Re: A variation on my current mirror low distortion sine wave oscillator - 10dB less distortion and much the same number of components

"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vp4ibd$28rod$1@dont-email.me...

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.
>

>

...

I think the circuit JM posted is likely to work.

>
Congratulations. You've said something  sensible for once.

Why do you like to talk down to other people Bill?
It doesn't make you look superior, far from it.

>

Two BCM61B devices would probably be fine for the current mirror.
A single LT1679 can be used for U1,5,6,8 with a cheaper device for the rest.

>
This is a less defensible observation.

After more simulation I agree that all devices may as well be LT1679.
Two quad op amp packages.
This produces the lowest possible distortion in simulation.

>
U5,U6 and U8 are the phase shift oscillator. U1 should a cheap device that is part of the network that generates the amplitude
feedback signal fed into the integrator wrapped around U7 to generate the gain control signal that modulate the level of the
feedback sine wave that adjusts the output amplitude.
>

And C6 can be made from two polarized capacitors.

>
Sadly, you can't buy a pair of 940uF polarised capacitors. Two 1000uf polarised capacitors would be quite close enough

There have been two polarised 1000uf capacitors on my schematic for the last few days.

>
https://4donline.ihs.com/images/VipMasterIC/IC/VISH/VISH-S-A0010924709
/VISH-S-A0010924709-1.pdf?hkey=6D3A4C79FDBF58556ACFDE234799DDF0
>
are offered at +/-10% and +/-20% tolerance and 940uF is within 10% of 1000uF.
>
I(R17) runs at about 100uA and I(R21) runs at about 145uA -that is about 25mV across R17 and 36mV across R21. A single polarised
capacitor isn't going to get depolarised by 11mV of bias.
>
Putting 100uF polarised caps in parallel to R17 and R19 delivers harmonics about -135dB below the fundamental rather more cheaply,
and won't upset people who get nervous about polarised caps.
>
As we know, LTSpice isn't all that credible when it predicts very low harmonic content, so 470uF is perhaps a a bit much.
>

Q5, R25, R7 and C9 can be removed but keeping R7, C9 and a base series resistor for Q1 does no harm.
The simulation result in LTSpice 24.1.2 is the same, about -142dB at 2kHz.
>
Do you have any productive comments yourself?
>

>

So getting worked up over its exact value isn't very productive, don't you agree?

>
If you are silly enough to think like that this isn't going to be a productive discussion, and your part of it hasn't been for
quite a while.

>
Are you done coming across as a grumpy old headmaster talking to a student who didn't do their homework?
My approach to such a situation would be to offer the student help and encouragement.

>
That has been my approach, and it is what I mostly post. Some students don't notice that they are being offered help.
>

But it appears that yours would be along the lines of "You obviously don't know what you're doing you stupid kid" and "What you
should have done is..."

>
There's little point in being diplomatic with poor students.

Who said anything about poor students Bill?
Talking to adults as thought they are children may cause issues for you.

Quite a few of them are resistant to the idea that they've got stuff wrong, and being diplomatic lets them skate around the
negative message.
>
I've have spelled this out quite explicitly from  time to time over the twenty years I've been posting here.

I've been reading and posting in this and other groups for way more than 20 years.

>
<snip>
>
Bill Sloman, Sydney
>