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?

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

The Class C transistor amplifiers I design and use most certainly
saturate. I believe that's standard practice for solid state amplifiers.
I don't think tube type class C amplifiers are or were typically driven
to saturation, but I honestly don't know for sure.

The classic definition of class C involves only the fraction of the
cycle during which it conducts ( 180 degrees). There's no restriction
on how hard it conducts.

Roy Lewallen, W7EL

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?

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

On 2006-07-12, Roy Lewallen wrote:
The Class C transistor amplifiers I design and use most certainly
saturate. I believe that's standard practice for solid state amplifiers.

And as a followup, is the power dissipation in the final controlled
by the duty cycle of the pulses (or number of degrees of conduction
more generally if the input is not a square wave)?

--
Ben Jackson AD7GD

http://www.ben.com/

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?

Saturation is itself a somewhat mushy point. There's a V_sat specified
on the datasheets but the actual definition of saturated is entirely
application-sensitive. As a practical matter as you add more base
current you will go further into saturation (up until you melt the
base-emitter junction and then all sorts of wacky things ensue).

Choice of drive level and output level and load impedance in a Class C
amplifier certainly will in many cases put the device into saturation.

Tim.

Ben Jackson wrote:
On 2006-07-12, Roy Lewallen wrote:
The Class C transistor amplifiers I design and use most certainly
saturate. I believe that's standard practice for solid state amplifiers.

And as a followup, is the power dissipation in the final controlled
by the duty cycle of the pulses (or number of degrees of conduction
more generally if the input is not a square wave)?

For operation at HF and higher, it's not always that simple.

For anything but the lowest-speed PWM/class C/class D/class E
operations, the transistor switching time (and the influence of the
load on the switching time) can be a most important contributor to
power dissipated in the device.

An indication of the "bare transistor" switching time can be found on
the datasheets as the delay time, rise time, and most importantly
storage time and fall time.

Very closely related if you are operating anywhere near max power
dissipation or max current is the SOA curve.

Tim.

Ben Jackson wrote:
On 2006-07-12, Roy Lewallen wrote:
The Class C transistor amplifiers I design and use most certainly
saturate. I believe that's standard practice for solid state amplifiers.

And as a followup, is the power dissipation in the final controlled
by the duty cycle of the pulses (or number of degrees of conduction
more generally if the input is not a square wave)?

Not really, although it's a factor. What counts more is the transistor
voltage when the current is maximum, and the relationship between V and
I during switching. This in turn depends on the nature of the output
network among other things. If you really want to find out more about
the efficiency of a saturating transistor amplifier, look up some of the
papers on "class E" operation by Sokal & Sokal and by Raab. They
basically took a time-domain approach to find the optimum conditions for
maximum efficiency of a saturating class C amplifier. I've never
consciously designed a "class E" amplifier, but have gotten more than
85% collector efficiency from saturating Class C amplifiers at the 10
watt input level.

Roy Lewallen, W7EL

What says the group? Do we saturate in Class C or not?
-----------------------

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


"Tim Shoppa" wrote in message
oups.com...
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?

Saturation is itself a somewhat mushy point. There's a V_sat specified
on the datasheets but the actual definition of saturated is entirely
application-sensitive. As a practical matter as you add more base
current you will go further into saturation (up until you melt the
base-emitter junction and then all sorts of wacky things ensue).

Choice of drive level and output level and load impedance in a Class C
amplifier certainly will in many cases put the device into saturation.

Tim.

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


"Ben Jackson" wrote in message
...
On 2006-07-12, Roy Lewallen wrote:
The Class C transistor amplifiers I design and use most certainly
saturate. I believe that's standard practice for solid state amplifiers.

And as a followup, is the power dissipation in the final controlled
by the duty cycle of the pulses (or number of degrees of conduction
more generally if the input is not a square wave)?

--
Ben Jackson AD7GD

http://www.ben.com/

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?

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.