I am using multisim to simulate an inverting summing op amp (LM741CN). I calculate my expected Vout to be the following: -Vout = ((20/2.5)V1) + ((20/5)V2) + ((20/10)V3) + ((20/20)V4). According to my calculations, my Vout with binary input of 0001 should be -1. However, when I measure in Multisim, that is not what I get. Are my calculations incorrect or am I measuring incorrectly?

circuit in multisim

  • \$\begingroup\$ It will be helpful for other people that may attempt to help you if you updated your question with the updated circuits. \$\endgroup\$ – Wesley Lee Oct 18 '16 at 1:43

The 741 can provide about 20mA, but you're asking for much more than that. Bump all your resistor values by a factor of at least 20, if not 100. Real circuits are not built with resistors that small. Also, the outputs of the 741 are not rail to rail, and can only reach within a few volts of the rails. Change the opportunity amp power supplies to +/- 15

Lastly, voltmeters need to be wired in parallel, not in series.

  • \$\begingroup\$ Thanks for the speedy reply. I bumped all resistor values up by 20 and then by 100 - my Vout was 0 constantly (after closing all switches). \$\endgroup\$ – Markovnikov Oct 17 '16 at 23:08
  • \$\begingroup\$ @Markovnikov, see my edit \$\endgroup\$ – Scott Seidman Oct 17 '16 at 23:11
  • \$\begingroup\$ And make sure the switch is closed \$\endgroup\$ – Scott Seidman Oct 17 '16 at 23:13
  • \$\begingroup\$ I bumped all the resistors up quite a bit and changed the op amp power supplies to +/- 15 and I still get 0 volts as my Vout. \$\endgroup\$ – Markovnikov Oct 17 '16 at 23:18
  • \$\begingroup\$ !new circuit \$\endgroup\$ – Markovnikov Oct 17 '16 at 23:18

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