I am using MCP662 op-amp as differential amplifier:

enter image description here

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?

  • \$\begingroup\$ If you know the Vcm will never exceed the maximum value, why would it violate it? \$\endgroup\$
    – Eugene Sh.
    Aug 10, 2015 at 21:07

1 Answer 1


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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.