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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: The low distortion oscillator problem
Date: Sat, 8 Feb 2025 01:46:37 -0000 (UTC)
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john larkin <JL@gct.com> wrote:
> On Fri, 7 Feb 2025 12:43:17 -0500, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
>> On 2025-02-06 00:44, john larkin wrote:
>>> On Thu, 06 Feb 2025 05:08:16 +0000, JM
>>> <sunaecoNoChoppedPork@gmail.com> wrote:
>>> 
>>>> On Thu, 6 Feb 2025 03:58:59 +1100, Bill Sloman <bill.sloman@ieee.org>
>>>> wrote:
>>>> 
>>>>> There have been quite a few postings about 1kHz low distortion sine wave
>>>>> oscillators.
>>>>> 
>>>>> The problem is that if you want a get stable output from a sine wave
>>>>> oscillator you have to add a non-linear element to control the gain
>>>>> around the oscillating circuit.
>>>>> 
>>>> 
>>>> You don't.
>>> 
>>> What limits the amplitude?
>>> 
>> We had a long discussion of this in one of the myriad other 1-kHz 
>> oscillator threads.  One approach is to use a comparator+integrator to 
>> control the tail current source (suitably cascoded).
>> 
>> The key is for the gain-setting mechanism to be outside the oscillator 
>> loop, so that it doesn't get run through its range on each cycle.  The 
>> bias of the active device does change some, of course, but that's harder 
>> to avoid.
>> 
>> Cheers
>> 
>> Phil Hobbs
> 
> But where can I buy those linear diodes?
> 
> The idea of using a s/h to pick off the sine amplitude, for level
> feedback, is interesting. Properly done, it should result in a
> zero-ripple amplitude signal.
> 
> Or use an active full-wave rectifier to get the average, and filter
> the heck out of that.
> 
> I suspect that nobody needs a way-sub-PPM THD sine wave, so it's
> pretty much a game.
> 
> One might Spice using an ohmic mosfet or two as a low distortion
> variable resistor. The i/v curves look awfully straight around zero.
> 
> 

I spent some quality time with that complementary Class AB car stereo amp
of JT’s last summer, and the more time I spent, the more impressed I was. 

Its bias loop used an LM311 comparator sensing the minimum collector
current at the zero crossing, and charged up a biggish cap that set the
voltage between the PNP and NPN bases. Every time it got too low, the
comparator dumped a bit of charge into the cap, and a bleed resistor took
it out again. 

Lots of us have done similar things, e.g. the class-H TEC driver in our
LC120 laser controller. The really nifty thing about Jim’s circuit was that
it measured what you actually care about, namely the minimum class-A
current right at the crossover point, rather than some DC average that
depends on the waveform, power supply droop, and other stuff with bupkis to
do with the crossover distortion. It worked brilliantly, according to the
spherical cows. 

Something like that, measuring the instantaneous peak voltage of our
oscillator, would do an excellent job of regulating the tail current to
keep the amplitude constant. 

Cheers 

Phil Hobbs 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics