The low distortion oscillator problem

You don't.

What limits the amplitude?

Counter-example?

Clipping is a non-linear process. The most linear op amp becomes non-linear as soon as its output hits the supply rails.

"JM" snipped-for-privacy@gmail.com wrote in message news: snipped-for-privacy@4ax.com...

There are plenty of examples out there claiming to be a stable output sinewave oscillator, with no obvious non-linear element.

Maybe folow that with a Chebychev low pass filter with a zero in the stop band at 4kHz.

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736 336 736 WIRE -16 768 -16 720 WIRE 416 768 416 736 WIRE 928 768 928 704 WIRE -240 816 -240 768 WIRE -208 816 -240 816 WIRE -160 816 -160 768 WIRE -160 816 -208 816 WIRE -160 864 -160 816 WIRE -16 864 -16 832 FLAG 336 208 0 FLAG 736 208 0 FLAG -400 176 0 FLAG -336 64 vcc FLAG -336 256 vee FLAG 1216 160 vout FLAG 192 672 vx FLAG -208 640 vt FLAG -208 816 vb FLAG -160 864 0 FLAG -16 864 0 FLAG 1216 688 0 FLAG 1120 688 0 FLAG 1040 688 0 FLAG 416 768 0 FLAG 928 848 0 FLAG -16 128 vcc FLAG -16 192 vee FLAG 416 128 vcc FLAG 416 192 vee FLAG 816 128 vcc FLAG 816 192 vee FLAG 304 688 vcc FLAG 304 752 vee FLAG -48 720 vg FLAG 928 704 vc SYMBOL res 48 16 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 10K SYMBOL res 304 16 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 16K SYMBOL res 704 16 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 16K SYMBOL cap 448 16 R90 WINDOW 0 0 32 VBottom 2 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SYMBOL res 416 576 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R12 SYMATTR Value 22k SYMBOL res -224 672 M0 SYMATTR InstName R8 SYMATTR Value 10Meg SYMBOL res 640 480 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R13 SYMATTR Value 47k SYMBOL res 128 704 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R11 SYMATTR Value 1k SYMBOL g 1216 544 M0 WINDOW 3 -255 133 Left 2 SYMATTR Value table{-2m,0,2m,1} SYMATTR InstName G1 SYMBOL cap 1136 576 M0 SYMATTR InstName C7 SYMATTR Value 2p SYMBOL res 1056 544 M0 SYMATTR InstName R14 SYMATTR Value 1 SYMBOL SpecialFunctions\\sample 768 480 M0 SYMATTR InstName A1 SYMBOL cap -32 768 R0 SYMATTR InstName C8 SYMATTR Value 10u SYMBOL voltage 928 752 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V4 SYMATTR Value PWL(0 0 3 -6) SYMBOL OpAmps\\LT1679 -16 160 R0 SYMATTR InstName U1 SYMBOL OpAmps\\LT1679 416 160 R0 SYMATTR InstName U2 SYMBOL OpAmps\\LT1679 816 160 R0 SYMATTR InstName U3 SYMBOL OpAmps\\LT1679 304 720 M0 SYMATTR InstName U4 SYMBOL bi -160 688 R0 WINDOW 3 -236 218 Left 2 SYMATTR Value I={v(vt,vb)/(-100*limit(-1m,v(vg)-0.7,-15))} SYMATTR InstName B1 SYMBOL diode 672 720 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D1 SYMATTR Value 1N914 SYMBOL diode 272 416 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D2 SYMATTR Value 1N914 TEXT 1048 -104 Left 2 !.tran 0 5 0 1u startup TEXT 1048 -128 Left 2 !.options plotwinsize=0 numdgt=15 TEXT 1056 -80 Left 2 !.save v(vout) v(vg) v(vc)

Very funny, The sample and hold at A1 is an obviously a non-liner element, and B1 is even more obscure, but "limit" is a non-linear operation.

Saving only the nodes voltages you want us to pay attention to is a pretty transparent trick. I saved a few more - not enough to have a particularly clear idea of what you are doing, but quite enough to be confident that it isn't any kind of counter example.

You can also use an LM13700 to build a "solid-state lightbulb"! A grounded virtual resistance with a logarithmic V/I curve.

It's obviously clipping on the supply rail. It's pretty subtle clipping

- the top of the sine wave is sightly, but perceptibly, flatter that the bottom, but it does stick out like a sore thumb in the Fourier transform.

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

My NMR gradient coil drivers had PPM current accuracy and microsecond settling. I used many parallel mosfets with an opamp per fet to turn each one into an essentially ideal device, zero threshold voltage. That's fairly easy to bias to zero-deadband class AB.

Looks like a beast.

Would have been hard to do in 1968, though!

Cheers

Phil Hobbs