AF Osc AGC alternatives to lamp/thermistor/FET

That's still limiting a Wein bridge, essentially a single-element resonator.

If you do the double-integrator type oscillator, you can do gain control on the first integrator, and then the second one adds 6 dB/octave harmonic reduction.

I think you can even get 12 dB/octave harmonic reduction if you do the amplitude limiter right. 12*3 = 36!

I believe Pye did something like that in one of their laboratory oscillators. As they were closely allied to Philips at the time, it wouldn't surprise me to find an earlier Philips oscillator with the same circuit.

There's probably a way to finagle the 4th section of an LM13700 to apply a bias current inversely proportional to the output level into pin IB1, here:

At some point, opamp distortion and slew rate will usually dominate THD. I'd guess that the LM13700 isn't very good.

It might be OK as the small-influence AGC gain tweak, ahead of two good integrating opamps.

"piglet" snipped-for-privacy@hotmail.com wrote in message news:vf2vp7$e446$ snipped-for-privacy@dont-email.me...

Conversion to single rail allows V3 to be a standard zener.

This circuit is probably more stable without manual adjustment than the FET version.

There's crud either side of 1k Hz if you believe the FFT on current zoom extent. I wonder what reality would say.

Version 4 SHEET 1 1368 896 WIRE 288 -208 -32 -208 WIRE 464 -208 288 -208 WIRE 608 -208 544 -208 WIRE 672 -208 608 -208 WIRE 880 -208 736 -208 WIRE 960 -208 880 -208 WIRE 1056 -208 960 -208 WIRE 608 -176 608 -208 WIRE -400 -144 -400 -176 WIRE -288 -144 -400 -144 WIRE 880 -144 880 -208 WIRE 960 -128 960 -208 WIRE -32 -112 -32 -208 WIRE -400 -96 -400 -144 WIRE -288 -96 -288 -144 WIRE 608 -64 608 -112 WIRE 1056 -64 1056 -208 WIRE 1200 -64 1200 -112 WIRE 880 0 880 -64 WIRE 960 0 960 -64 WIRE -288 16 -288 -32 WIRE -32 16 -32 -32 WIRE -400 32 -400 -16 WIRE 752 48 752 16 WIRE 1056 64 1056 16 WIRE 1056 64 784 64 WIRE 688 80 448 80 WIRE 720 80 688 80 WIRE 1104 96 784 96 WIRE 1200 96 1200 16 WIRE 1200 96 1104 96 WIRE 288 128 288 -208 WIRE 448 128 448 80 WIRE 176 144 176 112 WIRE 144 160 -160 160 WIRE 752 160 752 112 WIRE 288 176 288 128 WIRE 288 176 208 176 WIRE 1104 176 1104 96 WIRE 1200 176 1200 96 WIRE -688 192 -688 160 WIRE -32 192 -32 80 WIRE 32 192 -32 192 WIRE 144 192 32 192 WIRE -400 208 -608 208 WIRE 288 224 288 176 WIRE 688 240 688 80 WIRE 848 240 688 240 WIRE 1056 240 1056 64 WIRE 1056 240 912 240 WIRE 32 256 32 192 WIRE 176 256 176 208 WIRE 448 256 448 208 WIRE 448 256 384 256 WIRE 512 256 448 256 WIRE -512 272 -512 176 WIRE -608 288 -608 208 WIRE -544 288 -608 288 WIRE 384 288 384 256 WIRE 512 288 512 256 WIRE 1104 288 1104 240 WIRE 1200 288 1200 240 WIRE -400 304 -400 208 WIRE -400 304 -480 304 WIRE -288 304 -400 304 WIRE -240 304 -288 304 WIRE -32 304 -32 192 WIRE -688 320 -688 272 WIRE -608 320 -688 320 WIRE -544 320 -608 320 WIRE 688 336 688 240 WIRE -688 368 -688 320 WIRE -608 384 -608 320 WIRE 288 384 288 304 WIRE 384 384 384 352 WIRE 384 384 288 384 WIRE 512 384 512 352 WIRE 560 384 512 384 WIRE -512 400 -512 336 WIRE 384 400 384 384 WIRE 512 400 512 384 WIRE -32 432 -32 384 WIRE 32 432 32 320 WIRE 688 448 688 416 WIRE 896 448 688 448 WIRE -608 480 -608 448 WIRE -688 496 -688 448 WIRE -160 496 -160 160 WIRE 288 496 288 384 WIRE 288 496 -160 496 WIRE 688 496 688 448 WIRE 384 512 384 464 WIRE 448 512 384 512 WIRE 512 512 512 464 WIRE 512 512 448 512 WIRE 288 544 288 496 WIRE 448 560 448 512 WIRE 800 640 800 608 WIRE 896 656 896 448 WIRE 896 656 832 656 WIRE 448 672 448 640 WIRE 688 672 688 576 WIRE 688 672 448 672 WIRE 768 672 688 672 WIRE 288 688 288 624 WIRE 880 688 832 688 WIRE 896 688 880 688 WIRE 800 752 800 704 FLAG -400 -176 PRail FLAG 176 112 PRail FLAG -400 32 0 FLAG 288 128 OUT FLAG 752 16 PRail FLAG 800 608 PRail FLAG 960 0 0 FLAG 32 432 0 FLAG 176 256 0 FLAG -32 432 Half FLAG -288 304 Half FLAG -688 496 0 FLAG -688 160 PRail FLAG 288 688 Half FLAG 880 0 Half FLAG 752 160 0 FLAG 800 752 0 FLAG 880 688 Half FLAG 560 384 Half FLAG 1200 288 0 FLAG 1200 -112 PRail FLAG 608 -64 Half FLAG -512 400 0 FLAG -512 176 PRail FLAG -608 480 0 FLAG -288 16 0 FLAG 1104 288 0 SYMBOL voltage -400 -112 R0 WINDOW 123 0 0 Left 0 WINDOW 39 -147 49 Left 2 WINDOW 0 -78 25 Left 2 WINDOW 3 19 2 Left 2 SYMATTR SpiceLine Rser=0.1 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL res 272 208 R0 SYMATTR InstName R3 SYMATTR Value 3300 SYMBOL res 272 528 R0 SYMATTR InstName R4 SYMATTR Value 2200 SYMBOL diode 368 288 R0 SYMATTR InstName D1 SYMATTR Value 1N914 SYMBOL diode 368 400 R0 SYMATTR InstName D2 SYMATTR Value 1N914 SYMBOL diode 496 288 R0 SYMATTR InstName D3 SYMATTR Value 1N914 SYMBOL diode 496 400 R0 SYMATTR InstName D4 SYMATTR Value 1N914 SYMBOL res 432 112 R0 SYMATTR InstName R5 SYMATTR Value 100k SYMBOL res 432 544 R0 SYMATTR InstName R6 SYMATTR Value 100k SYMBOL res 704 432 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R8 SYMATTR Value 10k SYMBOL res 704 592 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R9 SYMATTR Value 10k SYMBOL res 560 -224 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 2200 SYMBOL res 896 -48 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R10 SYMATTR Value 470k SYMBOL cap 944 -128 R0 SYMATTR InstName C4 SYMATTR Value 1000n SYMBOL diode 624 -112 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D5 SYMATTR Value 1N914 SYMBOL diode 672 -192 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D6 SYMATTR Value 1N914 SYMBOL res -48 -128 R0 SYMATTR InstName R1 SYMATTR Value 15k SYMBOL cap -48 16 R0 SYMATTR InstName C1 SYMATTR Value 10n SYMBOL cap 16 256 R0 SYMATTR InstName C2 SYMATTR Value 10n SYMBOL res -48 288 R0 WINDOW 3 34 76 Left 2 SYMATTR Value 15k SYMATTR InstName R2 SYMBOL res 1040 -80 R0 SYMATTR InstName R11 SYMATTR Value 10k SYMBOL OpAmps\\LT1058 800 608 M0 WINDOW 0 29 34 Left 2 SYMATTR InstName U5 SYMBOL OpAmps\\LT1058 176 112 R0 SYMATTR InstName U1 SYMBOL OpAmps\\LT1058 752 16 M0 WINDOW 0 39 6 Left 2 WINDOW 3 33 33 Left 2 SYMATTR InstName U2 SYMBOL OpAmps\\LT1058 -512 240 R0 SYMATTR InstName U3 SYMBOL res -704 176 R0 SYMATTR InstName R12 SYMATTR Value 10k SYMBOL res -704 352 R0 SYMATTR InstName R13 SYMATTR Value 10k SYMBOL res 1184 -80 R0 SYMATTR InstName R14 SYMATTR Value 4.7k SYMBOL zener 1216 240 R180 WINDOW 0 24 64 Left 2 WINDOW 3 -149 63 Left 2 SYMATTR InstName D7 SYMATTR Value BZX84C6V2L SYMATTR Description Diode SYMATTR Type diode SYMBOL cap 912 224 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C3 SYMATTR Value 100n SYMBOL cap -624 384 R0 SYMATTR InstName C5 SYMATTR Value 100n SYMBOL polcap -304 -96 R0 SYMATTR InstName C7 SYMATTR Value 100µ SYMBOL cap 1088 176 R0 SYMATTR InstName C8 SYMATTR Value 100n TEXT -352 744 Left 2 !.tran 0 10 5 startup TEXT 984 792 Left 2 ;EPW SED OCT 2024 TEXT 568 840 Left 2 ;DIODE BRIDGE VARIOLOSSER AGC FOR WIEN BRDG OSC TEXT 576 880 Left 2 ;Single rail conversion by Edward Rawde, Oct 20 2024

At the cost of adding another component, how about adding another opamp solely to regulate the output signal level? Say you want 2V p-p output, sample some of the output, rectify it and compare it to a reference level which is applied to the other opamp input. Just my 2c.

Thanks, if you make the bridge diodes a pair in series (so 8 in the bridge instead of 4) then harmonic distortion should be improved somewhat.

You might be able to finesse the long tail pair idea into lower distortion too.

piglet

"piglet" snipped-for-privacy@hotmail.com wrote in message news:vf3nie$i4vk$ snipped-for-privacy@dont-email.me...

...

Adding more diodes seemed to reduce it by about 3dB So I went to extremes and also adjusted R5 and R6.

Like the FET circuit it seems that lowest distortion occurs when it's on the edge of not working at all.

Version 4 SHEET 1 1656 1236 WIRE 288 -208 -32 -208 WIRE 464 -208 288 -208 WIRE 608 -208 544 -208 WIRE 672 -208 608 -208 WIRE 880 -208 736 -208 WIRE 960 -208 880 -208 WIRE 1056 -208 960 -208 WIRE 608 -176 608 -208 WIRE -400 -144 -400 -176 WIRE -288 -144 -400 -144 WIRE 880 -144 880 -208 WIRE 960 -128 960 -208 WIRE -32 -112 -32 -208 WIRE -400 -96 -400 -144 WIRE -288 -96 -288 -144 WIRE 608 -64 608 -112 WIRE 1056 -64 1056 -208 WIRE 1200 -64 1200 -112 WIRE 880 0 880 -64 WIRE 960 0 960 -64 WIRE -288 16 -288 -32 WIRE -32 16 -32 -32 WIRE -400 32 -400 -16 WIRE 752 48 752 16 WIRE 1056 64 1056 16 WIRE 1056 64 784 64 WIRE 688 80 448 80 WIRE 720 80 688 80 WIRE 1104 96 784 96 WIRE 1200 96 1200 16 WIRE 1200 96 1104 96 WIRE 288 128 288 -208 WIRE 448 128 448 80 WIRE 176 144 176 112 WIRE 144 160 -160 160 WIRE 752 160 752 112 WIRE 288 176 288 128 WIRE 288 176 208 176 WIRE 1104 176 1104 96 WIRE 1200 176 1200 96 WIRE -688 192 -688 160 WIRE -32 192 -32 80 WIRE 32 192 -32 192 WIRE 144 192 32 192 WIRE -400 208 -608 208 WIRE 288 224 288 176 WIRE 688 240 688 80 WIRE 848 240 688 240 WIRE 1056 240 1056 64 WIRE 1056 240 912 240 WIRE 32 256 32 192 WIRE 176 256 176 208 WIRE 448 256 448 208 WIRE 448 256 384 256 WIRE 512 256 448 256 WIRE -512 272 -512 176 WIRE -608 288 -608 208 WIRE -544 288 -608 288 WIRE 384 288 384 256 WIRE 512 288 512 256 WIRE 1104 288 1104 240 WIRE 1200 288 1200 240 WIRE -400 304 -400 208 WIRE -400 304 -480 304 WIRE -288 304 -400 304 WIRE -240 304 -288 304 WIRE -32 304 -32 192 WIRE -688 320 -688 272 WIRE -608 320 -688 320 WIRE -544 320 -608 320 WIRE 688 336 688 240 WIRE -688 368 -688 320 WIRE -608 384 -608 320 WIRE -512 400 -512 336 WIRE 384 400 384 352 WIRE 512 400 512 352 WIRE -32 432 -32 384 WIRE 32 432 32 320 WIRE -608 480 -608 448 WIRE -688 496 -688 448 WIRE 384 512 384 464 WIRE 512 512 512 464 WIRE 288 608 288 304 WIRE 384 608 384 576 WIRE 384 608 288 608 WIRE 512 608 512 576 WIRE 560 608 512 608 WIRE 384 656 384 608 WIRE 512 656 512 608 WIRE 384 752 384 720 WIRE 512 752 512 720 WIRE -160 800 -160 160 WIRE 288 800 288 608 WIRE 288 800 -160 800 WIRE 288 848 288 800 WIRE 384 848 384 816 WIRE 512 848 512 816 WIRE 688 912 688 416 WIRE 896 912 688 912 WIRE 384 960 384 912 WIRE 448 960 384 960 WIRE 512 960 512 912 WIRE 512 960 448 960 WIRE 688 960 688 912 WIRE 288 992 288 928 WIRE 448 992 448 960 WIRE 800 1104 800 1072 WIRE 896 1120 896 912 WIRE 896 1120 832 1120 WIRE 448 1136 448 1072 WIRE 688 1136 688 1040 WIRE 688 1136 448 1136 WIRE 768 1136 688 1136 WIRE 880 1152 832 1152 WIRE 896 1152 880 1152 WIRE 800 1216 800 1168 FLAG -400 -176 PRail FLAG 176 112 PRail FLAG -400 32 0 FLAG 288 128 OUT FLAG 752 16 PRail FLAG 800 1072 PRail FLAG 960 0 0 FLAG 32 432 0 FLAG 176 256 0 FLAG -32 432 Half FLAG -288 304 Half FLAG -688 496 0 FLAG -688 160 PRail FLAG 288 992 Half FLAG 880 0 Half FLAG 752 160 0 FLAG 800 1216 0 FLAG 880 1152 Half FLAG 560 608 Half FLAG 1200 288 0 FLAG 1200 -112 PRail FLAG 608 -64 Half FLAG -512 400 0 FLAG -512 176 PRail FLAG -608 480 0 FLAG -288 16 0 FLAG 1104 288 0 SYMBOL voltage -400 -112 R0 WINDOW 123 0 0 Left 0 WINDOW 39 -147 49 Left 2 WINDOW 0 -78 25 Left 2 WINDOW 3 19 2 Left 2 SYMATTR SpiceLine Rser=0.1 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL res 272 208 R0 SYMATTR InstName R3 SYMATTR Value 3300 SYMBOL res 272 832 R0 SYMATTR InstName R4 SYMATTR Value 2200 SYMBOL diode 368 288 R0 SYMATTR InstName D1 SYMATTR Value 1N914 SYMBOL diode 368 848 R0 SYMATTR InstName D2 SYMATTR Value 1N914 SYMBOL diode 496 288 R0 SYMATTR InstName D3 SYMATTR Value 1N914 SYMBOL diode 496 848 R0 SYMATTR InstName D4 SYMATTR Value 1N914 SYMBOL res 432 112 R0 SYMATTR InstName R5 SYMATTR Value 75k SYMBOL res 432 976 R0 SYMATTR InstName R6 SYMATTR Value 75k SYMBOL res 704 432 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R8 SYMATTR Value 10k SYMBOL res 704 1056 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R9 SYMATTR Value 10k SYMBOL res 560 -224 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 2200 SYMBOL res 896 -48 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R10 SYMATTR Value 470k SYMBOL cap 944 -128 R0 SYMATTR InstName C4 SYMATTR Value 2000n SYMBOL diode 624 -112 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D5 SYMATTR Value 1N914 SYMBOL diode 672 -192 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D6 SYMATTR Value 1N914 SYMBOL res -48 -128 R0 SYMATTR InstName R1 SYMATTR Value 15k SYMBOL cap -48 16 R0 SYMATTR InstName C1 SYMATTR Value 10n SYMBOL cap 16 256 R0 SYMATTR InstName C2 SYMATTR Value 10n SYMBOL res -48 288 R0 WINDOW 3 34 76 Left 2 SYMATTR Value 15k SYMATTR InstName R2 SYMBOL res 1040 -80 R0 SYMATTR InstName R11 SYMATTR Value 10k SYMBOL OpAmps\\LT1058 800 1072 M0 WINDOW 0 29 34 Left 2 SYMATTR InstName U5 SYMBOL OpAmps\\LT1058 176 112 R0 SYMATTR InstName U1 SYMBOL OpAmps\\LT1058 752 16 M0 WINDOW 0 39 6 Left 2 WINDOW 3 33 33 Left 2 SYMATTR InstName U2 SYMBOL OpAmps\\LT1058 -512 240 R0 SYMATTR InstName U3 SYMBOL res -704 176 R0 SYMATTR InstName R12 SYMATTR Value 10k SYMBOL res -704 352 R0 SYMATTR InstName R13 SYMATTR Value 10k SYMBOL res 1184 -80 R0 SYMATTR InstName R14 SYMATTR Value 4.7k SYMBOL zener 1216 240 R180 WINDOW 0 24 64 Left 2 WINDOW 3 -149 63 Left 2 SYMATTR InstName D7 SYMATTR Value BZX84C6V2L SYMATTR Description Diode SYMATTR Type diode SYMBOL cap 912 224 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C3 SYMATTR Value 100n SYMBOL cap -624 384 R0 SYMATTR InstName C5 SYMATTR Value 100n SYMBOL polcap -304 -96 R0 SYMATTR InstName C7 SYMATTR Value 100µ SYMBOL cap 1088 176 R0 SYMATTR InstName C8 SYMATTR Value 100n SYMBOL diode 368 400 R0 SYMATTR InstName D8 SYMATTR Value 1N914 SYMBOL diode 496 400 R0 SYMATTR InstName D9 SYMATTR Value 1N914 SYMBOL diode 368 656 R0 SYMATTR InstName D10 SYMATTR Value 1N914 SYMBOL diode 496 656 R0 SYMATTR InstName D11 SYMATTR Value 1N914 SYMBOL diode 368 512 R0 SYMATTR InstName D12 SYMATTR Value 1N914 SYMBOL diode 496 512 R0 SYMATTR InstName D13 SYMATTR Value 1N914 SYMBOL diode 368 752 R0 SYMATTR InstName D14 SYMATTR Value 1N914 SYMBOL diode 496 752 R0 SYMATTR InstName D15 SYMATTR Value 1N914 TEXT -352 872 Left 2 !.tran 0 2 1 startup TEXT 992 704 Left 2 ;EPW SED OCT 2024 TEXT 992 744 Left 2 ;DIODE BRIDGE VARIOLOSSER AGC FOR WIEN BRDG OSC TEXT 992 784 Left 2 ;Single rail conversion by Edward Rawde, Oct 20 2024

You still need a multiplier of some sort to twaek loop gain. And multipliers are nonlinear!

My vote would go to a comparator sensing the peak of the AC waveform, and reducing the DC bias current a smidge each time it fires.

Changing the current on the peaks causes minimal change to the phase, assuming that you do it right, and the tiny nonlinearity goes away in between, rather than causing distortion all through the cycle.

Some months back, we talked about an audio amp design of Jim T’s that set it’s quiescent bias that way, except switching near the zero crossing. Turns out to be a startlingly good approach.

Cheers

Phil Hobbs

.

People don't much use Hajimiri for phase-noise estimates (Leeson being adequate and simpler), but this is relevant to the audio-amp question.

Joe Gwinn

Looks like a good read, thanks.

Cheers

Phil Hobbs

DC bias current of what?

An active device, specifically a bjt. You get transconductance proportional to collector current.

Cheers

Phil Hobbs

But it's precisely the non-linearity you want, where you want it, if you spend enough on a good quality multiplier.

That's actually unwise. Good quality Wein bridges get the harmonic content of their output about 120dB below the fundamental.

Comparators pull a spike of current from the supplies whenever they fire, which spread higher harmonics of the output frequency all around the circuit. Full wave rectifiers generate less harmonic noise in the power rails. You still need lots of power supply rejection, but lower levels of noise on the power rails mean you need less.

And the AD734 - and other well-designed Gilbert multipliers exploit that to the hilt.

How about using a lamp as the gain-controlling element at a low signal level, so it doesn't self-heat, then heating it with superimposed D.C. from some sort of voltage comparator/rectifier which is separate from the signal path ?

It might have to be in some sort of bridge circuit, so that any noise on the heating current doesn't appear in the signal.

How about the 'good old' lamp (LED) + LDR method? But I cannot find any linearity data of the LDR resistance itself (at constant illumination), important for harmonic distortion. Most searches end up in the audiophoolery corner.

Arie

My recollection is that they were good enough for amateur use but not up to professional quality (hence professional mixing desks not using them, despite how useful they might have been). I seem to remember they had distortion levels around 0.5%, but that might not be exactly right.

The photoconductive cells had a rather strange recovery curve which exhibited two time constants. When they were plunged from light into darkness their resistance rapidly increased to a percentage of the dark value, then slowly crept upwards to the final dark value over tens of seconds or even minutes.

If rapid control was needed, a pair of photoconductive cells was wired as a potential divider so that the much faster decrease in conductivity of one cell, when it was illuminated, would short-circut the other one while it recovered slowly after being darkened.

Optocoupler? There might be more detailed manufacturer data available for an appropriate device.