# Common mode voltage in op amps

Discussion in 'Electronic Basics' started by Amanda Robin, Nov 24, 2005.

1. ### Amanda RobinGuest

Hello all,

I am reviewing my old electronics notes in preparation for a
comprehensive exam.

In my notes I have written, a couple of times, that with a non-inverting
op amp configuration there is common mode voltage, and with an inverting
configuration there is no common mode voltage.

I know the definition of common mode voltage, but I look and look at the
circuit diagrams and can't see why one would have it and the other not.
One of the basic "rules" for op amps is that both input terminals are
ideally held to the equivalent voltage. So it seems to me that every op
amp in the world, however configured, would ideally have common mode
voltage. (I'm probably confusing two different things here, but maybe
someone can help me get unconfused.)

Is it because, with the inverting configuration, the input terminals are
held to ground? And zero V on two terminals doesn't count as common?

Thanks for any tips.

Amanda

2. ### BobGuest

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

3. ### John PopelishGuest

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.

4. ### Amanda RobinGuest

<<snipped good explanations to question about common mode voltage>>

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

Amanda