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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Low distortion sinewave oscillator without big capacitor.
Date: Wed, 2 Apr 2025 15:57:30 +1100
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On 1/04/2025 2:09 pm, Edward Rawde wrote:
> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vsdi3h$3nagd$2@dont-email.me...
>> On 31/03/2025 5:54 am, Edward Rawde wrote:
>>> Not long ago JM posted a 1KHz sinewave oscillator with very low distortion.
>>> It used a 470uF non polarized capacitor which in practice would probably be made from two 1000uF capacitors.
>>> There's nothing wrong with that but I wanted to see whether I could make a working circuit without needing such a large
>>> capacitor.
>>>
>>> What I have so far is below.
>>>
>>> Any comments?
>>
>> It relies on the Analog Devices MAT-02 dual transistor, which is now obsolete
>>
>> https://www.analog.com/media/en/technical-documentation/obsolete-data-sheets/mat02.pdf
>>
>> The .asc file shows eight NPN transistors labelled MAT-02, presumably in four pairs of the part, but it isn't clear which of the
>> eight transistors should be paired up.
> 
> BCM61B is available and very reasonably priced.
> https://www.digikey.ca/en/products/detail/nexperia-usa-inc/BCM61B-215/2119400
> It can be used for Q1 Q2 and Q3 Q4 in the circuit below.
> 
> The remaining four transistors can use MAT14
> https://www.digikey.be/en/products/detail/analog-devices-inc/MAT14ARZ-R7/2510588
> which although a bit pricey has four independent matched transistors.
> I can't find an LTSpice model for MAT14 so MAT02 is still shown in the circuit below.
> 
>>
>>> What's the best way to control the output level?
>>> Currently it's 5v pk-pk but I rather have half that.
>>
>> The rectified currents from the four phased shifted versions of of the output waveform flow through R13, R14, R15 and R16 into R11
>> and through it into the virtual earth set up at the inverting input of U3, where the summed current is compared with a fixed
>> current drawn from the +15V rail through D1 and R10.
>>
>> Doubling R10 from 330k to 680k would roughly halve the output amplitude.
>> One could be more precise, but it wouudl be hard to justify the extra effort.
> 
> Ah yes, that takes care of the output level.

Not all that well. Using the positive rail as your voltage reference 
suck, and including the diode drops of the rectifying diodes is even 
worse.There are precision rectifiers that use op amps to take out the 
diode drop, and synchromous rectifiers built around transmission gates 
can be even more precise.

> The revised circuit is below.
> Line 459 will need to be unwrapped.

I did that, and the circuit does work, after a fashion.

It still uses eight transistors to do what John May did with three 
separate discrete transistors.

Because he didn't use a matched pair, he had to use two 250R emitter 
resistors to get the operating conditions he needed to make them act as 
parts of a three-transistor asymmetric Wilson current mirror.

https://en.wikipedia.org/wiki/Wilson_current_mirror

He then had to by-pass the resistors with a big capacitor to get the 
effect he needed. This introduced a phase shift, but John May's 
phase-shift oscillator offers four different phases from which he could 
pick off the right phase to get the correction signal he needed.

By summing current from two adjacent phases you can get exactly the 
phase shift you need, but he didn't to be all that exact.

Using a matched pair for the two transistors at the bottom of the mirror 
you can get rid of one of the emitter resistors and make the other one 
small enough not to matter (so you don't need the capacitor), but you do 
need to pick off a different phase to get the right feedback.

The MAT04 is totally unnecessary.

Or it least that's the way it strikes me. I've yet to get a simulation 
to work to illustrate the point - it's a complicated circuit, and once 
it hits saturation the subtle effects that stabilise it get swamped.

I need a better grasp of what's going on in the circuit, and a way to 
start it up that doesn't let it slide over into saturation before it 
stabilises.

-- 
Bill Sloman, Sydney