I have a project to create circuit of inverting summing amplifier with 3 inputs and a simulation on it. I am using capture (OrCAD Pspice) and so far I got to the circuit you see below, at the end of this wall of text. The circuit is supposed to have the following parameters:

Av1 = 1
Av2 = 2
Av3 = 3
f max = 20kHz

And that's it. Since I do not have the value of any resistors I should use in my circuit I have picked a random number - in my case:

Rf = -6k Ohm
Ri1 = 6k Ohm
Ri2 = 3k Ohm
Ri3 = 2k Ohm

Please note that I am not even sure that Rf can have negative value, but if it doesn't I am not really sure how I can make the voltage gain positive as the equation is:

Av1 = -(Rf/Ri1)

The second problem is that I can not get the simulation to work. I am not sure which one should I use (probably the AC sweep?), but I have tried using Time Domain, DC Sweep, AC sweep, Bias points. Not even one of them worked. I tried changing the sources between AC and DC, but I keep getting flat only lines. The only thing that works in my simulation is that with increasing the voltage through resistors their current increases. I am pretty sure the simulation I get should look something like the one shown here - I tried to make my circuit diagram pretty much the same to the one in that post, but it still doesn't work.

Here is the diagram itself: enter image description here

  • \$\begingroup\$ I'm getting the impression that both your 12V rails are positive... \$\endgroup\$
    – Dzarda
    Commented Jan 19, 2014 at 21:45
  • 1
    \$\begingroup\$ And positive feedback. And a negative resistor. Definitely a few things to fix here. \$\endgroup\$
    – Ben Voigt
    Commented Jan 19, 2014 at 22:06
  • 1
    \$\begingroup\$ Compare your circuit to electronics.stackexchange.com/questions/58228/… \$\endgroup\$
    – Ben Voigt
    Commented Jan 19, 2014 at 22:10

1 Answer 1


Here's the pin-out of a 741: -

enter image description here

Please note the following: -

  • Positive supply is to pin 7 not pin 5
  • Negative supply is to pin 4 not pin 1
  • Negative feedback is from output to the inverting input NOT the non-inverting input
  • The gain is inverting i.e. the minus sign is associated with the gain being inverted as in +1v becomes -1v therefore Rf is a positive value.
  • \$\begingroup\$ Thank you! I have fixed my mistakes, but I still can not get any changes in the simulation. Here is how my circuit looks right now - I can see that there is a flowing current as all of the resistors change their voltage (comparing the voltage on both pins), but I can not get the sine wave that I showed in my first post. I have tried getting it on "out2" and "out" (although it should not be there), but still nothing. Do you have any idea what might be the problem? P.S. Do not mind the green circle \$\endgroup\$
    – A Petrov
    Commented Jan 20, 2014 at 10:23
  • \$\begingroup\$ @r0Lf try adjusting the amplitudes - at the moment you will get a massive signal that clips. Set two signals to zero amplitude and adjust one signal to check that it looks ok. \$\endgroup\$
    – Andy aka
    Commented Jan 20, 2014 at 10:26
  • \$\begingroup\$ Apparently the green circle was actually very important - I had to change the element, since there was no information in it for the simulation to work. Thank you! \$\endgroup\$
    – A Petrov
    Commented Jan 20, 2014 at 14:09

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