Class C and AM - Are they compatible?

Discussion in 'Wireless & RF Design' started by jamesinnewcastle, Nov 5, 2009.

  1. jamesinnewcastle

    Thread Starter New Member

    Nov 5, 2009
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    Hi

    My first post and I am not experienced in RF at all - please be gentle!

    Trying to understand how a Class C RF amplifier can produce an AM signal!

    As I understand so far the Class C RF amplifier will only faithfully 'reproduce' the top 'bit' of the input waveform (always shown as a sine wave I notice). The next bit of the explanation then details how this 'top bit' causes a tuned load to 'resonate' thereby restoring the signal. Like striking a bell.

    OK so you reconsitute a sine wave. Got that. Great if you just want a sine wave, say for morse code. BUT where does the modulation envelope of say my voice come into it?

    If you modulate the amplitude of the 'top bit' with a voice signal you surely have simply reproduced a Class A Amplifier!

    My question has resulted from a heated 'discussion' where I work as one party concludes that a Class C power amp cannot be used to boost a CB radio as it can't pass AM signals.

    Can anyone shed any light?

    James
     
  2. wr8y

    Active Member

    Sep 16, 2008
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    Correct. USING a Class C amp to "boost" an AM or SSB transmission is not going to work.

    However, MODULATING a class C amp is another discussion.

    I hate it when I see a thread like this JUST as I am headed out on a service call...
     
  3. aj_silverthunder

    Active Member

    Jun 6, 2009
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    yes,C-class amplifier is not suitable with AM,but its very good for FM transmitters.:)
     
  4. wr8y

    Active Member

    Sep 16, 2008
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    You might have fun reading this:
    http://www.w8ji.com/Vacuum_tube_amps.htm
    Some of his other links are good, too. It does not directly answer your questions, but makes good foundational reading.
     
  5. k7elp60

    Senior Member

    Nov 4, 2008
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    In a class C amplifier that is modulated by an AM signal the output of the carrrier wave will vary in amplitude with the modulating frequency. Depending upon the modulation techique the carrier frequency output amplitude my go to zero.
    Since a class C amplifier does not conduct for a full cycle, the tuned circuit on the output of the amplifer produces a full 360 degree wave. It is said that the AM rides on the RF. If you look at the output on an oscilloscope the RF carrier wave will be varying in amplitude with the AM frequency.
     
  6. jamesinnewcastle

    Thread Starter New Member

    Nov 5, 2009
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    OK - Here is a follow-up question.

    AM requires a carrier and a modulation frequency. When do you combine the two?

    Do you:

    1. Mix immediately at low level - then pass the signal through many stages of amplification - From what I can see you would not be able to use Class C for any of these stages or you lose the modulation - or distort it to a point of uselessness.

    2. Amplify the carrier only to the final aerial power (using Class C circuitry) and use a modification to the final stage to allow the modulating signal to control the output of the transmitter.

    (Naturally there are options in between, but bear in mind the original question)

    Also I have found many CB amplifiers that use a single Class C stage to 'boost' the output from a standard CB radio - how can that be possible if Class C can't pass AM?


    James
     
  7. ELECTRONERD

    Senior Member

    May 26, 2009
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    Usually you have a mixer whose inputs are from an audio signal (amplified of couse) and also an oscillator that sets the carrier frequency. That in turn goes to the RF amplifier. Below you'll find a block diagram, this version includes filtering and an I.F amp. The mixer is the cirle with the big "X" and notice that the oscillator and audio go into the inputs. Thus, you have a modulated signal. So you would use your Class-C amp for the audio and input that to the mixer and also the oscillator to set the carrier.

    [​IMG]

    Austin
     
  8. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Indeed. In fact to GENERATE an A.M. signal you MUST have a class C amplifier. The vast majority of A.M. broadcast transmitters use high level modulated Class C amplifiers.

    However, once you've generated an A.M. signal, you must use a LINEAR amplfiier for any subsequent amplification. This method is known as low-level modulation, and is used when high efficiency is not a real priority.


    Eric
     
  9. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    If you have a perfectly linear amplifier and inject an R.f. carrier and an audio signal, the result will be a simple algebraic sum of the two signals. There will be no modulation at all. Each input signal will retain its original identity. To generate the necessary sidebands, you must use a MULTIPLICATION of the two input signals, which a Class C amplifier does nicely.

    As you can see, a high levell modulated A.M. transmitter needs HUGE amounts of audio power. This is why broadcast transmitters have heavy iron transformers larger than anything you're likely to see in any other branch of electronics!

    In recent years, most modulation sections have been replaced with switching type audio modulators....similar to a switching regulator...but modulated at an audio frequency. This has eliminated much of the heavy iron. For many decades the standard A.M. modulation section used push-pull class B or class AB amplifiers.


    Eric
     
  10. jamesinnewcastle

    Thread Starter New Member

    Nov 5, 2009
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    KL7AJ

    Good point about multiplication - hadn't considered that.

    I've been doodling with transfer curves and I can see how a Class C amp with a tuned load could in fact amplify in a pseudo linear manner.

    Tha answer or 'trick' lies in the shape of the modulation envelope which I think K7elp60 was saying. As the envelope is effectively the modulating signal symmetrically arranged on the 'top' and the 'bottom' of the envelope you can effectively 'look' like a Class A amp. I've done a sketch.

    James

    contd......
     
  11. jamesinnewcastle

    Thread Starter New Member

    Nov 5, 2009
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    OK - But the output isn't exactly a classic 'modulated' AM signal, but it's amplitude does change in accordance with the modulation.

    Now if that signal is applied to a tuned load would that load not 'ring' in such a way that the classic AM signal is restored?

    There is a condition. As the Class C transfer curve does not let the modulation drop to zero the peak to peak of the modulating signal must not cause 100% AM modulation - I think that there is a term for this but I can't remember. (You can see some parts of the envelope that have been clipped in the example).

    How does that stack up? I think I've argued myself in a circle!!

    James
     
    Last edited: Nov 6, 2009
  12. jamesinnewcastle

    Thread Starter New Member

    Nov 5, 2009
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    In looking for any guidance on this whole subject what I find is lots of people talking about how 'kicking' the tuned circuit brings about reformed sine waves and how the harmonics can be reduced. But no-one addresses or shows how the AM envelope is maintained.

    Equally, discussions about Class B talk about 180 degree conduction and Class C being about less - BUT in the circuits shown I don't see any active bias present in the Class C examples (well just one, and that was by way of explanation!). Class C amps just seem to use the 'natural' bias that the amplifying devices have, ie transistors 0.6V needing to be applied before conduction starts. Sort of pseudo Class C or Class C by default.

    James
     
  13. wr8y

    Active Member

    Sep 16, 2008
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    You aren't far from the truth! If you REALLY want to understand all of this, you have some reading to do.

    I suggest going to eBay and picking up a copy of "The ARRL Radio Amateur's Handbook" or Bill Orr's Radio Handbook (a better choice, IMO - but out of print as he's gone!) Don't worry about getting THIS YEAR's edition. You will save a lot of money if you buy one a few years old.
     
  14. bertus

    Administrator

    Apr 5, 2008
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  15. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Yep, those are all classics. Also, R..L. Shrader's Electronic Comminucations is very good.


    I might add that in conventional high level A.M., you don't even consider any possible linear function of the final amplifier. The amplifier is always operated in a fully saturated state...that is, the r.f. drive signal is high enough that any INCREASE in drive causes no increase in output power. Any "identity"of the R.F. driving signal is totally lost....it is merely a "clock".

    A good high-efficiency A.M. broadcast transmitter runs close to 90% efficiency in the final stage, which means the grid is only conducting for 10% of the cycle...but during that 10% it's driven HARD...approaching a true switching mode.

    In a properly operating A.M. transmitter, the R.F. output power is always the exact square of the applied plate voltage.


    Eric
     
  16. jamesinnewcastle

    Thread Starter New Member

    Nov 5, 2009
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    Hmmm, where does the AM part come into play? How does the carrier level rise and fall in order to achieve amplitude modulation? Your description gives the impression of a screaming transmitter which has forgotten that it is supposed to modulate - This would be great for morse-code, but it doesn't seem to be amplitude controlled in any way?


    I've downloaded Orrs books, thanks guys, and there is a lot of stuff in there to read, perhaps the bits that answer my questions - lots on AM and some other great stuff as well.


    James
     
  17. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Hi James:

    You modulate the D.C. supply voltage to the final amplifier. This is what gives it the square law multiplication.

    By the way, all the modulation power is put into the sidebands. If you have a very narrow passband filter centered precisely at the carrier frequency, the power level stays the same. (The FCC has very stringent rules on "carrier shift", which require the average carrier power to remain within a couple of percent of licensed power with or without modulation)

    eric
     
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