Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: How 3-capacitor sine generator works really?
Date: Fri, 3 May 2024 16:58:50 +1000
Organization: A noiseless patient Spider
Lines: 74
Message-ID: <v121vo$db1f$1@dont-email.me>
References: <804485177e123558699ce859cc4d496a@www.novabbs.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Injection-Date: Fri, 03 May 2024 08:59:05 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="d89bb5212c74faa8a77b462c9204c15a";
	logging-data="437295"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/6EvUGu2CKprHdailgkTvTLL/bnKCK5EQ="
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:PtF3EaRjVEG5p9gVeGxHaXCzFBI=
Content-Language: en-US
In-Reply-To: <804485177e123558699ce859cc4d496a@www.novabbs.com>
Bytes: 4549

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