saturated the base region is stuffed full of carriers
(holes, for an NPN transistor). It takes a while for those carriers to
go away, during which the transistor stays on. This is a very nonlinear
effect, and can be very slow. The old 74Sxx series logic put a
schottkey diode from collector to base on the transistors to keep them
out of saturation, and sped them up considerably.

The very slow saturation recovery you see in saturated switch
applications (unless using a gold-doped transistor) is largely absent in
typical RF power applications. The reason is the bipolar drive usually
employed -- there's typically a large amount of negative base current
available to suck the stored charge out of the base region in a hurry.

. . .

Roy Lewallen, W7EL

Roy Lewallen wrote:
Tim Wescott wrote:

. . .
The "saturation" in bipolar transistor terminology means "current
saturation", but it could just as well mean "carrier saturation".
When the transistor is saturated the base region is stuffed full of
carriers (holes, for an NPN transistor). It takes a while for those
carriers to go away, during which the transistor stays on. This is a
very nonlinear effect, and can be very slow. The old 74Sxx series
logic put a schottkey diode from collector to base on the transistors
to keep them out of saturation, and sped them up considerably.


The very slow saturation recovery you see in saturated switch
applications (unless using a gold-doped transistor) is largely absent in
typical RF power applications. The reason is the bipolar drive usually
employed -- there's typically a large amount of negative base current
available to suck the stored charge out of the base region in a hurry.

. . .
I didn't think of that -- in spite of having applied it in a small
switching regulator application, per an ap note by Zetex.

D'oh.

Thanks for pointing it out -- perhaps I'll remember this second
application. Perhaps when the third one comes around I'll put one and
one together...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html