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.
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.
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.
It's a phase shift oscillator - one of many.
Here's a solution I came up with back in 1986
For oscillation you need to put the output back in phase to the input.The tansistor gives 180 degrees phese shift
and as gain is >1 it wil oscillate
The 100K base resistor was probably selected to match thebeta of the transistor
The lesson for your students is more general
Another phase-shift osc form uses three RC integrators
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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