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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: A variation on my current mirror low distortion sine wave
 oscillator - 10dB less distortion and much the same number of components
Date: Wed, 19 Feb 2025 23:20:49 +1100
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On 19/02/2025 5:40 am, Edward Rawde wrote:
> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vp1acj$1j5t7$1@dont-email.me...
>> On 18/02/2025 2:50 pm, Edward Rawde wrote:
>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vp0svp$1d8re$6@dont-email.me...
>>>> On 18/02/2025 3:54 am, Edward Rawde wrote:
>>>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:voujeq$11678$2@dont-email.me...
>>>>>> On 17/02/2025 3:53 pm, Edward Rawde wrote:
>>>>>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:voualf$rm6g$8@dont-email.me...
>>>>>>>> On 17/02/2025 2:14 am, Edward Rawde wrote:
>>>>>>>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vorsg8$emeo$7@dont-email.me...
>>>>>>>>>> On 16/02/2025 2:18 pm, Edward Rawde wrote:
>>>>>>>>>>> "JM" <sunaecoNoChoppedPork@gmail.com> wrote in message news:gp6vqjl5oma32tga136kspreh7a8182ofg@4ax.com...
>>>>>>>>>>>> On Mon, 10 Feb 2025 17:18:01 +1100, Bill Sloman <bill.sloman@ieee.org>
>>>>>>>>>>>> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> Basically same idea, but two separate controllable asymmetric current
>>>>>>>>>>>>> mirrors, rather than one, and no current steering. The half-wave
>>>>>>>>>>>>> rectifier still seems to be the source of the distortion in the
>>>>>>>>>>>>> stabilised output.
>>>>>>>>>>>>>
>>>>>>>>>>>>> C25 and C26 take out as much of it as I can. Increasing them - from 15nF
>>>>>>>>>>>>> to 33nF makes the distortion worse. Splitting the resistors into three
>>>>>>>>>>>>> rather than two and adding two more capacitors might help, but what this
>>>>>>>>>>>>> circuit needs is more insight, rather than more components.
>>>>>>>>>>>>
>>>>>>>>>>>> What is the point of a push-pull current mirror?  You don't need
>>>>>>>>>>>> response at dc.  A "class A" (for want of a better term) mirror with
>>>>>>>>>>>> minimal current deviation will have distortion levels orders of
>>>>>>>>>>>> magnitude less than the circuit you propose.
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Is there any specific reason for the npn Q5?
>>>>>>>>>>> Replacing it and R25 with a single 100k resistor from U2 to Q1 base seems to work just as well.
>>>>>>>>>>> 2kHz is 141dB down measured with cursors on a zoomed in FFT in LTSPice 24.1.2
>>>>>>>>>>
>>>>>>>>>> Complementary pairs often work better than simple emitter followers.
>>>>>>>>>
>>>>>>>>> But it's not a Sziklai pair. Both base-emiiter currents flow through R25
>>>>>>>>
>>>>>>>> It's still exploiting the same idea.
>>>>>>>>
>>>>>>>>> The Sziklai pair has been used for centuries.
>>>>>>>>
>>>>>>>> The Wikipedia page lists a 1957 patent. Transistors had been around for perhaps ten years by then. I got into electronics
>>>>>>>> around
>>>>>>>> 1966 (as a graduate student in chemistry) and knew about complementary Darlington pairs from early on, though nobody called
>>>>>>>> them
>>>>>>>> Sziklai pairs back then.
>>>>>>>>
>>>>>>>>> There's one on page 566 (Pdf page 16)
>>>>>>>>> https://www.worldradiohistory.com/UK/Wireless-World/60s/Wireless-World-1961-11.pdf
>>>>>>>>>
>>>>>>>>>> John May probably has a good reason for the choice. I've used them from time to time.
>>>>>>>>>>
>>>>>>>>>> https://en.wikipedia.org/wiki/Sziklai_pair
>>>>>>>>
>>>>>>>> John May's post makes it clear that he didn't have a good reason to go for that arrangement - it was cut and pasted from
>>>>>>>> from
>>>>>>>> a
>>>>>>>> earlier circuit where it did make more sense. He also make it clear that your modification wasn't well thought out - the
>>>>>>>> 100k
>>>>>>>> resistor isn't required at all, and would degrade the performance of the circuit (though not enough for anybody to notice).
>>>>>>>
>>>>>>> Bill. The current in the resistor is about 500 nA.
>>>>>>> Why would the resistor degrade the performance?
>>>>>>
>>>>>> The 2N38906 has 10pF of input capacitance and 4.5pF of output capacitance. The resistor introduces about 1usec of lag, which
>>>>>> degrades the high frequency performance.
>>>>>
>>>>> Which is irrelevant for this circuit.
>>>>>
>>>>>> In a 1kHz oscillator this isn't going to worry anybody,
>>>>>
>>>>> So why bother pointing it out?
>>>>>
>>>>>> and the LT1013 is slow enough that it won't matter - C9 kills any risk there - but the resistor clearly isn't doing anything
>>>>>> useful, so one has to wonder why you bothered to add it.
>>>>
>>>> That is the question that matters.
>>>
>>> But it doesn't matter to anyone else Bill.
>>
>> What makes you think that? You may find it a comforting thought, but it strikes me a self-serving delusion.
> 
> It strikes me as an obvious fact.
> I'm not expecting anyone else to offer any comment but it's interesting that they haven't.
> And it's not like this group is only for the discussion of electronic matters, as the "Cracking Speech by JDV" thread shows.
> You seem to be enjoying yourself there.
> 
>>
>>>>> In any real circuit I would generally not connect a low impedance output from an op amp directly to the base of a transistor,
>>>>> but
>>>>> this doesn't mean that there aren't cases where it's perfectly fine or desirable to do so.
>>>>> In this case it doesn't matter, so why bother pointing out that it doesn't matter?
>>>>
>>>> You've been complaining that my circuits include too many components, even though each one of the serves a purpose.
>>>
>>> So add another hundred ferrites and claim that each one serves a purpose if you want.
>>> I don't mind.
>>
>>
>> But you still complain about it.
> 
> I wouldn't call it complaining. Just pointing out the obvious.
> 
>> The ferrites do serve a purpose, even if you can't see the point.
> 
> Ok let me simulate your mode of response. This is just a simulation, it doesn't mean you actually said this. Here goes.
> 
> <Bill>
> 
> Why have you included useless ferrites in your circuit?
> Even a five year old should be able to see that a simulation with and without all nine ferrites produces exactly the same harmonic
> distortion result.
> (-57.5dB at 2kHz in LTSPice 24.1.2)
> You obviously don't know what those ferrites are doing, and didn't realise that you didn't need them.
> 
> </Bill>
> 
>>
>>> You're still only going to get 60dB down in LTSpice 24.1.2
>>
>> So LTSpice 17 and LTSpice 24 give different results - not a good reason fro trusting either of them. If you want to make a fuss
>> about harmonic levels you have to measure them in a real circuit, which is expensive and time-consuming. John May has done it -
>> neither of us have.
>>
>>>> You should expect me to complain when one of your circuits includes a useless component which degrades it's performance even it
>>>> it
>>>> is only a very minor degradation.
>>>>
>>>>> You can also argue that R7 isn't needed, but in any real circuit I would include both resistors.
>>>>> I can always put 0 ohm in.
>>>>
>>>> I automatically put a resistor in series with the gate of a power MOSFET.
>>>
>>> I do too. Then I can put any value resistor in place from 0 ohm to infinity ohm.
>>
>> But the circuit won't work if the resistance is too high.
> 
> Who cares Bill?
> I just put 1 Meg in there and ran a simulation. The result it exactly the same.
> I'm not suggesting I'd actually use 1 Meg in practice.
> 
>> And production want to buying and fit a single resistor value.
> 
> This circuit has nothing to do with production.
> It exists only in the mind of a few people and the memory of a few computers.
> 
>> Select on test resistors aren't popular - Cambridge Instruments used them from time to time, and production kept on proposing to
>> use a fixed resistor.
> 
> "From 1960 the company started to decline and struggled to turn a profit."
> Hmm
> 
>> We were buying parts in six month chunks, and for that six months production always fitted the same resistor (and got bored). A
>> new batch of parts would need a different resistor.
>>
>>> Just like the resistor I put between U6 and Q1.
>>
>> Far from it. You didn't know what it was doing, and didn't realise that you didn't need it.
> 
> Now you're starting to border on telling blatant lies Bill.
> For example, in the circuit here (which turned up in a search engine search).
> https://www.eeeguide.com/op-amp-regulators/
> The base is connected directly to the transistor.
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