Common mode voltage in op amps

Amanda,

I think you have it right. The term 'common mode' refers to the component of a signal that is equal, at two nodes or two branches. This can be a voltage or a current, depending on what's being described. The other component would be (might be) called the 'differential mode'. There are various terms used to describe these two components of a signal. In telephony, the 'common mode' part is called 'longitudinal', whereas the differential part is referred to as the 'metallic' component. You say potato, I say potahtoe...

In the inverting mode configuration, for an opamp, the + lead is held to some static voltage. Sometimes this is 'circuit common' (aka ground), but it doesn't have to be. So, there is no 'common mode' component, between the + and - inputs, because the + side doesn't move (with respect to either opamp supply voltage and/or circuit common). There is a differential component, albeit very small, between the + and - pins.

In the non-inverting case, the + pin is driven, so it's obviously moving with respect to the supplies/common. The - pin moves along with the + pin, so since both are moving it can be said that there is a 'common mode' component of the two pins. Also, just as in the inverting-mode case, there is a small differential component between the + and - pins.

Have fun, in school.

Bob

This has some truth in it. Enough to be dangerous. ;-) Common mode voltage is just the average voltage on the two inputs.

It is the voltage they have in common.

The danger is coming into focus.

All closed loop functions made with opamps keep the two inputs at almost exactly the same voltage with negative feedback. The tiny remaining differential voltage times the very high gain of the opamp produces the output voltage that drives the feedback. Any voltage the two inputs share in common does not involve any differential voltage, and, ideally, does not get amplified. Every opamp powered from a given supply voltage has a common mode input range over which differential voltages can be amplified, and the common voltage shared by both inputs can be ignored.

You are getting close. The inverting configuration has the + input held to a fixed reference voltage that may be ground. The - input receives both the input current and the feedback current and if these are not exact opposites of each other, the combination causes some voltage change at the - input. And any change from the reference voltage applied to the + input represents differential voltage and gets amplified largely and alters the feedback current to precisely match the input current and put the - input voltage back at the same voltage as is applied to the + input. So both inputs stay in a very small region of the common mode range.

The non inverting configuration applies the input signal to the + input, and the - input is forced to follow it (via the feedback path) wherever it goes as long as it stays inside the common mode range and the output can swing enough in the required direction. Anything else would produce a difference between the two inputs, and that would drive the output to wherever it has to go to reduce that difference to nearly zero (so that nearly zero times a lot of differential gain amounts to wherever the output has arrived).

So both configurations make use of the common mode range, but the non inverting configuration makes use of more of it.

For this reason, an opamp has to have very low common mode voltage gain (it has to ignore where the two inputs are and only be aware of their difference) in order to work well in the non inverting configuration, because there is a lot of common mode swing going on, and it has to be ignored, and only the difference between the two inputs being amplified, for the circuit to perform as expected.

Since only a tiny common mode swing (half of the tiny difference voltage) takes place with the inverting configuration, very low common mode gain is a lot less important for this configuration to work well.

Thanks, John and Bob, for your time and good explanations.

Amanda