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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Low distortion sinewave oscillator without big capacitor.
Date: Wed, 2 Apr 2025 14:13:00 -0400
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"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vsig45$vt0r$1@dont-email.me...
> 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.

Ok thanks for taking the time to look at it.
Let me know when you get it to work.

>
> -- 
> Bill Sloman, Sydney
>
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