Hi all,

Hi all,

As usual, thanks for the quick replies.

First of all, let me explain that I am not designing for a specific
application right now. 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.
Therefore, I don't want to just say "pick one which says it will work for
this application", I want to explain what is required for a transistor to
work well in a class C amplifier.

The whole point of my tutorial is to help make people "RF literate". It
does this, in part, by explaining how RF circuits work at the transistor
level. I think it is valuable to know how this works even if all you are
doing is just using prebuilt modules. In addition, the lessons learned in
trying to understand such RF circuits are useful in making sure that you
make intelligent design choices even in using off-the-shelf modules.
Besides, aren't you curious about how radio works at all levels? I
wouldn't be in the hobby or in the profession if I weren't.

Secondly, thanks for the book recommendations, but I already have
Motorola's RF Device Data book and Mini-Circuits RF designer's guide. In
fact, it was looking through Motorola's book that raised this question in
my mind to begin with.

After considering your responses, I think the problem is that "class C"
can mean several different things and the references I have give only one
meaning of it. They consider class C operation to be where the active
element (BJT,FET,tube,etc.) is only either completely on or completely
off. While in the on state, they consider it to act like a small
resistance. This gives you a behavior which can approach 100% efficiency
(looks like 85% max in their graph for a practical case) where the output
amplitude depends only on the conduction angle and supply voltage, not on
the input amplitude.

Apparently class C can also refer to the case where the transistor or
tube is not fully on in the conducting portion of the cycle. In this
case, the output amplitude depends on the input amplitude. Because the
average DC current can still be much smaller than for class A, it is more
efficient than class A. So, the a similar efficiency analysis applies as
applied above, but the circuit's output amplitude is now much less
dependent on the supply voltage and totally dependent on the input amplitude.

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. 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.

For BJTs, though, internal charge storage effects create speed penalties
for going into saturation. In general, these cannot be modeled as
capacitances because they invovle some pure delays (called Td and Tsd, Td
being the pure delay to begin to turn on and Tsd being the pure delay to
begin to turn off after being completely on). These delays depend only on
the amount of base overdrive, and are pure delays (a sudden change in the
input causes no change in the output until Td or Tsd time has elapsed),
so they do not act like capacitances. Therefore, S parameters or other
small-signal models would not model them. Yes, real switching circuits
also have capactive delays which would be modeled by S paramters, but
those are not the whole story for BJTs.

I think this now explains why the datasheets have only S parameters and
no timing parameters: they are for upper end HF,VHF, and UHF applications
where it is very difficult to make a BJT come out of saturation quickly
enough. So, they assume that you are thinking along the lines of the
"linear for part of the time" class-C model and give you the parameters
which help you to design such a circuit.

I just wish that the books I have made this distinction. It's because of
all of these little frustrating, subtle points that I have wanted to
write this tutorial in the first place. It will contain a lot of
explanations of such things which can often stump beginners.

Thanks again for your help,

Sean

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