Newsgroups: sci.electronics.design
Subject: Re: How 3-capacitor sine generator works really?
References: <804485177e123558699ce859cc4d496a@www.novabbs.com> <3at93j9tmlr3defa0rc7tfrs0n4ntnksul@4ax.com>
From: albert@spenarnc.xs4all.nl
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Date: Mon, 13 May 2024 11:39:01 +0200
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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 -