Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Bill Sloman 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: 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