# Understanding op-amp common mode voltage range when Vin+ is not equal Vin-

I am using MCP662 op-amp as differential amplifier:

First inputs' signal can vary from 0V to 3V and the second input signal can vary from 1V to 1.4V. MCP662 has common mode voltage range equal 1.7V (Vdd-1.3V). In this circuit Vin+ can exceed 1.7V (for example it is 2V when one input signal is 3V and another is 0). However, common mode voltage (Vin+ + Vin-)/2 will never exceed 1.7V.

Is the Vcm range be violated in this circuit? Can this circuit to be used when both input signals can vary between 0V and 3V?

• If you know the Vcm will never exceed the maximum value, why would it violate it? Commented Aug 10, 2015 at 21:07

## 1 Answer

If the op-amp is working as an amplifier Vin(+) = Vin(-), of course.

It cannot work as an amplifier when the input voltages are +3/0V since the output would have to be 6V to balance the amplifier and it will die at less than 3V.

$V_{OUT} = \frac{2K}{1K} (V_{TOP} - V_{BOTTOM})$ (when balanced)

Input common mode range is maybe 0-1.6V taking temperature into account. You will exceed the common mode input range of the amplifier when the top input exceeds about 2.4V (2.55V at >=25°C), period.