I am trying to make sense of the difference in non-inverting and inverting op-amp based audio "mixers". The circuits are basically summing op-amp amplifiers. Here is the part which I do not understand. (Figure from simulations below)
I simulated both and the main difference is that in the non-inverting case, the other sources act like shorts, but in inverting configurations they do not. Why is that so? Can someone explain?
So basically if I am summing 3 signals from sources and only one is generating a sinusoidal signal with 100 mV amplitude, I get 33 mV at the output of the non-inverting configuration and in the inverting case, I have 100 mV at the output. Why?
Best Answer
In the top amplifier the summing junction is a virtual ground so the feedback current exactly matches the current into the virtual ground and the output is the inverted sum of the input voltages (modified by the resistor ratio, - here unmodified because all the resistors are the same)
In the bottom amplifier the 470K does basically nothing, so I'm going to ignore it,
The three inputs flow into a three-way voltage divider so given that the resistors are equal they get averaged so 0,0,100mV gets you 33.3mV
the amplifier just buffers that gives a 33.3mV output.
If you want 100mV from the non-inverting amplifier configure it to have a gain of 3. eg: 2K from output to inverting input and 1K from there to ground.