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

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.
Yes I know that's not the same circuit as the one in the current mirror but here it's just an example of direct connection to the
base.
My instinct would be to include a base resistor to protect the op amp against failure of the pass transistor.
In the current mirror circuit it doesn't matter whether such a resistor is doing anything or not.
You can easily demonstrate that with a simulation or two.
 Why do you enjoy telling other people that they don't know what they're doing Bill?
 

>

In any case this resistor doesn't actually exist. As resistors go it's about as real as Ceci n'est pas une pipe.
It only exists in the limited mathematical imagination of the computer in front of

>
And it's cheaper and quicker to model a circuit that it is to build and test it. Within the limits of the model it's quite useful,
but the game to get to circuit that you can build, which is likely to work. No real circuit? No game.

 I think the circuit JM posted is likely to work.

Congratulations. You've said something  sensible for once.

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.
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
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. 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.
<snip>
Bill Sloman, Sydney