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'[EE]: Class C amps'
2001\07\09@033848 by shb7

face picon face
Hi all,

In the midst of my research for the topics in my RF
tutorial, I have found a question which really has me
stumped and for which I can't find any answers anywhere.

All the books on RF amplifier design say that class C
RF amps should be operated with the transistor going
fully between saturation and cutoff to maximize
efficiency. This makes sense to me because you don't
want the transistor conducting current AND having a
significant voltage drop at the same time.

What I don't understand, though, is this: when you
design a transistor saturated switching circuit, you
usually want to know the switching paramaters for the
transistor (Ton, Toff, etc.) Yet, whenever I look up
datasheets for RF power transistors intended for class
C operation, no such parameters are given.

What gives? How are you supposed to know if a given
transistor can handle class C operation at a given
frequency? I'm also puzzled as to why you never see
Shottkey-clamped transistors in class C RF amplifier
service, since that would surely allow class C
operation at higher frequencies, wouldn't it?

Thanks,

Sean

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2001\07\09@053956 by Scott Stephens

picon face
>All the books on RF amplifier design

Indeed, "All" the books?

sorry, I don't mean to be nasty, I can't help it. Being a cellular engineer
at Motorola, I attended to infamous "Dale Carnegie" course that learned me
to "Never Criticize, Condemn or Complain" (hear no evil, see no evil, speak
no evil - go along and get along with social and management Pravda!) To "Not
pay too much" ("I'll give you something to cry about") to passively accept
and "not worry" - internalize, to blame oneself for ones stress related
illness and poisonings - (if you have a problem, your the problem). This
last technique of blaming the victim finds widespread application in abusive
families, police, prisons, communist brainwashing camps, and cult religions
that demand its members faith (and personal identities) are sinfull if their
faith will not heal their illness, if they must turn to science and doctors.
A very clever, subtle and insidius double bind that dumbs down and destroys
independance and initiative.

Of course, I got it all wrong, misunderstood what they were trying to
'teach' me. After all authority and social concensus is 'always right'. So I
don't need the 'double whammy' of getting smacked and ridiculed. Anyone hear
anything about the Motorola engineer Scott Falater that drowned his wife
while he was asleep? Or that Hewlet Packard woman that jumped from several
thousand feet out of the corporate jet?

I think corprate security has too many former CIA mindcontrol experts. But I
digress.

> say that class C RF amps should be operated with the transistor going
>fully between saturation and cutoff to maximize efficiency.

And harmonic generation too!

>What I don't understand, though, is this: when you
>design a transistor saturated switching circuit, you
>usually want to know the switching paramaters for the
>transistor (Ton, Toff, etc.) Yet, whenever I look up
>datasheets for RF power transistors intended for class
>C operation, no such parameters are given.
>What gives?

For RF transistors, have you found those funky things called "Smith chats"
that show the "Complex Impedance" vs Frequency?

> How are you supposed to know if a given
>transistor can handle class C operation at a given
>frequency?

You use a spice model and spice. Or you use the smith chart to tell you the
complex impedance, and design a conjugate matching network for the
transistor, input and output. The hybrid or S (scattering) parameters will
allow you to calculate the gain too, which the data sheets usualy provide
(Hfe, for instance).

Simply, the t-rise and fall are determined by the capacitive load the on the
transistor. Depending on the amplifier configuration (common emitter) , the
'Miller effect' can increase the effective capacitance. So your t-rise and
fall depend on frequency, external circuit components and amplifier
configuration.

ARRL has lots of good literature on design, which I will dig up references
for if no-one else tells you better. Seem to remember some old articles in
RF Design too. I wonder if their online?

> I'm also puzzled as to why you never see
>Shottkey-clamped transistors in class C RF amplifier
>service, since that would surely allow class C
>operation at higher frequencies, wouldn't it?

What? Putting a sharper pulse into the amplifier (with a diode 'clipper'?)
is done to generate harmonics. It would not improve the gain of the
transistor. But it is done with pulse sharpeners (frequency multipliers).

Scott

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2001\07\09@054208 by Graeme Zimmer

flavicon
face
Hi,

> All the books on RF amplifier design say that class C
> RF amps should be operated with the transistor going
> fully between saturation and cutoff to maximize
> efficiency. This makes sense to me because you don't
> want the transistor conducting current AND having a
> significant voltage drop at the same time.

Err.... I'll likely be showing my ignorance here....

The way I understood it, in Class C the transistor is conducting over a
narrow conduction angle, but it is reatively linear during that section.

It conducts in a short "Raised Cosine" burst...  that is, a Class C
transistor is not hard limiting.

If it were being switched hard on and off it would be in Class "D" or
perhaps Class "E"......

......................... Zim

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2001\07\09@090656 by Olin Lathrop

face picon face
> I'm also puzzled as to why you never see
> Shottkey-clamped transistors in class C RF amplifier
> service, since that would surely allow class C
> operation at higher frequencies, wouldn't it?

The point of the shottkey clamp is to prevent the transistor from going into
saturation.  As you pointed out, you want the transistor to spend as much
time as possible fully on or off in class C operation.  If you had a
shottkey clamp, it might be able to switch off faster at the cost of more
dissipation in the on state.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, .....olinKILLspamspam.....embedinc.com, http://www.embedinc.com

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2001\07\09@090706 by Olin Lathrop

face picon face
> Simply, the t-rise and fall are determined by the capacitive load the on
the
> transistor. Depending on the amplifier configuration (common emitter) ,
the
> 'Miller effect' can increase the effective capacitance. So your t-rise and
> fall depend on frequency, external circuit components and amplifier
> configuration.

Is this a commercial or hobby project?  I wouldn't just hack something like
this on a commercial product, but I once built a small transmitter from
"parts bin" transistors for which the specs where rather unclear.  The basic
trick was to have a common emitter transistor drive one in the common base
configuration.  In other words, pulse goes into base of Q1.  Emitter to Q1
goes to ground, and the collector drives the emitter of Q2.  The base of Q2
is tied to a solid low voltage reference with good bypass caps.  The emitter
of Q2 drives the load.  The point of all this is to greatly reduce the
Miller effect.

This transmitter was built and worked very well.  This was many years ago in
a college dorm.  The signal was fed into the power wires, but you could pick
it up with a hand held radio up to about 30 meters from the building.  I'm
sure it was totally illegal, but we had great fun with it on Sunday nights.
One time we got everyone in the dorm to flush their toilets at the same
time.  (Don't do this with sinks or whatever.  Toilets usually had a "soft
off").  Apparently the higher elevations of Troy New York had an unexplained
water outage for up to 10 minutes.  Surprisingly, we never got into much
trouble about that.  We found out later that the signal hopped a few power
transformers and in one case we heard about was picked up over a mile away.
Good thing this ended soon afterwards when the semester ended.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamspam_OUTembedinc.com, http://www.embedinc.com

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2001\07\09@111542 by Jim

flavicon
face
 " How are you supposed to know if a given
   transistor can handle class C operation at a
   given frequency?


If you have to ask - the transisitor is probably not
designed for that type of service.

Normally any device intended for a class of service
will have parameters decribing various classes of
service and also present 'test circuits' for demonstrating
same.

I recommend viewing/getting ahold of some of the old
Motorola "RF Device Data" books for a good feel on
how the RF guys spec devices.

If you're working with general 'switching' type transistors
like the 2n2222a - these are not normally characterized for
RF service - but function quite adequately for low power
RF applications within their capability. It's the 'art' of RF
design that enters here  when devices are not fully
characterized for service in RF apps (like 'S parameters',
MUG, MAG, input and output Z at spot freqs, etc).

Another choice for some apps requiring RF amps/oscillators
are the family of products by http://www.minicircuits.com

They publish an "RF/IF Designer's Guide" and sell "Designer's Kits"
with an assortment of parts.

A large part of RF design with discrete parts involves designing
impedance matching circuits, designing for stability (amps that
won't oscillate, etc) and the use of such RF Modeling Applications
like Microwave Spice. It is not a trivial pursuit ...

Jim  (RF circuit design/test exper. from AM broadcast - Ku Band)



{Original Message removed}

2001\07\10@085016 by Russell McMahon

picon face
A surprisingly good book (despite being a SAMS book) on RF design which
unlocks many practical secrets (and even makes sense of Smith charts) is

   RF Circuit Design ((o surprise))
   Chris Bowick
   Howard W Sams & Co
   1st published 1982 (don't let that put you off)
    $US22.95 at one time in the US
       ISBN 0-672-21868-2
       Sams ref MAY be 21868

       176 pages A4



     Russell McMahon
_____________________________

----- Original Message ---    --
From: "Sean Breheny" <KILLspamshb7KILLspamspamcornell.edu>
To: <RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU>
Sent: Monday, 9 July 2001 19:37
Subject: [EE]: Class C amps


{Quote hidden}

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2001\07\10@094904 by Scott Stephens

picon face
From: Jim <TakeThisOuTjvpollEraseMEspamspam_OUTDALLAS.NET>

>I recommend viewing/getting ahold of some of the old
>Motorola "RF Device Data" books for a good feel on
>how the RF guys spec devices.

Good luck finding them. They have many good app notes, but you can find many
good app notes from the many excellent online and user-freindly web sites
from Siemens, Phillips, Harris, Zetex, et. And RF design & smith charting
software too. Checkout http://www.rfglobalnet.com for software, tutorials,
app-notes and website indexes.

The last time I tried to get an app note from Motorola (several years ago),
I had to have it faxed to me and it took 1/2 hour! I have found them
unfreindly in several ways, which I'll refrain from detailing as they have
been ranted about in this forum in the past, especially regarding getting
their parts. It was said they are made from pure 'unobtainium'. Maybe they
have to see if you are 'in the club' before they can sell to you? I don't
know.

>Another choice for some apps requiring RF amps/oscillators
>are the family of products by http://www.minicircuits.com

Their MMIC's or VNA's have constant 50 ohms in and out over their bandwidth,
so its a no-brainer, but for the layout. And those suckers run hot and are
inefficient. No substitute for good design, but cheap'n dirty.

Scott

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2001\07\10@105651 by Jim

flavicon
face
I guess I'll say it again - the task of RF design is NOT a
trivial undertaking. Many a *new* engineer has learned
the hard way to take the sage advice of 'the old timer'
in regards to hard-learned lessons and 'tricks of the
trade. For the novice who needs results NOW (in a
prototype, proof of concept or class project) there
is no substitute for the off-the-shelf time-tested product
such as those offered by minicircuits (an others).

IF you're going to build millions of units and the units are
going to be built in (formerly Red) China using low-cost
labor then the discrete design route makes economic
sense - but then you'll need vector network analyzers,
spectrum analyzers and extensive design expereince down
to the component level.

Minicircuits simply solves most of that - and won't require
you to learn the intricacies of biasing over temperture and
assuring a known level of stability in any amp particular
one employs in circuits they ahve designed.

I dare say that there isn't:

A) an RF power (output) level coupled with any
B) particular frequency band
C) at state of the art effeciencies

that I can't find suitable parts and app notes for in minutes.
Applying these designs for one's own purposes using these
parts won't require the 'need to know' such specific (and
irrelevant in the RF world) paramerters like device Ton
and Toff times. (Although, in the design of these transistors
themselves by the foundry, these parms are considered by the
solid state physics majors who oversee the fabrication of
such parts as 'product engineers'.)

As other posters have pointed out - it's input and output Z in
the form of S parameters (for direct use in uWave modelling
pgms) or "Smith Charts" that are of much greater importance
for the purpose of design LC (Inductor/Capacitor) Z (impedance)
matching circuits.

Other important parameters (for real power amps) are such
as things as maximum allowable output mismatch - this clues
one in as to where possible device destruction due to a
mismatched load may ocurr.

With the widespread proliferation of wireless 2-way as
we have seen today there exists a pleathora of integrated,
concisely packaged, nearly foolproof multifunction modules,
ICs and packaged amps, receivers, and transceivers that
can be employed for nearly every purpose under the sun.

It used to be that op-amps were discrete tube and transistor
designs too -  and those days have been long gone for quite
awhile now (Yogi Bera quote used without permission) save for
perhaps one or two EE lab courses in ...

But wait! Dicrete tube and transistor design is not dead! Witness
the new amps from guitar amp makers like Fender and others ... what
comes around goes around - but I don't think that discrete RF amp
design is really going to erupt as fad anytime soon.

Jim


{Original Message removed}

2001\07\10@121942 by Sean Breheny

face picon face
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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2001\07\10@125614 by SkinTech

flavicon
face
Sean,

I think you mix up class C with class D. Class D is where the active device
is either fully on or off. This is sometimes referred as 'digital amps'; in
fact it is some form of pulse modulation (pulse width modulation, duty cycle
modulation, whatever). Think of it as a power ADC. You need a filter at the
output to reconstitute the original (analog) signal.

Class C is when the active device conduct for less than 180 degrees of the
signal cycle (assuming a sinewave signal). The output will be the top (or
bottom) of the sinewave, but for less than half a cycle. This is of course a
grossly distorted replica of the input signal. Therefor, a tuned filter
(also called a 'tank', normally a parallel LC circuit) at the signal
frequency is added at the onput. The filter starts to 'oscillate' at the
signal frequency, thus providing an amplified version of the input signal.
One consequence is that class c has very narrow bandwidth, unless the
reconstitution filter is tunable or has low Q; in the latter case the
efficiency suffers. In fact, one can argue that the output filter takes the
power of the output 'pulse' and transforms it into a sinewave. The effective
output power is the area under the output 'pulse' minus the losses in the
output filter.

As for the transistor to use, it must a) be a power device, b) have good
high-frequency specs (Fc, low capacitances, low rise/fall times, the usual
stuff). By nature it does not have to be particularly linear.

Cheers, Jan Didden


{Original Message removed}

2001\07\10@183214 by Jim

flavicon
face
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


{Original Message removed}

2001\07\10@190106 by Sean Breheny

face picon face
Hi Jim,

Thanks for all the effort that you put into this explanation, but yes, I
am aware of all the considerations that you mention. I am not confused
about what a class C amp is or what it is used for, I just wondered about
why RF power transistors were not speced for switching performance, and I
now realize that it is because class C amps are usually not run
completely into saturation.

Yes, I planned on discussing the appropriate applications for each
amplifier class. I don't expect my tutorial to make seasoned RF designers
out of people, just to help beginners over some of the "humps" that they
encounter in trying to understand RF work.

Incidentally, I would appreciate your comments on the tutorial when it is
finished.

Thanks again,

Sean


On Tue, 10 Jul 2001, Jim wrote:

{Quote hidden}

> {Original Message removed}

2001\07\10@190515 by Sean Breheny

face picon face
Hi Jan,

Perhaps you are right, but the book from which I learned the most about
class C and D amps (Radio Frequency Electronics by Jon Hagen, not a
really great book, although I don't know it to be flat out wrong
anywhere) describes class C amps as saturated switches and class D amps
as totem-pole saturated switches (two transistors, like a digital inverter).

Sean


On Tue, 10 Jul 2001, SkinTech wrote:

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2001\07\10@205933 by Scott Stephens

picon face
>Yes, I planned on discussing the appropriate applications for each
>amplifier class. I don't expect my tutorial to make seasoned RF designers
>out of people, just to help beginners over some of the "humps" that they
>encounter in trying to understand RF work.

This is what the ARRL Handbood, the Ham's 'Bible', is about. The older ones
are less technical and are worth checking out - circa 1980

>> 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.

Ham's, Hackers and Hobbyists don't need that. BTW got my latest Globalnet
newsletter today:

"Ever have a problem needing a solution? Have you ever been in need of
advice?
RF Globalnet provides Discussion Forums to foster the community and enable
continuous interaction and collaboration within our industry.
Post your questions in Discussion Forums at any time:
http://rf.rfglobalnet.com/forums/home.htm "

I should drop in and query the experts about radio-solitons, Bessel beams
and x-waves. Be nice if an engineer could explain the U1 and SU(2) groups.
Perhaps its just including the transient response of space-time with
Maxwell's steady state equations?

Scott

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2001\07\11@090604 by Olin Lathrop

face picon face
> Perhaps you are right, but the book from which I learned the most about
> class C and D amps (Radio Frequency Electronics by Jon Hagen, not a
> really great book, although I don't know it to be flat out wrong
> anywhere) describes class C amps as saturated switches and class D amps
> as totem-pole saturated switches (two transistors, like a digital
inverter).

I always thought that class C meant using pulses to excite a tank circuit to
resonance, and class D is some form of pulse modulation with the output
signal derived from the pulses average.  Class D usually implies filtering
to get rid of the high frequency pulses, but is intended to produce some
arbitrary waveform, not to resonate a tank circuit.

The college dorm AM transmitter I mentioned in an earlier post had a class C
push-pull output.  The switching elements alternately pulled down on
opposite ends of a center tapped primary.  The modulation signal acted like
the power supply at the center tap.  Both halves of the primary and the
secondary were carefully adjusted for resonance at the desired frequency.
Each switch was only on a small fraction of the cycle.  About 10% if I
remember right, for a total of 20% "on".  The tank circuit was free wheeling
the remaining 80% of the time.  A push-pull driver like this has the
advantage of generating far less power in the even harmonics.


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2001\07\11@091634 by Alan B. Pearce

face picon face
I suspect some confusion is originating because people have forgotton the
definitions of amplifier classes. My memory is that the classes are
generally defined as follows

Class A - conduction is required for 100% of cycle
Class B - conduction is for 50% of cycle (idealised case)
class C - conduction is for <50% of the cycle (requires tank circuit of some
kind, eg rf stages and TV horizontal output stages
Class D - conduction is for varying percentage of cycle as waveform is
defined by PWM.

I do not claim that these are definitive, but this is my memory of them.

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2001\07\11@100128 by Graeme Zimmer

flavicon
face
> 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

it says here.......

"Class C is less well defined but is characterized by
conduction angles of less than 1800. There is usually
no bias voltage provided except by the drive signal, and
the efficiency is as much as 90%.

There are more classes. Saturated class C, sometimes
called C-E is a higher drive version of the same thing.
It can be 95% efficient.

Class D is a particular configuration using two transistors as switches.
Its use is limited by the parasitics of passive and active parts to
about 10MHz. It can be nearly 100% efficient.

Class E is a special zero voltage switching mode. It relies on
the proper phase relationship between drain voltage and
current to provide no overlap. The efficiency can be
100% with perfect parts. There are other classes,
somewhere between C and E........... "

see
www.advancedpower.com/TechnicalSupport/ApplicationNotesData/APT0001.pdf
and
http://iroi.seu.edu.cn/jssc9899/33ssc98/33ssc12/pdf/33ssc12-su.pdf

I've just leared there are Classes F1, F2, F3 and even Class S !!

............................... Zim

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2001\07\11@121901 by Harold M Hallikainen

picon face
       Something that started showing up in AM broadcast transmitters several
years ago is a third harmonic resonator on class C amps. This vastly
improves the efficiency of these amplifiers. Without it, the final tank
circuit tries to force the plate voltage to be a sine wave, pulling the
tube out of saturation "at the edges" of the conduction pulse. Putting in
the resonator allows the tube to stay in saturation (at least much more
so), decreasing plate dissipation and improving efficiency.
       This was introduced something like 50 years after class C amplifiers
were first used in AM transmitters. You'd have thought it was a mature
technology by then!

Harold

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2001\07\11@123145 by David VanHorn

flavicon
face
>
>         This was introduced something like 50 years after class C amplifiers
>were first used in AM transmitters. You'd have thought it was a mature
>technology by then!

How is it that AM transmitters use class C?
I thought that would destroy the AM, or do they do that in front of a
modulator??


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2001\07\11@131937 by Jim

flavicon
face
The final RF stage (in this case) is operated class C and
the collector (or plate) supply (Vcc or B+) is "modulated" - a
process called "high-level modulation".

Any succeding stages, however, must be LINEAR in
nature and therefore cannot be "class C" stages ...

Jim

{Original Message removed}

2001\07\11@133228 by David VanHorn

flavicon
face
At 12:18 PM 7/11/01 -0500, Jim wrote:
>The final RF stage (in this case) is operated class C and
>the collector (or plate) supply (Vcc or B+) is "modulated" - a
>process called "high-level modulation".
>
>Any succeding stages, however, must be LINEAR in
>nature and therefore cannot be "class C" stages ...

Ah.. Variable power supply.

Interesting approach, I bet it has some interesting kinks, as the
transistor's parameters change over applied VCE.

You could also call it PAM I guess.


As an aside, why are we hams so stuck on 12V and 50 ohms?
48V and 75 ohms look a lot better to me.


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2001\07\11@150645 by Olin Lathrop

face picon face
> How is it that AM transmitters use class C?
> I thought that would destroy the AM, or do they do that in front of a
> modulator??

Very roughly, the AM modulation signal is the "power supply" input to the
class C stage.


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2001\07\11@152721 by Harold M Hallikainen

picon face
On Wed, 11 Jul 2001 11:31:20 -0500 David VanHorn <EraseMEdvanhornspamspamspamBeGoneCEDAR.NET>
writes:
> >
> >         This was introduced something like 50 years after class C
> amplifiers
> >were first used in AM transmitters. You'd have thought it was a
> mature
> >technology by then!
>
> How is it that AM transmitters use class C?
> I thought that would destroy the AM, or do they do that in front of
> a
> modulator??
>

       All current AM transmitters use high level modulation. The modulator, by
various means, just varies the power supply voltage to the final
amplifier. The old way of doing this was to just put the audio in series
with the plate voltage (generally using a modulating reactor or choke to
carry the DC with a modulation transformer capacitor coupled across the
modulation reactor). The modulator was just a big class B audio
amplifier. Newer transmitters use PWM to generate the varying power
supply voltage to the RF amplifier. They are now using multiphase PWM so
the ripple frequency is higher making it easier to filter out. The low
pass filter after the PWM modulator needs suppress the switching
frequency enough to meet the FCC bandwidth requirments (the switching
frequency causes sidebands at the switching frequency and its harmonics
away from the carrier). Attenuating the switching frequency while not
messing up the audio is a trick, especially with 50 to 100,000 watts of
DC plus audio. Phase linearity of the filter is also important since
standard audio processing clips the audio peaks to avoid overmodulation.
Nonlinear phase response turns these clipped peaks on their side, causing
more overmodulation.
       Several years ago (maybe 20), Harris Corporation patented a new AM
modulator called the Progressive Series Modulator. It was a very simple
circuit (two transistors and a diode), but no one had thought of it
before. In a standard series modulator, you have a pass element
(transistor or tube) through which the DC goes from the supply to the RF
amplifier. Varying this pass element varies the "DC" to the RF amplifier,
modulating it. However, a series modulator is very inefficient. With no
modulation, at least 50% of the power is dissipated by the modulator (the
other 50% going to the RF amplifier). With the progressive series
modulator, the two transistors are connected in series. A diode is
inserted between a 1/2 voltage supply and the junction of the two
transistors. With no modulation, the top transistor is off and the bottom
transistor is saturated, giving close to 100% efficiency. The bottom
transistor varies towards off to modulate down. The top transistor varies
towards on to modulate up. Very clever!  Still not as efficient as PWM,
but it made possible high level transmitters "with no iron" giving very
low distortion and wide bandwidth.  I think Harris manufactured one model
with this scheme, then went to PWM for everything.

Harold




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Lighting control for theatre and television at http://www.dovesystems.com

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2001\07\12@140632 by Peter L. Peres

picon face
> I always thought that class C meant using pulses to excite a tank
> circuit to resonance, and class D is some form of pulse modulation
> with the output signal derived from the pulses average.  Class D
> usually implies filtering to get rid of the high frequency pulses, but
> is intended to produce some arbitrary waveform, not to resonate a tank
> circuit.

In my experience class C amps do drive the output transistor as hard as
possible for less than 180 degrees of the input signal, but they have a
couple of obvious items that make them special, at least at a second look:

First, all class C amps have a loaded Q of at least 2 and often 10. This
means that the collector voltage becomes negative vs. the emitter (and
base) half of the time. Then, the current phase vs. voltage phase in a
loaded RLC resonant tank circuit does not allow usual Ton Toff etc
calculations. Only impedance and S parameter etc calculations make sense.
And last, the C21 and Miller capacitances of the transistor play a
significant role in the operation of the circuit, requiring
'neutrodynation' (ac positive feedback) in some applications.

The bottom line is, that getting some unknown transistors to work in class
C requires an inordinate amount of lab work with a spectrum analyzer,
especially to remove any 'birdies' and parametric oscillations that may
appear and to get harmonics under control. This has been my experience so
far. Also unspecified transistors will often try hard to oscillate by
themselves in this role (and self destruct if allowed to). All modern
designs I've seen move away from class C to class AB (for low harmonics)
or D (switching) using push-pull outputs and untuned balun transformers.
The only people who still use class C are probably amateurs and fixed
frequency stations (like remote control Txes and radio stations). The
reward for all the effort is low cost, simplicity, and good efficiency.

Peter

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2001\07\12@150236 by Peter L. Peres

picon face
> How is it that AM transmitters use class C?

Uh, afaik, the class C amplifier is THE reason for the infamous 30% AM
modulation usual in broadcast afaik. You simply modulate the plate DC with
audio and you get AM 30% (they could have chosen up to 80% AM without
problems otherwise). If you search the web for homebrew small AM
transmitters (favored by old radio collectors etc) you will find schemes
that exemplify this. The simplest scheme (which I've tried myself) can be
made from parts from a tiny AM radio.

You use the L.O. (complete with variable), substituting the ferrite
antenna for the AM LO tuning circuit (red core coil). You may have to
rewind the ferrite with a tapped coil - standard tap is 0% power 30%
collector 100% tuning cap - i.e. 25+55 turns on a normal ferrite antenna
that works with a 270pF tuning cap, using the original wire). Then you
wire the audio amp output secondary (which must be transformer type) in
series with the collector power of the LO, instead of the IF1 coil
(yellow) if it's a self-oscillating converter. Audio in is at the hot side
of the volume pot. That's it.

Please note that this may be slightly illegal in some countries. Do not
attempt to run high power into this, it will not work. Mine barely worked
across a room on the original 3V batteries. Man this was *years* ago.

Peter

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2001\07\12@152118 by Jim

flavicon
face
  "Uh, afaik, the class C amplifier is THE reason for the
    infamous 30% AM modulation usual in broadcast
   afaik. You simply modulate the plate DC with audio
   and you get AM 30% (they could have chosen up
   to 80% AM without problem otherwise"

Whoa!

Where do you come UP with this stuff Peter?

I've held a First Class Phone License now for over 25 years,
have worked in the field in the broadcast on the transmitter
end and this, as posted above, is simply NOT true!

A few minutes with an O-scope connected to the IF strip
in any AM radio will show you FAR in excess of "30 %
AM modulation".

Your previous observation on the usefulness of class C
amps is vastly understated too. Every 2-way FM land
mobile transceiver ever built over the last twenty years
has used a succession of class C stages to derive the
legal max power of 110 W for a mobile or 330W for a
base station. The handheld radios also use "class C PAs"
as well as a driver stage or two running class C service.

To utilize the PAs (power amplifiers) in amatuer service (as
in boosting the power output from a 20 W PEP SSB rig) from
any of these 'monster' transmiter amps requires adding
"forward bias" to get then to do class AB service ...

Jim  (RF design, test and analysis from DC through Ku Band,
        microwatts through 1000's of watts)


{Original Message removed}

2001\07\12@155446 by Olin Lathrop

face picon face
> Uh, afaik, the class C amplifier is THE reason for the infamous 30% AM
> modulation usual in broadcast afaik.

Could you elaborate on this?  I thought the legal AM modulation limit was,
sensibly enough, 100%.  The little class C transmitter I built only put out
a few watts, but could easily be driven to 100% modulation.  Where does the
30% come from?


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2001\07\13@031416 by Peter L. Peres

picon face
>> Uh, afaik, the class C amplifier is THE reason for the infamous 30% AM
>> modulation usual in broadcast afaik.
>
> Could you elaborate on this?  I thought the legal AM modulation limit
> was, sensibly enough, 100%.  The little class C transmitter I built
> only put out a few watts, but could easily be driven to 100%
> modulation.  Where does the 30% come from?

When you have (+/-) 30% voltage modulation the power changes between 15%
and 100% peak. (power on an indle AM Tx is 50% peak power afair).
Increasing the index more will bring very little range gain, it will also
reduce the modulation index reserve under 15% which means that very ugly
distortions will happen the moment the index reaches 100% (clipping +
inversion). Also the power on one half of each modulation waveform may be
so low that background unintended radiation will be heard and demodulated
(esp. adjacent channel etc).

In the days when receivers were made of a garnet rock and a sewing needle
on a wooden stand this made perfectly good sense.

The good side is, that if you have a class C transmitter with collector
(or plate) modulation you will only need an AC source that supplies 60%
(pk-pk = +/-30%) of the supply voltage in modulation, in series with the
plate/collector source. This makes it more affordable, especially if you
are moving towards higher powers.

Peter

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2001\07\13@040246 by Peter L. Peres

picon face
> Where do you come UP with this stuff Peter?

The books I have say that standard AM broadcast modulation is 30%. The
scope patterns (using XY and detector method) for transmitter adjustment
specify 30%.

The explanation we were given was, that this was the modulation level that
could be used without requiring exceedingly complex receivers in tube days
(and still sound good and quiet in breaks).

I haven't built and adjusted any AM transmitters lately though, just lab
equipment ;-). So if you know different I stand corrected. Oh by the way
that's 30% amplitude, yes ? (which is 85% power).

And I seem to remember distinctly that AM IF and AM RF is definitely a
stable sine wave with not-so-significant modulation on it (the peak
amplitude changes are about +/-30% of the shown peak-to-peak sine).

Peter

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2001\07\13@042129 by Alan B. Pearce

face picon face
>The books I have say that standard AM broadcast modulation is 30%. The
>scope patterns (using XY and detector method) for transmitter adjustment
>specify 30%.

>The explanation we were given was, that this was the modulation level that
>could be used without requiring exceedingly complex receivers in tube days
>(and still sound good and quiet in breaks).

I suspect this is more a case of allowing 100% modulation on music peaks
without the signal distorting, or overmodulating. around 30% average
modulation sounds about right to do this for broadcast use.

When it comes to radio telephone use there is often a limiter to clip signal
peaks in a controlled way so the average modulation level can be increased
significantly closer to 100% to get higher readability at the receiver under
low signal strength conditions.

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2001\07\13@043827 by Graeme Zimmer

flavicon
face
> When it comes to radio telephone use there is often a limiter to clip
signal
> peaks in a controlled way so the average modulation level can be increased
> significantly closer to 100% to get higher readability at the receiver
under
> low signal strength conditions.

Actually, more like 150%.

Most AM Coms TX's have heavy "upward modulation".

On positive peaks, the modulation extends well above 100%.

...................... Zim

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2001\07\13@102601 by Jim

flavicon
face
   "When you have (+/-) 30% voltage modulation
     the power changes between 15% and 100%
     peak. (power on an idle AM Tx is 50% peak
     power afair).

Peter - your recollection on this subject appears to be a
bit fuzzy and the resulting conclusions lead to implications
that, in practice, are simply not the case ...

I HIGHLY suggest you and others perform the afore-
mentioned procedure wherein I recommended that an
attachment of an O-scope be made to a radio receiver's
IF strip and a first hand observation be made of the voltage
waveform present there - a waveform which, I might add, is
one-to-one proportional to the voltage waveform both
transmitted and as applied to the base of the antenna from
the transmitter via the feedline.

What you will find is that nominal un-modulated carrier
occupies, say (after adjustment of the scope variable
attenuation control),  2  (TWO)  major scope graticule
divsions, and upon 100% modulation the waveform will
occupy a grand total of 4 (FOUR) divisions. Also note that
'downward' modulation approcahes zero graticule divisions ...

THIS IMPLIES that peak power is 4 (FOUR) times nominal
carrier power - not just a simple 100% increase as you seem
to imply. Remember - double the voltage and quadruple the
power ...

This is common mistake that all *newbies* to RF make
when interpreting observations on *voltage measuring*
test instrumentation and their subsequent attempt to make
inferences on associated corresponding 'power' levels.

Recall this applicable portion of Ohm's law: (E*E) / R = P ?

It figures into the PEP power calculation, which is necessary
when computing the PEP of the envelope of an AM station
on 100% upward modulation:

Given a load of known resistance, the PEP is calculated as
over a single RF (sinusoid) cycle as:

       PEP = Erms*Erms/R = Epk*Epk/R/2

This will yield 4 times the 'power' (PEP) over an unmodulated
carrier, not just double ...

Reference, Analysis of an AM wave:
          http://www.tpub.com/neets/book12/48l.htm

More references: http://www.tpub.com/neets/book12/

Jim


{Original Message removed}

2001\07\15@232831 by Peter L. Peres

picon face
Jim, I think that we have a case of my saying that the bottle is half
empty and you saying it is half full.

The way I learned it was, that transmitter peak power - 100% - is the
reference. This is achieved with 100% voltage. Idle (quiet) operating
point is 70% voltage which corresponds to 0.7*0.7 = 49% power. The minimum
power is another 30% voltage down from 70%, at 40% voltage, which
corresponds to 0.4*0.4 = 16% power.

This means that the pk-pk amplitude of the transmitted sine wave will vary
with modulation between 40% and 100%, with the average (when quiet) at
70%, and not more than that. Or, if you want to, 100% +/- 30% for another
way of referencing. This is also what you will see with a scope in the IF
strip of a receiver, and what I see with a scope in the IF strip of a
receiver. Note: NOT in a communications receiver.

I hope that I made myself clear enough that this is about broadcast
service, i.e. AM and SW radio stations you tune into using a commercial
receiver.

Peter

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