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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Low distortion sinewave oscillator without big capacitor.
Date: Tue, 8 Apr 2025 11:56:57 -0400
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"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vt2bck$1gmbm$1@dont-email.me...
> On 7/04/2025 11:10 pm, Edward Rawde wrote:
>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vsvspr$33gdd$1@dont-email.me...
>>> On 7/04/2025 3:25 am, Edward Rawde wrote:
>>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vsuacb$1d4ec$1@dont-email.me...
>>>>> On 7/04/2025 12:39 am, Edward Rawde wrote:
>>>>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vsu092$14oc7$1@dont-email.me...
>>>>>>> On 6/04/2025 2:12 am, JM wrote:
>>>>>>>> On Fri, 4 Apr 2025 23:55:11 -0400, "Edward Rawde"
>>>>>>>> <invalid@invalid.invalid> wrote:
>>>>>>>>
>>>>>>>>> "JM" <sunaecoNoChoppedPork@gmail.com> wrote in message news:uop0vjp3d13t441ujfboi5aeeg08anm1je@4ax.com...
>>>>>>>>>> On Fri, 4 Apr 2025 16:29:27 -0400, "Edward Rawde"
>>>>>>>>>> <invalid@invalid.invalid> wrote:
>>>>>>>>>>
>>>>>>>>>>> "JM" <sunaecoNoChoppedPork@gmail.com> wrote in message news:add0vjdh2gcma0n9pfunq76n04cfbkhtnj@4ax.com...
>>>>>>>>>>>> On Fri, 4 Apr 2025 14:25:29 -0400, "Edward Rawde"
>>>>>>>>>>>> <invalid@invalid.invalid> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vsnufh$2ou7j$1@dont-email.me...
>>>>>>>>>>>>>> On 4/04/2025 11:33 am, JM wrote:
>>>>>>>>>>>>>>> On Thu, 3 Apr 2025 19:25:33 -0400, "Edward Rawde"
>>>>>>>>>>>>>>> <invalid@invalid.invalid> wrote:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> "JM" <sunaecoNoChoppedPork@gmail.com> wrote in message news:qq8tujlpciqc2jrd0ibljmjr9pd37ip6hi@4ax.com...
>>>>>>>>>>>>>>>>> On Sun, 30 Mar 2025 14:54:56 -0400, "Edward Rawde"
>>>>>>>>>>>>>>>>> <invalid@invalid.invalid> 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.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> You will need to adjust the feedback to suit.  Start with a -5 or -6
>>>>>>>>>>>>>>> gain block after the integrator and adjust it's gain until the startup
>>>>>>>>>>>>>>> is clean (no saturation).
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Here's my version of John May's variation.
>>>>>>>>>>>>>>
>>>>>>>>>>>>> ...
>>>>>>>>>>>>>
>>>>>>>>>>>>> Yes, that works but only 120dB down at 4KHz.
>>>>>>>>>>>>> I put the damping resistor back to 47k since I don't care what happens during the first few seonds as long as it
>>>>>>>>>>>>> happens.
>>>>>>>>>>>>> If it's necessary to wait one minute for the purest tone, that's fine with me.
>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Linewraps are going to be a problem - delete all "\n" from the last few lines
>>>>>>>>>>>>>
>>>>>>>>>>>>> You don't want to remove \n just remove the wraps.
>>>>>>>>>>>>>
>>>>>>>>>>>>> BCM61B does not have two independent transistors.
>>>>>>>>>>>>>
>>>>>>>>>>>>> So you probably want BCM847BS which has two independent matched transistors and a very low price at digikey, so may as
>>>>>>>>>>>>> well
>>>>>>>>>>>>> use
>>>>>>>>>>>>> two
>>>>>>>>>>>>> of them as shown below.
>>>>>>>>>>>>>
>>>>>>>>>>>>> I took the model from
>>>>>>>>>>>>> https://github.com/peteut/spice-models/blob/master/nxp/complex_discretes/complex_discretes.txt
>>>>>>>>>>>>>
>>>>>>>>>>>>> It should only be necessary to unwrap the last line of the following.
>>>>>>>>>>>>> Don't remove \n just remove the wraps, you may need to use a horizontal scroll bar.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Version 4.1
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Best to just offset the integrator output so the amplitude is brought
>>>>>>>>>>>> under control sooner.
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Wow. That has much lower distortion too.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> If you just replace Q1,2 duals with a simple long tailed pair I think
>>>>>>>>>> you will get better performance.
>>>>>>>>>
>>>>>>>>> I'm not sure I understand how the multiplier could be implemented with just a long tailed pair.
>>>>>>>>>
>>>>>>>>
>>>>>>>> Just modulate the tail current and select how much to steer to the
>>>>>>>> output by directly driving the bases rather than indirectly as in your
>>>>>>>> circuit.  The following link shows one example topology, and a four
>>>>>>>> quadrant differential I/O version.  Compare the linearity of each of
>>>>>>>> them.
>>>>>>>>
>>>>>>>> https://1drv.ms/u/c/1af24d72a509cd48/EWVCUG7-jFJMu7-01VczCRcBzEC9JPHrV45x7TOunN90Gg?e=GXbvX5
>>>>>>>>
>>>>>>>> It could be used as shown here.
>>>>>>>>
>>>>>>>> https://1drv.ms/u/c/1af24d72a509cd48/EVmMVrvUD15GutoR5nCJ7QEBSeZsHWpHudqR0b8XtTLMLw?e=HIV74I
>>>>>>>
>>>>>>> As I've already said, I like it. I've played with it a bit.
>>>>>>>
>>>>>>> The ON-Semiconductor NSS40301MDR2G NPN dual comes with a 2mV guaranteed maximum difference in base-emitter voltages (at the
>>>>>>> same
>>>>>>> emitter current). Edward Rawde's Nexperia BCM61B dual part has matched current gain, but no guarantee on the Vbe. The 
>>>>>>> Nexperia
>>>>>>> BCM847BS does offer 2mV base-emitter matching, and would presumably work just as well.
>>>>>>>
>>>>>>> Putting in the ON-Semiconductor dual means that your gain control circuit doesn't have to waste output swing coping with
>>>>>>> part-to-part variation.
>>>>>>>
>>>>>>> I've added a cascode transistor (Q1, it should be Q4) to minimise any Early effect distortion.
>>>>>>>
>>>>>>> I've snipped out the op amp driving the base of Q3. Once you'd gone AC-coupled, it wasn't doing anything useful.
>>>>>>>
>>>>>>> And I've put a string of eight diodes in series with R10. They nominally compensate for the temperature dependence 
>>>>>>> introduced
>>>>>>> by
>>>>>>> the four rectifier diodes D2, D12, D13 and D14. In this version of the circuit the rectifiers knock about 0.6 volts off a 
>>>>>>> sine
>>>>>>> wave that peaks at 3.8V, about a quarter of the 15V rail. I haven't run the numbers to fix the best number of diodes, but
>>>>>>> something between six and eight looks okay.
>>>>>>>
>>>>>>> The harmonics aren't great - most of them are about 90dB below the fundamental, but the seventh is only 85dB down.
>>>>>>>
>>>>>>
>>>>>> Not sure I see the point if it's only 80dB down Bill.
>>>>>> @ 7Khz in LTSPice 24.1.5
>>>>>
>>>>> LTSpice isn't all that reliable as predictor of low level distortion. Having an armoury of different circuits to try when you
>>>>> finally get around to building  and testing something real may be useful.
>>>>>
>>>>>> I can almost get that from a simple phase shift oscillator and a 1KHz tuned circuit.
>>>>>
>>>>> I'm sure that you think so. John Larkin thought that a bang-bang amplitude control was worth suggesting...
>>>>>
>>>>>> The last time I included a diode string like that in one of my circuits (which I seem to recall had better than 80dB
>>>>>> performance)
>>>>>> you told me it was nuts.
>>>>>
>>>>> It probably was. In this case there are better ways of getting a rectified output than a simple series diode - I've posted
>>>>> circuits which incorporate precision rectifiers which get rid of the forward drop through the diode, and I've built circuits
>>>>> that
>>>>> used synchronous rectifiers built around transmission gates where the output isn't shifted by a temperature dependent diode
>>>>> drop.
>>>>> It went into a GaAs single crystal puller as a retrofit.
>>>>>
>>>>> The main point of the diode string was as a satirical comment on that aspect of the design you posted, which probably counts 
>>>>> as
>>>>> being hostile, but I am hostile to ill-thought out designs, hard though it is to get the design time to sort them out.
>>>>
>>>> I don't see anything particularly hostile there Bill. Just different points of view.
>>>>
>>>>> I've got stuck with sorting out other peoples half-baked designs often enough, but only after my bosses had had their noses
>>>>> rubbed
>>>>> in the unfortunate consequences.
>>>>
>>>> When I started work as a fresh graduate (but one who also had practical experience with everything from TV antenna systems in
>>>> hospitals to AY-3-8500 based games) I was amazed at some of the analogue circuit design blunders I encountered.
>>>> In one case I built a piece of custom test equipment which needed +15V and -15V. An available transformer had two suitable
>>>> isolated
>>>> secondary windings so I just used two 7815 devices.
>>>> Only to be told by a more experienced "Designer" that connecting the output of a 7815 to ground would short it out and I had to
>>>> use
>>>> 7915 for that.....
>>>
>>> What he should have said was the you were messing up the ground returns by hooking up the +15V output of the second 7815 to the 
>>> 0V
>>> rail.
>>>
>>> He was avoiding spending a long time talking about grounding and shielding, which is a rather specialised subject.
>>
>> I see. Thank you for letting me know what he was thinking Bill.
>> Do you have some kind of time machine that you travel around in so you can have a better knowledge of what happened in a past
>> situation when compared with someone who was actually there at the time?
>>
>>>
>>> In this particular thread your eight transistor is bonkers, but it
>>> works - not that I can see how.
>>
>> Seems pretty obvious to me how it works Bill.
>
> But John May could take out half the transistors and produce a circuit that still worked.

One reason for that is that a current source/sink can be approximated by a high value resistor so instead of using a current mirror 
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