The first op amp's output is 18 V and second's output is 27 V but when I sum those two outputs with a summing op amp, it gives -8.53 V at output. What is the wrong with my circuit? Am I not supposed to read "45 V" at output?

R1 = 1k, R2 = 1k, (non-inverting amplifier)

R3 = 2k, R4 = 1k, (non-inverting amplifier)

R5 = 1k, R6 = 1k, R7=1K (adder)

  • \$\begingroup\$ You are certainly not supposed to see a voltage outside the op-amp supply rails! \$\endgroup\$ – Brian Drummond Nov 2 '13 at 12:26
  • \$\begingroup\$ Is the simulation model behind those op-amps an ideal op-amp model or a real circuit model of a '741? \$\endgroup\$ – The Photon Nov 2 '13 at 15:35
  • \$\begingroup\$ It's ideal I think..The simulation software is ISIS. \$\endgroup\$ – whocares Nov 2 '13 at 15:40

The adder circuit does add but it inverts the summation about the voltage applied to it's non-inverting input. So an input of A and an input of B becomes C - (A plus B), where C is the voltage on the non-inverting input.

Another problem is that none of the opamps have power rails connected.

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