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CLapp LC oscillation

Discussion in 'Electronic Design' started by jason, Mar 19, 2005.

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  1. jason

    jason Guest

    Hello All

    I am studying about Clapp LC oscillator.
    Given below link is the circuit of interests

    May I know how do we calculate by hand if this circuit gonna oscillate
    or not?

    I learnt in lecture that I must find Av multiply by beta(feedback
    factor). Av is always given by gm multiply by R efficient , which is
    Reff is the sum of series and parallel resistances of all emitter
    resistance, ac emitter resistance , load resistance and also reflected
    resistances from the tap of capacitors. Emitter is mentioned here
    because I learnt about bipolar transistor configuration.

    For the circuit in the link given, I do not understand why the same
    approach is use?
    Why we can use gmReff as the Av since it is a common drain
    configuration? Is it because we have already sum all the net
    resistances in Reff , therefore we can use gmReff as the Av?

    So for other type of configuration of mosfet such as common gate and
    common source? We can use the same Av=gmReff ?
    If possible show me the correct way of checking if the circuit can
    oscillate by manual least the steps or methodology.
    Really appreciate your help in advance.

    Kindly enlighthen

    rgds and thanks
  2. Don Pearce

    Don Pearce Guest

    First can you tell us the purpose of C1, C2 R5 and R6. The capacitors
    are unnecessary, and the resistors will degrade the quality of the


    Pearce Consulting
  3. jason

    jason Guest

    Dear Don

    Thanks for the response.
    C1 and C2 should be for coupling purposes. Then R5 and R6 should be for
    high input and output impedance to and from the amplifier.
    That's my guess. If wrong, kindly correct me.

    My main question is why my lecturer can use Av=gmReff for both the case
    of bipolar transistor and also mosfet for different
    configuration(common gate, common drain, common source, EC, CB or CC)

    Kindly enlighthen please

    rgds and thanks
  4. In oscillator circuits, it's not correct to use the terms 'common
    drain', common gate, common source'. This is because the apparent
    'common' electrode is determined by the *external* circuit, usually
    where the earth/ground connection is made, and that doesn't affect the
    conditions for oscillation.
    Yes, if you choose to take for the 'input' and 'output' of the
    amplifier, the points in the circuit which makes the equation true.
    An oscillator circuit is a loop, and you can break the loop at *any*
    point in order to determine the loop gain. But the calculation is easier
    for some places in the loop than for others.
  5. There is no DC path through the Clapp network, so you don't need C1 and
    You don't need them either. The three capacitors in the Clapp network
    are not only for tuning; they are also for impedance transformation. If
    the inductor were tapped at two places, you would see it as a
    transformer. Well, you can do (almost) exactly the same by connecting
    two or more capacitors (three in the Clapp circuit) in series across an
    untapped inductor and taking taps off between the capacitors.
  6. jason

    jason Guest

    Dear John

    Thank you so much. You have taught so much I will never have in class
    But can you further explain about the followings?
    I do not understand it

    <<In oscillator circuits, it's not correct to use the terms 'common
    drain', common gate, common source'. This is because the apparent
    'common' electrode is determined by the *external* circuit, usually
    where the earth/ground connection is made, and that doesn't affect the
    conditions for oscillation. >>

    The common gate drain or source is refer to the amplifier stage(the
    simple mosfet in my case). Is that incorrect? Or you mean amplifer is
    the external circuit?

    <<Yes, if you choose to take for the 'input' and 'output' of the
    amplifier, the points in the circuit which makes the equation true. >>

    The main problem is I do not know which is the point of the "input" and
    "output" that makes the equation true.
    Can you explain about this for the picture I sent in the link?
    Or if you have any online document thats explains this well and comes
    with example.

    Kindly enlighthen
    Thank you all

  7. jason

    jason Guest

    Dear All

    Please help in this enquiry. Kindly explain in details as I am not IC
    designer but a student taking IC design course. I am new in this.
    Kindly explain in details if possible.
    I know I am bold and ashame for asking this but I hope you can give me
    a helping hand. Please share me me if u have any good online material
    on this topic
    Thank you so much in advance
    My apology for any inconveniences caused
    Thank you

  8. Reg Edwards

    Reg Edwards Guest

    All LC and crystal oscillators are the same.

    Tubes or transistors

    They all behave in the same way.

    Changing the point of the ground connection has no electrical effect.
    It merely changes the name.

    The only way to understand how they work is to study the simple
    elementary arithmetic involved. Otherwise one must suffer in

    Unfortunately, in the Western World's schools, the standards of
    education in elementrary arithmetic during the last 30 years has
    fallen to abysmally low levels.

    I once met a prospective member of parliament, no doubt with visions
    of becoming the Chancellor of the Exchequer, who was unable to
    calculate that 7 times 9 equals 63. I'm pleased to report he
    forfeited his £500 deposit.

    No chance of HIM ever understanding a Clapp or a Collpits or a
  9. Fabio G.

    Fabio G. Guest

    Linearize it with the small signal analysis and write the expression of
    the loop gain in the Laplace domain.

    If a frequency in which the phase shift is zero exists, the circuit may
    If at that frequency the loop gain is >= 1, the circuit will oscillate.
    This is the basic methodology to analyze oscillators.

  10. Jim Thompson

    Jim Thompson Guest

    I think that's a necessary condition for oscillation, but not a
    sufficient condition for starting. Probably requires application of
    one of Lyopanov's tests.

    ...Jim Thompson
  11. Genome

    Genome Guest

    Hi Cheang Long, can I take a guess that you are Chinese?
    Absolutely no offence meant. It's just for the next bit....
    Perhaps, if you are Chinese you should be referring to it as the Crapp
    Oscillator. Go on, smile.....?

    I know nothing about them either but I was so intrigued that I thought I'd
    have a go.

    Mind you, you have to promise that your real name is not Paul Burridge....
    that's an 'in' joke by the way.

    I notice, from your website schematic that you are using LTspice. That is
    way cool because I do too. I am the most excellent person on the planet so
    you can't be too bad yourself.
    Well, if I figure it out I'll tell you.
    That sounds about right or rather, that's the sort of stuff the old bloke at
    the front of the room has being burbling on about for so long that he missed
    the point in the first place and isn't very creative sexually. I guess he
    doesn't excite students either.
    But, you don't want to do that.....

    What you want to do is a loop analysis that runs around things from the
    input back to the input (the loop) without quite getting there. Scary stuff
    but that's the way it is.... Anything else is chips cooked in old vegetable
    Ah but,

    First you have to look at your circuit and figure out what it thinks it's
    doing. I am anally retentive so I like pretty circuit diagrams.

    Since you have LTspice you will be able to do something with this.....I have

    Select the text. Copy it to the clipboard. Open Notepad and paste the
    results into it. Save the file with a suitable name with a .asc extension
    instead of .txt

    Version 4
    SHEET 1 880 680
    WIRE 496 80 528 80
    WIRE 528 496 336 496
    WIRE -32 496 -32 464
    WIRE -32 272 -32 208
    WIRE -32 128 -32 80
    WIRE -32 80 336 80
    WIRE -32 80 -160 80
    WIRE -160 80 -160 400
    WIRE -160 464 -160 496
    WIRE -160 496 -32 496
    WIRE -160 0 368 0
    WIRE 368 0 368 80
    WIRE 368 80 416 80
    WIRE -160 528 -160 496
    WIRE 528 304 528 80
    WIRE 528 384 528 496
    WIRE -32 336 -32 368
    WIRE -32 368 112 368
    WIRE -32 368 -32 400
    WIRE 336 496 112 496
    WIRE 336 304 336 80
    WIRE 336 80 368 80
    WIRE 336 400 336 496
    WIRE 112 400 112 368
    WIRE 112 368 256 368
    WIRE 112 480 112 496
    WIRE 112 496 -32 496
    WIRE 256 368 256 32
    WIRE 256 368 288 368
    WIRE 256 32 -160 32
    FLAG -160 0 VD
    IOPIN -160 0 Out
    FLAG -160 528 0
    FLAG -160 32 VG
    IOPIN -160 32 Out
    SYMBOL res 512 64 R90
    WINDOW 0 -27 62 VBottom 0
    WINDOW 3 -30 61 VTop 0
    SYMATTR InstName R1
    SYMATTR Value 1K
    SYMBOL ind -48 112 R0
    WINDOW 0 42 45 Left 0
    WINDOW 3 41 69 Left 0
    SYMATTR InstName L1
    SYMATTR Value 100µ
    SYMBOL cap -176 400 R0
    WINDOW 0 45 23 Left 0
    WINDOW 3 45 45 Left 0
    SYMATTR InstName C1
    SYMATTR Value 220p
    SYMBOL cap -48 272 R0
    WINDOW 0 40 24 Left 0
    WINDOW 3 41 47 Left 0
    SYMATTR InstName C2
    SYMATTR Value 1n
    SYMBOL cap -48 400 R0
    WINDOW 0 47 23 Left 0
    WINDOW 3 48 47 Left 0
    SYMATTR InstName C3
    SYMATTR Value 1n
    SYMBOL voltage 528 288 R0
    WINDOW 0 40 43 Left 0
    WINDOW 3 46 67 Left 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName VDD
    SYMATTR Value 20V
    SYMBOL njf 288 304 R0
    WINDOW 0 56 37 Left 0
    WINDOW 3 56 62 Left 0
    SYMATTR InstName J1
    SYMATTR Value 2N3819
    SYMBOL res 96 384 R0
    WINDOW 0 39 43 Left 0
    WINDOW 3 39 63 Left 0
    SYMATTR InstName R2
    SYMATTR Value 1E6
    TEXT -168 576 Left 0 !.tran 0 200u 0 uic

    And you should be able to open it up in LTSpice and run it.
    Now, once you have seen MY circuit oscillate you should begin to realise
    that I have an Humungeously (JW) Enormous Penis and I am Gods gift to

    OK, I was lucky.

    I knew a bit and fiddled a bit and it didn't work. So I thought something
    else and took a guess and it had a go. So I changed something else and off
    it went. And that reminded me of something else.

    What I will say for now is L1 and C1 form a parallel resonant network.
    You may beat me up later.

    Go on, write me the impedance of a parallel LC network. I'm not being a
    bastard here, if you can't do it then you haven't been taught it....

    I've been taught it but, in the existensial sense.... it doesn't (make

    And yes, I am making this up as I go along. I apologise for my liberal use
    of English thingummyjigs.

  12. Reg Edwards

    Reg Edwards Guest


    Touch an RF point with a damp finger. If it stops oscillating then it
    shows it's working and there's not much to worry about.


    I do wish you guys would stop referring to Laplace. It's an indication
    you are not fully aware of what you are waffling about.

    It was Heaviside's revolutionary mathematics, 100 years later than
    Laplace, which solved the trivial yet-to-appear problem of how
    oscillators work - and a great number of other fundamental, physical,
    hitherto insoluable engineering problems.

    It was Laplace's relatively trivial piece of work, of not much
    consequence at the time, which by chance provided the link between
    Heaviside's masterpiece and the rest of the World of Mathematics. It
    was the missing link, discovered more or less by accident by the early
    practical communications engineers, needed to convince the university
    academics and blockheads that the self-educated Heaviside's p and D
    gave the right answers.

    But it was the blockheads, confidence tricksters, obtaining money
    under false pretences, not the engineers, who wrote the subsequent
    text books. And so Laplace's s highjacked Heaviside's p and D.

    Laplace, honoured, of the Napoleonic era, who survived Madam
    Guilotine, is amongst the world's greatest mathematicians. No doubt an
    honest man. He must be turning over in his grave at the injustice.

    Heaviside, a self-educated young genius, died with little recognition
    of the fundamental mathematical and engineering importance of his work
    after many years as a lonely recluse in 1925 aged 75.

    He will be remembered for his mathematical prediction of "The
    Heaviside Ionospheric Layer" via which world-wide HF communications
    were to blossom a few years afterwards.

    Quotation (to the blockhead university professors) : "Shall I refuse
    to eat my dinner because I do not understand the processes of
    digestion? "
  13. jason

    jason Guest

    I would like to thank people who gives constructive ideas. Thank you
  14. Robert Baer

    Robert Baer Guest

    C10 and C14 do the DC blocking, so C1 and C2 are redundant.
    R5 *decreases* the coupling (drive) to the LC network, thereby
    increasing the feedback loss.
    R6 is non-functional unless the value approaches 10% of R1 or more -
    in which case it also would increase the feedback loss *and* add a phase
    shift that can be undesirable.
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