1. Home
  2. Electronics Design
  3. really?

really?

Hi Friends!

Schematic / simulation in "Falstad online simulator":

formatting link
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.

Reply to
RodionGork
Loading thread data ...

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

formatting link
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.

formatting link

Reply to
Bill Sloman

On a sunny day (Fri, 3 May 2024 06:14:54 +0000) it happened RodionGork snipped-for-privacy@github.com wrote in snipped-for-privacy@www.novabbs.com>:

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.

Reply to
Jan Panteltje

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.

Reply to
John Larkin

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!

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

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.

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.

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).

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 :)

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

Reply to
RodionGork

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.