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Thread: Class C amps
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face BY : Jim email (remove spam text)



Hello again ...

 "First of all ... I am not designing for a specific
  application ... . I am just trying to write a short
  explanation of how class C amplifiers are designed.
  I want to make sure I explain, in this writeup, *how
  to select a transistor* for such an amplifier."

A lot of competant men (and women) schooled in solid
state physics and semiconductor fabrication as well as
practical engineers versed in the practical experience of
RF amp design have spent countless hours putting
together those design guides and data manuals. The
'selection' of device based on arbitrary parameters
described coldly in a text on design or in a short paper
will be pressed to do really do this topic justice (no
offense) without 'shorting' some aspect of this art.

I wonder, do you plan on desribing what applications
demand what style/type of amplifier: class A, class AB,
class B and class C?

Each class has it's *prime* application - but special
treatment of RF amplifiers (as opposed to audio amps)
and each classification demands special descriptions
that audio applications usually do not need.

For instance: a class C amp is not sutable for low RF power
apps like RF front ends! This should make sense as the
*driving* signal is insufficient most all of the time to even
cause the device to conduct.

A class C amp *is* suitable, however, as the final PA (power
amplifier) in an FM transmitter such as those used by hams
in the 2M (144 MHz) band.

A class C 'amp' may also be plate, collector, or drain modulated
(called hi-level modulation) as the output stage in an AM
transmitter.

A class C amp used as an outboard amp following either an AM
or SSB (both are considered linear modulation techniques) exciter
would prove to be the wrong move as it would either be full on
with just the carrier driving the device into conduction or RF
would only be generated on modulation peaks of the AM/SSB
signal  envelope.

A class A - or more likely, as is done in the actual practical case,
a class AB amp (a 'linear' amp) would be used following an AM
or SSB exciter/transmitter.

Two devices operating in class B (working in push-pull) would
also be suitable following an AM or SSB transmitter.

Did you have a particular environment that you were slating
this paper for: low power consumer unlicensed devices (milliwatts)
or ham class (2 to 3 watts on up to 1000 watts) or commercial
(1000 watts and over)?

As an aside - there are many ways to also generate
"AM" signals besides the hi-level technique described
above - there is also "low-level" technique would require
all stages following the modulated/modulated stage (of
course) be 'linear' (not class C) in nature. There is also
the pulsed-modulated technique that is beyond the current
scope of this description.

I guess you do also understand the important aspect that LC
networks (like plate or drain/collector 'tanks') play in 'completeing'
the portions of the waveform where the class C device is not
conducting (on) in an RF amplifier?

This  'flywheel' effect is a very important aspect of insuring that
sinusoids (as opposed to square waves) with sufficient purity
are the ultimate result (besides the matching duties these LC
'tanks' provide in performing impedance matching).


   "If you are using a FET or tube, then, AFAIK, there is no
     speed-reduction penalty for going to the completely on
    (ohmic) region of operation, so the "saturated" model of
    class C operation makes complete sense for FET or tube
    circuits."

There are other factors that come into play that serve to limit
performance with these devices. There are still no miracle devices,
but devices have improved in the last decade.

    "It also has the advantage that you can do AM modulation
      by changing the supply voltage. I think this is why my
      references use this model."

This trick has been played with bipolar devices since
their introduction - and with good results (millions of
design/production cost effective CB radio designs use
this technique, aircraft transmitters utilize AM and have
use high-level style modulation as well).

I don't want to pound this point to death, but there is little
substitute for consulting someone in the field who has the
background and knowledge to understand the intricacies
involved and the pitfalls lurking just around the corner
when if comes to designing things 'RF' ...

Jim


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