Five transistor version of the low distortion sine-wave oscillator

Edward Rawde posted an eight-transistor low distortion sine wave oscillator circuit recently, and John May pointed out that you could leave out half the transistors.
I couldn't immediately see exactly how either of the circuits worked, though I could get the simulations to run under LTSpice and see roughly what was going on.
I've now dug a bit deeper. Here is a five transistor version of John May's four transistor version.
Version 4
SHEET 1 3020 2548
WIRE 80 -208 -288 -208
WIRE 800 -208 160 -208
WIRE -288 -96 -288 -208
WIRE 80 -96 -288 -96
WIRE 400 -96 160 -96
WIRE -288 32 -288 -96
WIRE -176 32 -288 32
WIRE -32 32 -96 32
WIRE 48 32 -32 32
WIRE 208 32 128 32
WIRE 256 32 208 32
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WIRE 464 32 400 32
WIRE 608 32 544 32
WIRE 656 32 608 32
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WIRE 800 32 720 32
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WIRE 400 160 320 160
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WIRE 624 208 624 176
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WIRE 272 1184 240 1184
WIRE 368 1184 336 1184
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WIRE 576 1184 544 1184
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FLAG 224 208 0
FLAG 624 208 0
FLAG 832 160 vout
FLAG -272 1168 vcc
FLAG -272 1232 vee
FLAG -2528 1232 0
FLAG -2480 1104 vcc
FLAG -2480 1328 vee
FLAG -144 128 vcc
FLAG -144 192 vee
FLAG 288 128 vcc
FLAG 288 192 vee
FLAG 688 128 vcc
FLAG 688 192 vee
FLAG 336 464 0
FLAG 416 528 vcc
FLAG 352 528 vee
FLAG -2224 656 vcc
FLAG -1968 144 vcc
FLAG -2224 720 vee
FLAG -1360 480 vcc
FLAG -2432 496 vcc
FLAG -2432 1008 0
FLAG -1360 1440 0
FLAG -704 256 vcc
FLAG -704 320 vee
FLAG -816 336 0
FLAG -1664 1504 vee
FLAG -1904 1888 0
FLAG -1664 1040 vcc
FLAG -2160 1040 vcc
FLAG -208 1328 0
FLAG -2544 960 0
FLAG 816 1168 vee
FLAG -2240 1488 vee
SYMBOL res -80 16 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 10K
SYMBOL res 144 16 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 16K
SYMBOL res 560 16 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R3
SYMATTR Value 16K
SYMBOL cap 320 16 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 10n
SYMBOL cap 720 16 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C2
SYMATTR Value 10n
SYMBOL res 176 -224 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 10K
SYMBOL res 176 -80 M270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R5
SYMATTR Value 82k
SYMBOL cap -256 976 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C3
SYMATTR Value 1000n
SYMBOL res 32 1200 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R10
SYMATTR Value 680K
SYMBOL diode 176 1200 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D1
SYMATTR Value 1N914
SYMBOL res -48 960 R0
SYMATTR InstName R11
SYMATTR Value 40.2K
SYMBOL res 560 592 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R18
SYMATTR Value 10K
SYMBOL res 416 432 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R19
SYMATTR Value 10k
SYMBOL diode -16 656 M0
SYMATTR InstName D2
SYMATTR Value 1N914
SYMBOL diode 224 656 M0
SYMATTR InstName D12
SYMATTR Value 1N914
SYMBOL diode 368 656 R0
SYMATTR InstName D13
SYMATTR Value 1N914
SYMBOL diode 816 656 M0
SYMATTR InstName D14
SYMATTR Value 1N914
SYMBOL res -48 736 R0
SYMATTR InstName R13
SYMATTR Value 68k
SYMBOL res 192 736 R0
SYMATTR InstName R14
SYMATTR Value 68k
SYMBOL res 400 736 M0
SYMATTR InstName R15
SYMATTR Value 68k
SYMBOL res 784 736 R0
SYMATTR InstName R16
SYMATTR Value 68k
SYMBOL voltage -2480 1088 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 15
SYMBOL voltage -2480 1232 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V4
SYMATTR Value 15
SYMBOL OpAmps\LT1679 -144 160 R0
SYMATTR InstName U5
SYMBOL OpAmps\LT1679 288 160 R0
SYMATTR InstName U6
SYMBOL OpAmps\LT1679 688 160 R0
SYMATTR InstName U8
SYMBOL res -112 272 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R8
SYMATTR Value 10k
SYMBOL res -320 272 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R9
SYMATTR Value 560
SYMBOL cap -480 272 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C8
SYMATTR Value 1000n
SYMBOL res -272 1088 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R6
SYMATTR Value 100k
SYMBOL cap -304 1056 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C4
SYMATTR Value 10000n
SYMBOL OpAmps\LT1679 384 528 R90
WINDOW 3 37 22 VRight 2
SYMATTR InstName U1
SYMBOL pnp -2032 736 M180
WINDOW 3 84 0 Left 2
SYMATTR Value 2N3906
SYMATTR InstName Q4
SYMBOL res -1984 208 R0
SYMATTR InstName R12
SYMATTR Value 82k
SYMBOL res -1760 384 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R25
SYMATTR Value 2.7Meg
SYMBOL cap -2176 544 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C7
SYMATTR Value 10p
SYMBOL npn -2096 1152 M0
SYMATTR InstName Q1A
SYMATTR Value NSS40301MDR2G
SYMBOL npn -1728 1152 R0
SYMATTR InstName Q1B
SYMATTR Value NSS40301MDR2G
SYMBOL npn -1424 1232 R0
SYMATTR InstName Q2B
SYMATTR Value NSS40301MDR2G
SYMBOL npn -1968 1712 R0
SYMATTR InstName Q2A
SYMATTR Value NSS40301MDR2G
SYMBOL res -1552 1632 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R28
SYMATTR Value 100k
SYMBOL res -1376 560 R0
SYMATTR InstName R29
SYMATTR Value 100k
SYMBOL res -2448 528 R0
SYMATTR InstName R32
SYMATTR Value 10k
SYMBOL res -2448 848 R0
SYMATTR InstName R33
SYMATTR Value 15k
SYMBOL cap -944 256 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C12
SYMATTR Value 1000n
SYMBOL res -656 144 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R7
SYMATTR Value 14k
SYMBOL cap -688 32 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C5
SYMATTR Value 10p
SYMBOL cap -1600 384 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C6
SYMATTR Value 100n
SYMBOL res -1648 1456 R180
WINDOW 0 31 76 Left 2
WINDOW 3 31 40 Left 2
SYMATTR InstName R17
SYMATTR Value 220k
SYMBOL OpAmps\LT1679 -704 288 R0
SYMATTR InstName U2
SYMBOL OpAmps\LT1679 -272 1200 M0
SYMATTR InstName U3
SYMBOL OpAmps\LT1679 -2224 688 R0
SYMATTR InstName U4
SYMBOL cap -2560 864 R0
SYMATTR InstName C9
SYMATTR Value 10n
SYMBOL res -2224 1440 R180
WINDOW 0 31 76 Left 2
WINDOW 3 31 40 Left 2
SYMATTR InstName R20
SYMATTR Value 360k
SYMBOL npn -1424 880 R0
SYMATTR InstName Q3
SYMATTR Value 2N3904
SYMBOL diode 272 1200 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D3
SYMATTR Value 1N914
SYMBOL diode 368 1200 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D4
SYMATTR Value 1N914
SYMBOL diode 480 1200 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D5
SYMATTR Value 1N914
SYMBOL diode 576 1200 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D6
SYMATTR Value 1N914
SYMBOL diode 688 1200 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D7
SYMATTR Value 1N914
TEXT -2792 1560 Left 2 !.tran 0 10 0 1u startup
TEXT -2792 1504 Left 2 !.options plotwinsize=0 numdgt=15
TEXT -2248 -112 Left 2 ;Low distortion 1KHz oscillator. Edward Rawde 30 March 2025.\nBased on designs by JM and BS. Re-worked by JM, getting rid of half the transistors,\nand again by Bill Sloman, making Q1A/B and Q2A/B close matched transistor pairs\nand adding the cascode transistor at Q3, 12th April 2025..
TEXT -2896 16 Left 2 !.MODEL NSS40301MDR2G NPN\n+is=6.87023e-12 bf=445.496 nf=1.08926 vaf=60.529\n+ikf=7.23313 ise=2.38192e-09 ne=4 br=23.6872\n+nr=1.10701 var=8.89608 ikr=1.25064 isc=1e-16 \n+nc=1.13174 rb=905.334,irb=2.30349e-07 rbm=1e-10\n+re=0.00600548 rc=0.0300274 xtb=1.2219 xti=4,\n+eg=1.05 cje=3.80477e-10 vje=0.912237 mje=0.397194,\n+tf=5.90916e-10,xtf=0.0998483,vtf=7.09172,itf=0.010481,\n+cjc=8.35472e-11 vjc=0.702862 mjc=0.43477 xcjc=0.899998\n+fc=0.414631 cjs=0 vjs=0.75 mjs=0.5 \n+tr=5.64658e-08 ptf=0,kf=0 af=1
The circuit works by adding a portion of the sine wave output from U6 inside the ring oscillator to bigger DC current, and delivering as current from Q4 into an asymmetric current mirror formed by the dual transistor Q2A and Q2B.
Q2A and Q1A turned it into a voltage at V(n022) - once the circuit has settled this sits around 904mV about ground with a 727uV 1kz ripple, driven by the 1.83uA 1kHz current ripple going through Q2A
The actual voltage is set by the stable (but adjustable) base-emitter voltage of Q2A. The stable base-emitter voltage of Q1B applies a slightly smaller voltage (but with much the same variation) to the base of Q1B, whose output then has the 1.63uA 1kHz current ripple needed to keep the ring-oscillator running at a stable amplitude.
Q3 is a cascode to minimise Early effect distortion of the current coming out of Q1B.
The output of U3 adjusts base emitter voltage drop of Q2A by changing the current through it via R28.
During startup Q1B delivers more current to build up the oscillation.
The circuit take about 5 seconds to stabilise, and the output sine wave has a little harmonic content - the worst offender is a 4hKz spur about 140dB below the fundamental. Or at least this is what LTSpice 17.0.36 tells me.
At this distortion level LTSpice isn't all that credible.
The only other significant change to the circuit is the string of six diode (D1, D3, D4, D5, D6, and D7) which are intended to balance out the temperature dependent forward voltage drops across the rectifier diodes D2, D12, D13 and D14, which sit in series with a couple of volts of signal, rather less than the 15V from the negative rail.
--
Bill Sloman, Sydney