Class C amps and saturation (again)
 

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?
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?

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!"

73 from London
Bill M0HBR N2CQR CU2JL
http://www.gadgeteer.us

Class C amps and saturation (again)
 

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?
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?

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!"

73 from London
Bill M0HBR N2CQR CU2JL
http://www.gadgeteer.us

The definition I've always heard for class C is that conduction of the
active device is for less than half of each cycle. Nothing is said
about saturation.

Of course, if the active device conducts when there is appreciable
voltage across it, it will dissipate more power than if it were
conducting the same current at lower voltage.

Cheers,
Tom

Class C amps and saturation (again)
 

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

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

Class C amps and saturation (again)
 

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

Class C amps and saturation (again)
 

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?
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?

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!"

73 from London
Bill M0HBR N2CQR CU2JL
http://www.gadgeteer.us


As far as I'm concerned... saturation has nothing to do with the class
the amplifier is running. I can saturate an amplifier in any class.
Saturation is most ofen an undesireable effect... that causes
distortion in amplifiers.

www.telstar-electronics.com

Class C amps and saturation (again)
 

ORIGINAL MESSAGE:

On 5 Nov 2006 10:26:11 -0800, "Telstar Electronics"
wrote:

Saturation is most ofen an undesireable effect... that causes
distortion in amplifiers.

------------ REPLY FOLLOWS ------------

Saturation is only undesirable in LINEAR amplifiers. In non-linear
amps which are often used for CW, RTTY or FM, saturation is good
because it improves efficiency. Distortion in those applications is
unimportant.

Bill, W6WRT

Class C amps and saturation (again)
 

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

Class C amps and saturation (again)
 

"Telstar Electronics" wrote in
ps.com:

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?
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?

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!"

73 from London
Bill M0HBR N2CQR CU2JL
http://www.gadgeteer.us


As far as I'm concerned... saturation has nothing to do with the class
the amplifier is running. I can saturate an amplifier in any class.
Saturation is most ofen an undesireable effect... that causes
distortion in amplifiers.

www.telstar-electronics.com


Don't worry about saturation. CB'ers think over saturation produces
a better sounding signal. Of course, on ham radio we would call
them Lids.

SC

Class C amps and saturation (again)
 

"Telstar Electronics" wrote in message
ps.com...
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?
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?

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!"

73 from London
Bill M0HBR N2CQR CU2JL
http://www.gadgeteer.us


As far as I'm concerned... saturation has nothing to do with the class
the amplifier is running. I can saturate an amplifier in any class.
Saturation is most ofen an undesireable effect... that causes
distortion in amplifiers.

www.telstar-electronics.com


It can be messy. The class of amplifier is determined by the cut-off (or
lack thereof). A class A amplifier neither hits saturation nor hits
cut-off. Maximum theoretical efficiency (of a sinusodal waveform) of a
class A amp is 50%, but 25% is typical. Very low distortion, of course.

Class B amplifiers are biased at cutoff and only conduct for 50% of the
cycle. They shouldn't saturate, however. Efficiencies are around 60%.

Class C amps are typically biased well into cut-off and only conduct for
perhaps 90 degrees (25%) of the cycle and can run 70% efficient.

However, there are class D and E amplifiers and they are switchmode
amplifiers. They run on or off (cut off or in full saturation). They are
normally used for CW or FM and there was a circuit (and components)
available from a university to build a cw transmitter using switchmode. The
thing ran about 93% efficient!

To add to the confusion, with solid state it *is* possible to use the darn
things as a *linear* amplifier! Yep, commercial radio stations now use
these things. The problem is that the control circuitry (which, I believe,
controls the voltage fed to the final) is very complex and expensive and
will be found neither in amateur nor cb equipment for a long time.

73 from Rochester, NY
Jim

Class C amps and saturation (again)
 

Thanks to all who responded. I think I'm starting to understand this.
LTSpice helps a lot. I set up a class C amp and looked out power
dissipated in the transistor vs. power dissipated in the load. The big
efficiency gains that come with saturation were very apparent.

To better understand WHY this happens, I set up a spreadsheet that
looked at power dissipated in a variable resistor as it swept from .1
ohms to 10 ohms. It had a fixed 10 ohm resistor in series and 10 volts
DC across both of them. Yes indeed, the power disspated in the
variable resistor drops off dramatically when the resistance (and hence
the voltage across it) gets very low. I guess is why this happens in
the saturating Class C amp, Right?

But I still have some questions. When we design a Class A amp, the
familiar formula Rload = (Vcc-Ve)^2/2Pout allows us to come up with a
load value that will prevent the amplifier from saturating. A load of
this value will cause the voltage across the transistor (collector to
emitter) to vary from zero to twice Vcc. But it won't go into
saturation.

Why then do so many of the books (EMRFD, SSDRA, the W1FB books) call
for the use of essentially the same formula for the load when selecting
a load for Class C amps? We're no longer worried about staying out of
saturation, correct? In fact, we want to saturate. So why the same
formula? In fact, it seems to me that if you have a Class C amplifier
that is designed with this formula and is operating just below
saturation, you can get it to saturate just by increasing the value of
the load presented to the collector. Power out and efficiency
immediately improves. Linearity, of course, does not.

Thanks, 73 Bill M0HBR
http://www.gadgeteer.us





On Nov 4, 6:34 pm, 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?
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?

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!"

73 from London
Bill M0HBR N2CQR CU2JLhttp://www.gadgeteer.us

Class C amps and saturation (again)
 

wrote:
Thanks to all who responded. I think I'm starting to understand this.
LTSpice helps a lot. I set up a class C amp and looked out power
dissipated in the transistor vs. power dissipated in the load. The big
efficiency gains that come with saturation were very apparent.

To better understand WHY this happens, I set up a spreadsheet that
looked at power dissipated in a variable resistor as it swept from .1
ohms to 10 ohms. It had a fixed 10 ohm resistor in series and 10 volts
DC across both of them. Yes indeed, the power disspated in the
variable resistor drops off dramatically when the resistance (and hence
the voltage across it) gets very low. I guess is why this happens in
the saturating Class C amp, Right?

But I still have some questions. When we design a Class A amp, the
familiar formula Rload = (Vcc-Ve)^2/2Pout allows us to come up with a
load value that will prevent the amplifier from saturating. A load of
this value will cause the voltage across the transistor (collector to
emitter) to vary from zero to twice Vcc. But it won't go into
saturation.

Why then do so many of the books (EMRFD, SSDRA, the W1FB books) call
for the use of essentially the same formula for the load when selecting
a load for Class C amps? We're no longer worried about staying out of
saturation, correct? In fact, we want to saturate. So why the same
formula? In fact, it seems to me that if you have a Class C amplifier
that is designed with this formula and is operating just below
saturation, you can get it to saturate just by increasing the value of
the load presented to the collector. Power out and efficiency
immediately improves. Linearity, of course, does not.

....
In Class A, the active device is conducting the entire cycle, and
conducting in proportion to where you are in the cycle. Because of
that, you can drive a purely resistive load--that is, a non-resonant
load. Class A is used in single-ended broadband linear amplifiers,
such as receiver stages, audio amplifiers, scope vertical amplifiers,
....

In Class B, each active device is conducting for half the cycle. If
you use a single active device, you need a resonant tank to complete
the cycle. You can also have push-pull class B where one device
handles output voltages of one polarity, and the other handles output
voltages of the other polarity. It is usual to bias the active devices
in such a case slightly into class A, so that the nonlinearities right
at the half-cycle point can be smoothed over. But class B in a single
ended RF amplifier, with a resonant tank at the output, is still
capable of linear performance, because you can reduce the conduction
during the active half-cycle and reduce the output amplitude in
proportion.

In both Class A and Class B, it's possible for the output to peak with
the active device having a very low voltage across it, so that the the
output peak-to-peak amplitude at that point is still two times the
supply voltage, or nearly so, assuming it's a good approximation of a
sine wave there. For class A and B amplifiers, though, the output
amplitude may be less than that. The efficiency obviously goes toward
zero as the output amplitude goes toward zero, since some conduction in
the active devices is required to drive any output, and the dissipation
in the active device is proportional essentially just to the current at
low output swings since the voltage on the device is nearly constant;
but the power to the load is proportional to the square of the current.

In Class C, the active device is hard-on for a relatively small
fraction of the cycle. It always drives the output to about two times
the supply voltage, peak to peak, if it's properly designed for low
dissipation in the active device. But a key difference between it and
class A is that the active device is conducting essentially no current
except when there is only a very low voltage across it. In class A,
there's current through the device at all output voltages.

So at full output in all those cases, the peak-to-peak voltage is the
same, and for a given output POWER, that tells you what the effective
resistive load should be at that point.

(In Class D, the voltage at the output of the active devices swings
very quickly between power rails, spending most of the time at one rail
or the other, and the output is varied according to the duty cycle of
the rectangular waveform there. That waveform is filtered to remove
the switching frequency and its harmonics. The result can be a very
efficient amplifier. They are becoming common in audio applications
these days.)

Clearly there's a bit more to it than that simple explanation, but it's
a first approximation that may get you going...

Cheers,
Tom

Class C amps and saturation (again)
 

K7ITM wrote:
. . .
In Class C, the active device is hard-on for a relatively small
fraction of the cycle. It always drives the output to about two times
the supply voltage, peak to peak, if it's properly designed for low
dissipation in the active device. . .

I've designed saturating RF amplifiers of a few watts output in which
the device is on for well over half the cycle, and which have an
efficiency of around 90%. The peak-peak collector voltage is nearly 40
volts when run from a 12 volt supply. The high efficiency implies
relatively low dissipation in the active device. But perhaps this falls
outside some definitions of class C.

Roy Lewallen, W7EL

Class C amps and saturation (again)
 

Roy Lewallen wrote:
K7ITM wrote:
. . .
In Class C, the active device is hard-on for a relatively small
fraction of the cycle. It always drives the output to about two times
the supply voltage, peak to peak, if it's properly designed for low
dissipation in the active device. . .

I've designed saturating RF amplifiers of a few watts output in which
the device is on for well over half the cycle, and which have an
efficiency of around 90%. The peak-peak collector voltage is nearly 40
volts when run from a 12 volt supply. The high efficiency implies
relatively low dissipation in the active device. But perhaps this falls
outside some definitions of class C.

Roy Lewallen, W7EL

A flyback switching RF amplifier, Roy? ;-) One can also use active
devices that pull hard to ground and to a rail, delivering a square
wave output at high efficiency. That square wave can then be filtered
with a network that presents a high impedance for harmonics and passes
the fundamental, and achieve very high efficiency. A square wave is
nice because the even harmonics are theoretically zero, and in practice
can be much lower than the odd, so the filter doesn't have to have
super-steep cutoff. I guess that would be some variation on class D.
It's actually the way the HP8640B generates its signal: divide-by-2
stages from the master oscillator, followed by filters. I suppose it's
really best to describe the operation of any amplifier in some detail,
and not just rely on "Class A" or the like.

Cheers,
Tom


Cheers,
Tom