total closed loop gain of summing amplifier

[sorbik]

I'm trying to find the closed loop gain of the circuit overall, but now sure how to calculate it. The op amp becomes saturated at 16V when the VCC amount is 18V. From 18/16, that gives 1.125 gain.  However, the gain for the 10V source branch is 0.33 and the gain for the 5V source branch is 3.3. Is there a way to calculate the overall gain of a summing amplifier without forcing the op-amp into sautration?

[johnsold]

1. The gain is not well defined for a circuit in saturation. Saturation is a non-linear process and gain is typically applied to linear circuits.

2. Gain is not simply defined for a summing amplifier. The gain for each input may be different.  A simple manual calculation of the transfer function will show this. (That calculation assumes that the amplifier is operating in the linear region.)

That calculation is left to the student.

3. The typical way to measure gain in a multiple input circuit is to set one input to zero and measure the gain from the other input. The reverse the process by setting the second input to zero and measuring the gain from the first. The output will be the superposition of the outputs from the two steps, so long as the amplifire stays in the linear region in both the cases of one input being zero and in the combined case with both inputs non-zero.

In your circuit V4 saturates the amplifier whether VI is present or not. 

It appears that this is some kind of school assignment. I suggest that you first do the manual calculation mentioned in item 2. Verify the saturation for the values in the image. Show that the circuit works as expected for suitable values for the inputs.

Lynn