• How 3-capacitor sine generator works =?UTF-8?B?cmVhbGx5Pw==?=

    From RodionGork@21:1/5 to All on Fri May 3 06:14:54 2024
    Hi Friends!

    Schematic / simulation in "Falstad online simulator": https://tinyurl.com/23hcg8np

    This is probably very old and widely known schematic of single-transistor generator which
    requires no inductance, but instead uses three capacitors - actually it
    seems a chain
    of high-pass single-stage filters with transistor serving as feedback from output to input.

    One can find it, for example, in classic stylophone schematic (the part creating low-frequency
    oscillations for "vibratto" effect).

    I teach it to my pupils for years probably and I always thought I less or
    more understood what
    is happening inside - each filter stage gives shift in phase and hence when amplifying feedback
    is added there happen harmonic oscillations.

    However on the schematic given above I added 4 scopes over the length of
    the filter (potentials
    at the points A, B, C, D according to labels - here A and B are potentials
    at points between capacitors, C is at the base and D at collector) - I
    suddenly found that intermediate voltages are
    not pretty harmonic! They could be distorted by the current drawn into transistor base though. And
    I'm not sure the output is exactly sine now. Though probably it is a matter
    of adding some resistor to improve input impedance of transistor cascade?

    Regretfully I can't find any thorough explanation of the schematic
    (probably due to keywords being too general and I don't know if this design
    has fancy proper name). So I would be grateful either
    for links or for verbal clarifications.

    --
    to email me substitute github with gmail please

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  • From Bill Sloman@21:1/5 to RodionGork on Fri May 3 16:58:50 2024
    On 3/05/2024 4:14 pm, RodionGork wrote:
    Hi Friends!

    Schematic / simulation in "Falstad online simulator": https://tinyurl.com/23hcg8np

    This is probably very old and widely known schematic of single-transistor generator which requires no inductance, but instead uses three capacitors - actually it seems to be a chain
    of high-pass single-stage filters with transistor serving as feedback from output to input.

    One can find it, for example, in classic stylophone schematic (the part creating low-frequency
    oscillations for "vibratto" effect).

    I teach it to my pupils for years probably and I always thought I less or more understood what is happening inside - each filter stage gives shift in phase and hence when
    amplifying feedback is added you get harmonic oscillations.

    However on the schematic given above I added 4 scopes over the length of
    the filter (potentials
    at the points A, B, C, D according to labels - here A and B are potentials
    at points between capacitors, C is at the base and D at collector) - I suddenly found that intermediate voltages are
    not pretty harmonic! They could be distorted by the current drawn into transistor base though. And
    I'm not sure the output is exactly sine now. Though probably it is a matter of adding some resistor  to improve input impedance of transistor cascade?

    Regretfully I can't find any thorough explanation of the schematic
    (probably due to keywords being too general and I don't know if this design has fancy proper name). So I would be grateful either
    for links or for verbal clarifications.


    It's a phase shift oscillator - one of many.

    https://en.wikipedia.org/wiki/Phase-shift_oscillator

    The amplitude is limited by clipping in the single transistor amplifier.
    If you model it with LTSpice, you can get the program to produce a
    Fourier transform of the output waveform and it is going to have all the harmonics out to the cut-off frequency of the transistor.

    You can do better, but it takes more components.

    Here's a solution I came up with back in 1986, developed as a retrofit
    to excite a linear variable differential transformer used to measure the progressively increasing mass of a single crystal of gallium arsenide
    (GaAs) being grown in the Metals Research GaAs Liquid-Encapsulated
    Czochralski (LEC) crystal puller. The circuit it replaced had been
    developed a decade earlier and used components that had become obsolete
    in 1986. The new circuit replaced it in new machines and was retrofitted
    to some older machines.

    Only about 50% of the power fed into the oscillator ends up in the load,
    rather than the better than 90% transfer you can get with a classic
    Class-D oscillator – but it’s quite a lot more efficient than any of the low distortion oscillators I know about, and it lends itself to very
    precise control of the output amplitude. I’ve generated quite a few
    Spice models of various implementations of the idea, but I’ve yet to get around to building a current version of the real circuit – the 1986
    version worked fine, but at that time I wasn’t aware how good the
    circuit could be and didn’t have any reason to check out its performance
    in detail.

    Here's a proof-of-principle simulation - which doesn't have anything in
    common with the 1986 circuit.

    http://sophia-electronica.com/BillsBaxandall.html

    --
    Bill Sloman, Sydney

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  • From Jan Panteltje@21:1/5 to rodiongork@github.com on Fri May 3 07:51:23 2024
    On a sunny day (Fri, 3 May 2024 06:14:54 +0000) it happened RodionGork <rodiongork@github.com> wrote in <804485177e123558699ce859cc4d496a@www.novabbs.com>:

    Hi Friends!

    Schematic / simulation in "Falstad online simulator": >https://tinyurl.com/23hcg8np

    This is probably very old and widely known schematic of single-transistor >generator which
    requires no inductance, but instead uses three capacitors - actually it
    seems a chain
    of high-pass single-stage filters with transistor serving as feedback from >output to input.

    One can find it, for example, in classic stylophone schematic (the part >creating low-frequency
    oscillations for "vibratto" effect).

    I teach it to my pupils for years probably and I always thought I less or >more understood what
    is happening inside - each filter stage gives shift in phase and hence when >amplifying feedback
    is added there happen harmonic oscillations.

    However on the schematic given above I added 4 scopes over the length of
    the filter (potentials
    at the points A, B, C, D according to labels - here A and B are potentials
    at points between capacitors, C is at the base and D at collector) - I >suddenly found that intermediate voltages are
    not pretty harmonic! They could be distorted by the current drawn into >transistor base though. And
    I'm not sure the output is exactly sine now. Though probably it is a matter >of adding some resistor to improve input impedance of transistor cascade?

    Regretfully I can't find any thorough explanation of the schematic
    (probably due to keywords being too general and I don't know if this design >has fancy proper name). So I would be grateful either
    for links or for verbal clarifications.

    For oscillation you need to put the output back in phase to the input.
    The tansistor gives 180 degrees phese shift
    (when base goes up the collector goes down)
    the RC networks that follow give together an other 180 degrees at some specific frequency.
    so at the base now the feedback is in phase
    and as gain is >1 it wil oscillate at that frequency set by the R and C values.

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  • From John Larkin@21:1/5 to All on Fri May 3 07:43:36 2024
    On Fri, 3 May 2024 06:14:54 +0000, RodionGork <rodiongork@github.com>
    wrote:

    Hi Friends!

    Schematic / simulation in "Falstad online simulator": >https://tinyurl.com/23hcg8np

    This is probably very old and widely known schematic of single-transistor >generator which
    requires no inductance, but instead uses three capacitors - actually it
    seems a chain
    of high-pass single-stage filters with transistor serving as feedback from >output to input.

    One can find it, for example, in classic stylophone schematic (the part >creating low-frequency
    oscillations for "vibratto" effect).

    I teach it to my pupils for years probably and I always thought I less or >more understood what
    is happening inside - each filter stage gives shift in phase and hence when >amplifying feedback
    is added there happen harmonic oscillations.

    However on the schematic given above I added 4 scopes over the length of
    the filter (potentials
    at the points A, B, C, D according to labels - here A and B are potentials
    at points between capacitors, C is at the base and D at collector) - I >suddenly found that intermediate voltages are
    not pretty harmonic! They could be distorted by the current drawn into >transistor base though. And
    I'm not sure the output is exactly sine now. Though probably it is a matter >of adding some resistor to improve input impedance of transistor cascade?

    Regretfully I can't find any thorough explanation of the schematic
    (probably due to keywords being too general and I don't know if this design >has fancy proper name). So I would be grateful either
    for links or for verbal clarifications.

    That sim makes a suspiciously nice sine wave, for a phase-shift
    oscillator. The 100K base resistor was probably selected to match the
    beta of the transistor, and if so it wouldn't be as good in
    production, where betas vary.

    If that resistor is too big or too small, it won't oscillate. Try
    varying it.

    There is some AGC effect from base rectification biasing the transisor
    off, which increases beta tolerance.

    The lesson for your students is more general: the amplitide of a
    linear oscillator increases exponentially until something nonlinear
    kicks in to reduce the overall gain to unity. The nonlinearity makes distortion.

    Your phase shifter is three differentiators, so magnifies harmonics.
    Another phase-shift osc form uses three RC integrators, so can
    attenuate harmonics and make a better sine.

    Another lesson for students is that a hand-selected set of values may
    not be a reproducible, sellable circuit.

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  • From RodionGork@21:1/5 to All on Sat May 4 21:00:23 2024
    It's a phase shift oscillator - one of many.

    Thanks a lot, so the "keywords" were on the surface, I just missed. It
    could be googled by
    phase shift or even by RC-oscillator. Great!

    Here's a solution I came up with back in 1986

    Wow, thanks for curious story. I haven't yet went to school then :) Golden
    age of electronics!

    For oscillation you need to put the output back in phase to the input.
    The tansistor gives 180 degrees phese shift

    Thanks, I see I was mistaken thinking that each stage "shifts" phase by 90 degrees (obviously I forgotten university lectures) and that confused me.

    and as gain is >1 it wil oscillate

    supposedly, it is about overall gain - transistor gain multiplied by (less
    than 1) gains of filter stages - it seems they "eat" quite a lot of an amplitude.

    The 100K base resistor was probably selected to match the
    beta of the transistor

    please note here is some cheat - resistor is connected not to the positive supply but to the collector - it reduces the pull-up effect, but allows for wider range of resistance (effectively removing necessity of adding proper pull-down resistor at the base and small another one at emitter).

    The lesson for your students is more general

    Thanks, it is a good lesson for myself - as for the students, they are a
    bit too beginners to
    get into such depth of idea, but I'll try to communicate your explanations
    :)

    Another phase-shift osc form uses three RC integrators

    That's curious, I'll look for this variant.

    --
    to email me substitute github with gmail please

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  • From albert@spenarnc.xs4all.nl@21:1/5 to xx@yy.com on Mon May 13 11:39:01 2024
    In article <3at93j9tmlr3defa0rc7tfrs0n4ntnksul@4ax.com>,
    John Larkin <xx@yy.com> wrote:
    On Fri, 3 May 2024 06:14:54 +0000, RodionGork <rodiongork@github.com>
    wrote:

    Hi Friends!

    Schematic / simulation in "Falstad online simulator": >>https://tinyurl.com/23hcg8np

    This is probably very old and widely known schematic of single-transistor >>generator which
    requires no inductance, but instead uses three capacitors - actually it >>seems a chain
    of high-pass single-stage filters with transistor serving as feedback from >>output to input.

    One can find it, for example, in classic stylophone schematic (the part >>creating low-frequency
    oscillations for "vibratto" effect).

    I teach it to my pupils for years probably and I always thought I less or >>more understood what
    is happening inside - each filter stage gives shift in phase and hence when >>amplifying feedback
    is added there happen harmonic oscillations.

    However on the schematic given above I added 4 scopes over the length of >>the filter (potentials
    at the points A, B, C, D according to labels - here A and B are potentials >>at points between capacitors, C is at the base and D at collector) - I >>suddenly found that intermediate voltages are
    not pretty harmonic! They could be distorted by the current drawn into >>transistor base though. And
    I'm not sure the output is exactly sine now. Though probably it is a matter >>of adding some resistor to improve input impedance of transistor cascade?

    Regretfully I can't find any thorough explanation of the schematic >>(probably due to keywords being too general and I don't know if this design >>has fancy proper name). So I would be grateful either
    for links or for verbal clarifications.

    That sim makes a suspiciously nice sine wave, for a phase-shift
    oscillator. The 100K base resistor was probably selected to match the
    beta of the transistor, and if so it wouldn't be as good in
    production, where betas vary.

    If that resistor is too big or too small, it won't oscillate. Try
    varying it.

    There is some AGC effect from base rectification biasing the transisor
    off, which increases beta tolerance.

    The lesson for your students is more general: the amplitide of a
    linear oscillator increases exponentially until something nonlinear
    kicks in to reduce the overall gain to unity. The nonlinearity makes >distortion.

    Your phase shifter is three differentiators, so magnifies harmonics.
    Another phase-shift osc form uses three RC integrators, so can
    attenuate harmonics and make a better sine.

    Another lesson for students is that a hand-selected set of values may
    not be a reproducible, sellable circuit.


    On the other hand restricting amplification with a ptc resistor,
    or a 50 ma 6 V incandescent light bulb could result in a decent
    (less that 1% distortion) sine wave.

    Groetjes Albert
    --
    Don't praise the day before the evening. One swallow doesn't make spring.
    You must not say "hey" before you have crossed the bridge. Don't sell the
    hide of the bear until you shot it. Better one bird in the hand than ten in
    the air. First gain is a cat purring. - the Wise from Antrim -

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