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Uni Project FM TRnasmitter

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by meeral20, Jul 24, 2009.

  1. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Well it certainly ISN'T class C :p
     
  2. marcbarker

    marcbarker New Member

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    Why do you say that?
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Because class C is biased past the point where the device is cut OFF completely - the example shown is biased ON a good amount.
     
  4. dave

    Dave New Member

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  5. marcbarker

    marcbarker New Member

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    So? Are you saying it's not oscillating in class C ?

    What about the negative gate voltage* ?, isn't that indicative of "class c" ?

    Don't forget this is an RF circuit! :)


    *the j-fet is only at maximum gain when the osc first starts up (class a). Start up begins by self-noise being amplified and fed back. Thereafter, the j-fet pinches itself off by the RF voltage it makes (& negative voltage for the Varicap). Because the j-fet only conducts during part of the oscillator cycle, is why it's Class C.
     
    Last edited: Jul 26, 2009
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    The 'class' is a static measurement, if it's in class C it can't start oscillating, as it passes no current whatsoever.
     
  7. marcbarker

    marcbarker New Member

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    So is it oscillating in class c or not? :)
     
    Last edited: Jul 27, 2009
  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    How can it?, class is a passive mode, not an active one - and the circuit isn't in class C or anywhere near it.

    You're just talking complete rubbish.
     
  9. marcbarker

    marcbarker New Member

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    How are you so sure of this? Have you ever built (or at least analysed) this circuit?
     
    Last edited: Jul 27, 2009
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    What's to build or analyse?, the circuit isn't biased in class C.

    You appear to be nothing but a troll, and keep repeating the same rubbish - I warn you now, I'll be keeping an eye on your future posts, and also looking back over your previous ones.
     
  11. marcbarker

    marcbarker New Member

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    Accusing me of a 'Troll' does not change the fact that it is a "Class C oscillator".

    How many times do I have to repeat myself?, this is a "Class C Oscillator". Because it is self-biased to cutoff, and is why the Varicap Diode gets it's reverse bias.

    If my word isn't good enough, try the University of Denver, Colorado. Electronics 16 Scroll down to Clapp Oscillator

    "The average gate voltage was about -4.5 V,<..> The JFET is operating as a Class C amplifier in this circuit. This makes an excellent RF oscillator "

    "class c oscillator" - Google Search

    the mechanical analogy of this oscillator is found in a mechanical (tick-tick) wristwatch: the 'hair-spring' / flywheel escapement.
     
    Last edited: Jul 26, 2009
  12. Hero999

    Hero999 Banned

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    Whether you're right or not doesn't matter, just try not to come across as being cocky and smug.

    I can picture you laughing out loud as you read this post but I'm being serious.
     
  13. marcbarker

    marcbarker New Member

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    see how the majority were wrong at the beginning of this thread, each condemming a circuit posted: http://www.electro-tech-online.com/threads/uni-project-fm-trnasmitter.95651/#post769042

    How many times did I need to say: the varicap gets it's reverse bias

    Not even an apology, I get accused of being a 'troll', 'talking rubbish', and now 'cocky and smug'! :)

    I thought the idea of this Forum was to encourage Design Ideas and Reviews?, not gang up on those who don't agree with the majority, who were wrong anyway

    Supposing I did not speak up?, how long would it of taken that majority to realise they were wrong ?
     
    Last edited: Jul 26, 2009
  14. mneary

    mneary New Member

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    I did simulate this circuit. For all values I tried, the transistor draws current for the full cycle for just the first few cycles of oscillation.

    Thereafter, the transistor is biased deep into cutoff.

    I thought that circuits were classified by their operation after they stabilized, and with signals applied. If this is wrong, I would gratefully accept enlightenment.
     
  15. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I don't see how it could be, the 'class' is dependent on the biasing, which is a static condition. If it's different statically and dynamically, how can you decide what 'class' it might be in.

    A class C amplifer, dynamically, will change class continuously from class C, to class B, to class A - which is why you can only measure the class statically.
     
  16. marcbarker

    marcbarker New Member

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    Where does the reverse-bias for the varicap come from if it's not self-biased in Class C?

    1942 definition of Class C: "The Class C Amplifier is an amplifier which the grid bias voltage are such that the Plate current flows for considerally less the 180 degrees of cycle" http://www.electro-tech-online.com/custompdfs/2009/07/0017.pdf

    Anyway, if it wasn't operating in Class C, the fet would get hot, probably fail. And the varicap wouldn't be reverse-biased.

    If it took a 'troll' to notice one person after another condemn the OP circuit as 'wrong', maybe we need more 'trolls' on here? :)

    EDIT 30/07/09: I can see now where Nigel was coming from. I did not realise this at the time, we were divided by how the circuit Class is defined: Is 'Class' applicable to the Passive or Active condition? There probably is no definative answer but conform to a consensus of opinion.
     
    Last edited: Jul 29, 2009
  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    What has class C got to do with it?.

    Sounds a fine definition to me, and proves that the oscillator isn't class C.

    Class A amplifers don't usually get hot, which almost all (if not all) sinewave oscillators are - you certainly don't want them hot, for stability reasons.

    The oscillation of the FET 'may' alter it's bias conditions somewhat, but it's not going to be class C as it doesn't meet your 1942 definition, or my more universal one.

    You haven't done anything yet to prove it's a 'good' circuit, which it probably isn't anyway.
     
  18. JimB

    JimB Super Moderator Most Helpful Member

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    Osillator circuits like this one, whether built with a JFET or a good old triode valve, do develop a negative bias on the gate/grid which controls the amplitude of the oscillation.
    The 1N4148 is there to conduct on positive half cycles allowing the negative half cycles to bias the transistor off.
    The diode was not necessary with valves because grid current would flow when the grid voltage was positive.

    Back in the days when all I had by way of test equipment was an AVO model 7 and a general coverage shortwave receiver, the quick and dirty way of finding out whether the oscillator I had just built was oscillating or not was to short the grid pin to chassis while monitoring the anode current.
    If the anode current increased when the short was in place, I had made an oscillator, well done!
    Now tune the receiver to find the frequency.

    Whether the oscillator is running on class C, maybe it is, but to quote the man in the film "Franky my dear, I dont give a damn!"

    Going back to the circuit in post 1 of this thread, where the varicap bias is provided by the oscillator gate bias, my own view is that it is a clever wheeze to minimise the number of components. However it is not the basis of a well engineered repeatable design.
    But for the kiddies noise toy, it is probably OK.

    JimB
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I am amazed that the JFET oscillator oscillates in class-C.
    But I also don't care a damn because I will never make a JFET RF oscillator with vari-cap FM modulation.
     
  20. marcbarker

    marcbarker New Member

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    Advantanges: Superior frequency stability. Less phase noise. Very stable RF amplitude due to the self-regulating Class C bias. Superior to a Colpitts oscillator for VFO applications, because the self-bias allows accommodation of frequency-dependent loop gain. Superior to Hartley oscillator because less spurious frequencies, because single Inductor. Reliable. Simple to construct. This is a favoured choice for signal generators and transmitters.

    Because this thread has been primarily about: "where does the reverse-bias for the varicap come from, if it's not self-biased in Class C?"

    Definition of Class C agreed. Good. Found some common ground :)

    Denying the evidence of Class C operation doesn't make it 'go away'....

    Exhibit 1: Id=0.6 mA = Class C
    Exhibit 2: Reports of Negative Gate voltage = Class C
    Exhibit 3: Data from Denver University Electronics 16 = Class C

    How much more evidence of Class C is required?

    Class C Oscillators "class c oscillator" - Google Search operate in Class C, and this is what makes them run colder than Class A oscillators. Because Class C means: 'biased beyond cutoff', is where the varicap gets its reverse bias.

    Here is some analysis of this Clapp "Class C oscillator":
    http://www.electro-tech-online.com/custompdfs/2009/07/322Lecture24.pdf

    Conclusion: The OP circuit contains an example of an oscillator which stabilises in Class C. For a few cycles during start-up, the circuit operates in Class A. Stable Class C operation in oscillators provides advantages over Class-A biased oscillators.

    And of course... because the j-fet is biased beyond cutoff, means a negative voltage is present on the gate. It's enough to bias a varicap, connected with 'pointed end' to ground. [edit]

    That's it, I've had quite enough of this 'Class C' and 'varicap bias' thread now.
     
    Last edited: Jul 27, 2009
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The FET should be a bipolar transistor, the circuit should be Colpitts and the vari-cap diode should have a variable reverse DC bias for tuning the transmitter.
    The FM transmitter should have a voltage regulator and have pre-emphasis.
     
    Last edited: Jul 27, 2009

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