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What should I add here to make it work? The output should be 3 * 5 = 15V.

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  • \$\begingroup\$ by adding the same identical question 4 times, you somehow broke your question. Please repair it, we can't see anything. Also, whatever this is, you need to explain what you've tried and where you're stuck, and ask a precise question. \$\endgroup\$ Jan 14, 2021 at 8:12
  • \$\begingroup\$ What models are you using for D1 and D2? What do you expect those models to produce at the intermediate log voltages? What do you expect those intermediate voltages to produce at the output, given the model for D3 that you're using? This looks like a 'back of a napkin' sketch of how you might think about starting a mulitplier. However, you have some offset voltages to take into account, you need to understand the effect of those offsets. \$\endgroup\$
    – Neil_UK
    Jan 14, 2021 at 10:30

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what i should add here to make it work .... the output should be 35 =15v*

It looks like you have got U5's inputs swapped around so it's acting as an inverting comparator and not a linear circuit with positive gain: -

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You should also ensure that the op-amps have enough power supply headroom to be able to generate +15 volts on their output. Maybe retest but using smaller voltages that give a product that is less than 10 volts.


I also think that your output stage needs some refinement to make it a unity gain buffer. For instance, I believe that the 100 kΩ resistor (R7) should connect to 0 volts and the node between D3 and R7 should be the output that feeds the unity gain buffer.

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