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From: "Edward Rawde" <invalid@invalid.invalid>
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: Mon, 17 Feb 2025 11:54:13 -0500
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"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.

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 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.

>
> -- 
> Bill Sloman, Sydney
>
>