Re: How 3-capacitor sine generator works really?

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 -