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I am trying to make a analog circuit that can multiply two numbers which are input into the circuit as voltages. I came across the following schematic:

enter image description here

Here V+ and V- are +15 and -15 V respectively. The transfer function for the circuit is \$V_{out} = \frac{V_1V_2}{V_R}\$.

Before assembling the circuit physically, I decided to simulate it in LTSpice. I made the following circuit:

enter image description here

        Version 4
    SHEET 1 1356 760
    WIRE -16 0 -48 0
    WIRE 112 0 48 0
    WIRE -272 16 -368 16
    WIRE 48 16 -208 16
    WIRE -368 64 -368 16
    WIRE 48 64 48 16
    WIRE -208 80 -240 80
    WIRE -48 80 -48 0
    WIRE 0 80 -48 80
    WIRE 112 96 112 0
    WIRE 112 96 64 96
    WIRE 160 96 112 96
    WIRE -240 112 -240 80
    WIRE 0 112 -32 112
    WIRE -32 128 -32 112
    WIRE 128 128 96 128
    WIRE 320 128 288 128
    WIRE 432 128 400 128
    WIRE 608 128 576 128
    WIRE 720 128 688 128
    WIRE 944 128 912 128
    WIRE 1056 128 1024 128
    WIRE 672 160 384 160
    WIRE 1008 160 672 160
    WIRE 1296 160 1008 160
    WIRE 384 176 384 160
    WIRE 672 176 672 160
    WIRE 1008 176 1008 160
    WIRE 1296 176 1296 160
    WIRE 240 192 240 96
    WIRE 288 192 288 128
    WIRE 288 192 240 192
    WIRE 336 192 288 192
    WIRE 576 192 576 128
    WIRE 624 192 576 192
    WIRE 912 192 912 128
    WIRE 960 192 912 192
    WIRE 432 208 432 128
    WIRE 432 208 400 208
    WIRE 496 208 432 208
    WIRE 576 208 576 192
    WIRE 576 208 560 208
    WIRE 720 208 720 128
    WIRE 720 208 688 208
    WIRE 1056 208 1056 128
    WIRE 1056 208 1024 208
    WIRE 1120 208 1056 208
    WIRE -16 224 -48 224
    WIRE 112 224 48 224
    WIRE 336 224 304 224
    WIRE 624 224 592 224
    WIRE 720 224 720 208
    WIRE 768 224 720 224
    WIRE 912 224 912 192
    WIRE 912 224 848 224
    WIRE 960 224 928 224
    WIRE 304 240 304 224
    WIRE 592 240 592 224
    WIRE 592 240 368 240
    WIRE 656 240 592 240
    WIRE 928 240 928 224
    WIRE 928 240 656 240
    WIRE 992 240 928 240
    WIRE 1200 240 992 240
    WIRE -272 256 -368 256
    WIRE 48 256 -208 256
    WIRE 1200 256 1200 240
    WIRE -368 288 -368 256
    WIRE 48 288 48 256
    WIRE -208 304 -240 304
    WIRE -48 304 -48 224
    WIRE 0 304 -48 304
    WIRE 112 320 112 224
    WIRE 112 320 64 320
    WIRE 160 320 112 320
    WIRE 240 320 240 192
    WIRE -240 336 -240 304
    WIRE 0 336 -32 336
    WIRE -32 352 -32 336
    WIRE 112 352 96 352
    WIRE 112 384 112 352
    WIRE -272 496 -304 496
    WIRE -144 496 -208 496
    WIRE 0 512 -32 512
    WIRE 112 512 80 512
    WIRE -528 528 -624 528
    WIRE -208 528 -464 528
    WIRE 64 528 -208 528
    WIRE -624 560 -624 528
    WIRE -208 560 -208 528
    WIRE 64 560 64 528
    WIRE -464 576 -496 576
    WIRE -304 576 -304 496
    WIRE -256 576 -304 576
    WIRE -32 576 -32 512
    WIRE -32 576 -64 576
    WIRE 16 576 -32 576
    WIRE -144 592 -144 496
    WIRE -144 592 -192 592
    WIRE -64 592 -64 576
    WIRE 112 592 112 512
    WIRE 112 592 80 592
    WIRE 144 592 112 592
    WIRE 240 592 240 320
    WIRE 240 592 224 592
    WIRE -496 608 -496 576
    WIRE -256 608 -288 608
    WIRE 16 608 -16 608
    WIRE -288 624 -288 608
    WIRE -16 624 -16 608
    WIRE -224 656 -224 624
    WIRE 48 656 48 624
    WIRE 48 656 -224 656
    WIRE 64 656 48 656
    WIRE 144 656 128 656
    WIRE 144 672 144 656
    FLAG -32 128 0
    FLAG -240 112 0
    FLAG -32 352 0
    FLAG -240 336 0
    FLAG 304 240 0
    FLAG 592 240 0
    FLAG 928 240 0
    FLAG 1120 208 Vout
    FLAG -368 144 0
    FLAG -368 368 0
    FLAG 1296 256 0
    FLAG 1200 320 0
    FLAG -288 624 0
    FLAG -496 608 0
    FLAG -624 640 0
    FLAG -16 624 0
    FLAG 144 736 0
    FLAG 128 208 0
    FLAG 112 432 0
    SYMBOL OpAmps\\LM308 32 32 R0
    SYMATTR InstName U1
    SYMBOL res -32 64 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R1
    SYMATTR Value 4.32k
    SYMBOL voltage -112 80 R90
    WINDOW 0 -32 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value 1
    SYMBOL res 256 80 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R2
    SYMATTR Value 4.32k
    SYMBOL diode -16 16 R270
    WINDOW 0 32 32 VTop 2
    WINDOW 3 0 32 VBottom 2
    SYMATTR InstName D1
    SYMATTR Value 1N4148
    SYMBOL OpAmps\\LM308 32 256 R0
    SYMATTR InstName U2
    SYMBOL res -32 288 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R3
    SYMATTR Value 4.32k
    SYMBOL voltage -112 304 R90
    WINDOW 0 -32 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V2
    SYMATTR Value 2
    SYMBOL res 256 304 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R4
    SYMATTR Value 4.32k
    SYMBOL diode -16 240 R270
    WINDOW 0 32 32 VTop 2
    WINDOW 3 0 32 VBottom 2
    SYMATTR InstName D2
    SYMATTR Value 1N4148
    SYMBOL OpAmps\\LM308 368 144 R0
    SYMATTR InstName U3
    SYMBOL res 416 112 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R5
    SYMATTR Value 4.32k
    SYMBOL diode 496 224 R270
    WINDOW 0 32 32 VTop 2
    WINDOW 3 0 32 VBottom 2
    SYMATTR InstName D3
    SYMATTR Value 1N4148
    SYMBOL OpAmps\\LM308 656 144 R0
    SYMATTR InstName U4
    SYMBOL res 704 112 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R6
    SYMATTR Value 4.32k
    SYMBOL res 864 208 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R7
    SYMATTR Value 4.32k
    SYMBOL OpAmps\\LM308 992 144 R0
    SYMATTR InstName U5
    SYMBOL res 1040 112 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R8
    SYMATTR Value 4.32k
    SYMBOL Misc\\battery -368 48 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V3
    SYMATTR Value 15
    SYMBOL Misc\\battery -368 272 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V4
    SYMATTR Value 15
    SYMBOL Misc\\battery 1296 160 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V7
    SYMATTR Value 15
    SYMBOL Misc\\battery 1200 336 R180
    WINDOW 0 24 104 Left 2
    WINDOW 3 24 16 Left 2
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V8
    SYMATTR Value 15
    SYMBOL OpAmps\\LM308 -224 528 R0
    SYMATTR InstName U6
    SYMBOL res -288 560 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R9
    SYMATTR Value 4.32k
    SYMBOL voltage -368 576 R90
    WINDOW 0 -32 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName VR
    SYMATTR Value 2
    SYMBOL diode -272 512 R270
    WINDOW 0 32 32 VTop 2
    WINDOW 3 0 32 VBottom 2
    SYMATTR InstName D4
    SYMATTR Value 1N4148
    SYMBOL Misc\\battery -624 544 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V10
    SYMATTR Value 15
    SYMBOL res -48 576 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R10
    SYMATTR Value 4.32k
    SYMBOL OpAmps\\LM308 48 528 R0
    SYMATTR InstName U7
    SYMBOL res 96 496 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R11
    SYMATTR Value 4.32k
    SYMBOL res 240 576 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R12
    SYMATTR Value 4.32k
    SYMBOL Misc\\battery 144 752 R180
    WINDOW 0 24 104 Left 2
    WINDOW 3 24 16 Left 2
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V11
    SYMATTR Value 15
    SYMBOL cap -208 0 R90
    WINDOW 0 0 32 VBottom 2
    WINDOW 3 32 32 VTop 2
    SYMATTR InstName C1
    SYMATTR Value 10µ
    SYMBOL cap -208 240 R90
    WINDOW 0 0 32 VBottom 2
    WINDOW 3 32 32 VTop 2
    SYMATTR InstName C2
    SYMATTR Value 10µ
    SYMBOL cap -464 512 R90
    WINDOW 0 0 32 VBottom 2
    WINDOW 3 32 32 VTop 2
    SYMATTR InstName C3
    SYMATTR Value 10µ
    SYMBOL cap 128 640 R90
    WINDOW 0 0 32 VBottom 2
    WINDOW 3 32 32 VTop 2
    SYMATTR InstName C4
    SYMATTR Value 10µ
    SYMBOL cap 96 112 R90
    WINDOW 0 0 32 VBottom 2
    WINDOW 3 32 32 VTop 2
    SYMATTR InstName C5
    SYMATTR Value 10µ
    SYMBOL Misc\\battery 128 224 R180
    WINDOW 0 24 104 Left 2
    WINDOW 3 24 16 Left 2
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V5
    SYMATTR Value 15
    SYMBOL cap 96 336 R90
    WINDOW 0 0 32 VBottom 2
    WINDOW 3 32 32 VTop 2
    SYMATTR InstName C6
    SYMATTR Value 10µ
    SYMBOL Misc\\battery 112 448 R180
    WINDOW 0 24 104 Left 2
    WINDOW 3 24 16 Left 2
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V6
    SYMATTR Value 15
    TEXT -260 376 Left 2 !.tran 0 1 0 startup

The circuit is supposed to give me 8 V as output because V_1 is 4 V and V_2 is 2 V and V_R is 1 V, but I get the following output:

enter image description here

I added the capacitors hoping to get rid of that voltage spike but it didn't work. Irrespective of what input I give and even without the capacitors, I get the same output every time.

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1 Answer 1

4
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First, please read this thread.

Then, once you tidy up your schematic, you'll see that you shorted out the negative supplies of U3, U4, U5 to ground when you tried to ground the non-inverting inputs. You'll also see how you inserted a series capacitor in the rails of U1, U2, U6, U7. After correcting those, it won't hurt to reduce the clutter by adding only two supplies and only labels to the power pins of the opamps. The result is this and, you'll notice, the output is very close to 3*4/2=6:

it works now

I haven't completel tidy up the schematic sicne that is your job. And it looks like the solver needs the startup flag (or uic). Otherwise .op would have sufficed.

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