3
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I am running a simulation on the Moog Minimoog Model D oscillator in LTSpice. The circuit with the models needed is attached below.

Mini MOOG Oscillator

The square wave and triangular wave generation work as expected. The problem I am having is with the capacitor C4 charging. The charging ramp does not go to 5V as per the original schematic description, but instead reaches only ~4V. The schematic description (section 2.3) reads:

Schematic Description

With no voltage applied on the base of the transistor Q3, 5uA is negatively charging the bottom side of capacitor C4. This ramp is buffered by the U6 and Q5 stage and presented at the comparator U5 negative input. When the ramp reaches 0V, the comparator triggers and produces a +10V pulse that turns on the JFET J1 and brings the bottom side of the capacitor back to 5V. Well, at least in theory.

I have checked the schematic many times and verified that it (including the models used) are the same as the Model D's. I have also simulated the circuit with a 5V-0V ramp at the input of the buffer U6, and this produces correct results.

If anyone has any ideas why there is a residual ~1V across the capacitor, and if there is a way to fix it, I would be grateful.

FullOscillator.asc

Version 4
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FLAG -1424 704 -10V
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WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
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SYMATTR Value 5
SYMBOL voltage 1024 0 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
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SYMATTR Value 10
SYMBOL voltage 1136 0 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
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SYMBOL res -464 -32 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
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SYMBOL res -320 -32 R90
WINDOW 0 0 56 VBottom 2
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WINDOW 3 32 56 VTop 2
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SYMATTR Value 10K
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SYMATTR Value 10K
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SYMBOL res 528 512 R180
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SYMATTR Value 7.5K
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SYMATTR Value 100K
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WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R16
SYMATTR Value 100K
SYMBOL cap -352 112 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 0.01µ
SYMATTR SpiceLine V=4 Irms=162m Rser=0.395426 Lser=0 mfg="KEMET" pn="C0402C103K7PAC" type="X5R"
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SYMATTR Value LM393A
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SYMATTR Value 47K
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WINDOW 3 32 56 VTop 2
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WINDOW 3 0 56 VBottom 2
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SYMATTR Value 100K
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WINDOW 3 0 32 VBottom 2
SYMATTR InstName D1
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SYMATTR Value 2K
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SYMATTR InstName R23
SYMATTR Value 100K
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SYMATTR Value 2Meg
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WINDOW 3 32 32 VTop 2
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SYMATTR Value 0.001µ
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SYMBOL diode -1568 704 R0
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL Opamps\\opamp2 -1056 -192 R180
SYMATTR InstName U5
SYMATTR Value LM393A
SYMBOL cap -1616 -80 R180
WINDOW 0 24 57 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C4
SYMATTR Value 0.001µ
SYMATTR Description Polarized Capacitor
SYMATTR Type polcap
SYMATTR SpiceLine V=6.3 Irms=25.7m Rser=3.62516 Lser=0 mfg="KEMET" pn="C0402C102K9RAC" type="X7R"
SYMBOL njf -1408 -64 R180
SYMATTR InstName J1
SYMATTR Value J112
SYMBOL res -992 -416 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R27
SYMATTR Value 1Meg
SYMBOL res -784 -368 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R28
SYMATTR Value 100
SYMBOL cap -1328 -176 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName C5
SYMATTR Value 100p
SYMATTR Description Polarized Capacitor
SYMATTR Type polcap
SYMATTR SpiceLine V=25 Irms=0 Rser=0 Lser=0 mfg="Murata" pn="GRM0335C1E101JA01" type="C0G"
SYMBOL cap -1024 -512 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C6
SYMATTR Value 18p
SYMATTR Description Polarized Capacitor
SYMATTR Type polcap
SYMATTR SpiceLine V=25 Irms=0 Rser=0 Lser=0 mfg="Murata" pn="GCM0335C1E180JA16" type="C0G"
SYMBOL npn -1168 48 R0
SYMATTR InstName Q5
SYMATTR Value 2N3904
SYMBOL Opamps\\opamp2 -1280 -48 R0
SYMATTR InstName U6
SYMATTR Value TL081
SYMBOL Opamps\\opamp2 -1424 608 M0
SYMATTR InstName U4
SYMATTR Value TL081
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SYMATTR InstName R26
SYMATTR Value 5K
TEXT 904 232 Left 2 !.tran 0 10ms 0 0.5ms startup
TEXT 904 288 Left 2 !.include TL081.301
TEXT 904 336 Left 2 !.include LM393A.5_1
TEXT 904 392 Left 2 ;.wave "C:\\Users\\Boris\\Desktop\\Sawtooth.wav" 16 44.1k V(saw)\n.wave "C:\\Users\\Boris\\Desktop\\Square.wav" 16 44.1k V(squ)\n.wave "C:\\Users\\Boris\\Desktop\\Bent.wav" 16 44.1k V(bent)
TEXT 912 528 Left 2 !.options cshunt = 1e-13
TEXT 904 480 Left 2 ;.wave "C:\\Users\\Boris\\Desktop\\Triangle.wav" 16 44.1k V(tri)

LM393A.5_1

* LM393A VOLTAGE COMPARATOR "MACROMODEL" SUBCIRCUIT
* CREATED USING PARTS VERSION 4.03 ON 03/07/90 AT 14:17
* REV (N/A)
* CONNECTIONS:   NON-INVERTING INPUT
*                | INVERTING INPUT
*                | | POSITIVE POWER SUPPLY
*                | | | NEGATIVE POWER SUPPLY
*                | | | | OPEN COLLECTOR OUTPUT
*                | | | | |
.SUBCKT LM393A   1 2 3 4 5
*
  F1    9  3 V1 1
  IEE   3  7 DC 100.0E-6
  VI1  21  1 DC .75
  VI2  22  2 DC .75
  Q1    9 21  7 QIN
  Q2    8 22  7 QIN
  Q3    9  8  4 QMO
  Q4    8  8  4 QMI
.MODEL QIN PNP(IS=800.0E-18 BF=2.000E3)
.MODEL QMI NPN(IS=800.0E-18 BF=1002)
.MODEL QMO NPN(IS=800.0E-18 BF=1000 CJC=1E-15 TR=807.4E-9)
  E1   10  4  9  4  1
  V1   10 11 DC 0
  Q5    5 11  4 QOC
.MODEL QOC NPN(IS=800.0E-18 BF=20.29E3 CJC=1E-15 TF=942.6E-12 TR=543.8E-9)
  DP    4  3 DX
  RP 3  4 46.3E3
.MODEL DX  D(IS=800.0E-18)
*
.ENDS

TL081.301

* TL081 OPERATIONAL AMPLIFIER "MACROMODEL" SUBCIRCUIT
* CREATED USING PARTS RELEASE 4.01 ON 06/16/89 AT 13:08
* (REV N/A)      SUPPLY VOLTAGE: +/-15V
* CONNECTIONS:   NON-INVERTING INPUT
*                | INVERTING INPUT
*                | | POSITIVE POWER SUPPLY
*                | | | NEGATIVE POWER SUPPLY
*                | | | | OUTPUT
*                | | | | |
.SUBCKT TL081    1 2 3 4 5
*
  C1   11 12 3.498E-12
  C2    6  7 15.00E-12
  DC    5 53 DX
  DE   54  5 DX
  DLP  90 91 DX
  DLN  92 90 DX
  DP    4  3 DX
  EGND 99  0 POLY(2) (3,0) (4,0) 0 .5 .5
  FB    7 99 POLY(5) VB VC VE VLP VLN 0 4.715E6 -5E6 5E6 5E6 -5E6
  GA    6  0 11 12 282.8E-6
  GCM   0  6 10 99 8.942E-9
  ISS   3 10 DC 195.0E-6
  HLIM 90  0 VLIM 1K
  J1   11  2 10 JX
  J2   12  1 10 JX
  R2    6  9 100.0E3
  RD1   4 11 3.536E3
  RD2   4 12 3.536E3
  RO1   8  5 150
  RO2   7 99 150
  RP    3  4 2.143E3
  RSS  10 99 1.026E6
  VB    9  0 DC 0
  VC    3 53 DC 2.200
  VE   54  4 DC 2.200
  VLIM  7  8 DC 0
  VLP  91  0 DC 25
  VLN   0 92 DC 25
.MODEL DX D(IS=800.0E-18)
.MODEL JX PJF(IS=15.00E-12 BETA=270.1E-6 VTO=-1)
.ENDS
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0
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After looking over the schematic there is one thing that I could see that could be affecting the simulation, you don't have real NPN's, but ideal spice NPN's. Put in the real NPN's and that might fix your problem. If it doesn't then I'll simulate the circuit and see if there is another problem.

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0
1
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Which original schematic description? According to FantasyJackPalance, the swing should be +3v to 0v:

MOOG Minimoog D

I was able to force a ~5v swing by changing C4 from X7R to C0G (lower parasitics) and the J112 to a J108 (lower beta) but this is technically a hack.

\$\endgroup\$
2
  • \$\begingroup\$ Hm, I haven't noticed that on the schematic. I have some difficulty to believe that the swing should be +3V, but then at +5V at the input of the comparator as the TL081 and 2N3904 only act a buffer. I've managed to track down the original explanations in the Minimoog manual: imgur.com/a/PLm5A5E \$\endgroup\$ – Boris Yanchev Aug 29 '19 at 15:42
  • \$\begingroup\$ Interesting. I wonder if someone has a real board they could 'scope to give us actual values. \$\endgroup\$ – rdtsc Aug 30 '19 at 12:53

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