# Simulation of inverting summing amplifier not working and finding it's parameters

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:

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