Calculating voltage at non-inverting input of the op-amp

Question

I am asked to calculate the voltage at the non-inverting input of the op-amp if v1=4.78V and v2=0v.

I thought that the answer would be zero, as the voltage at the non-inverting input must be approximately equal to the voltage at the inverting input, and the inverting input is connected to ground. However, this is incorrect.

I would appreciate it if you could help me understand what is happening here. I have gone through some practice questions relating to op-amps with circuits constructed similar to this using the assumption that the voltage at the input equals that at the output, however, i am getting this question wrong.

Thank you very much for your time,

Answer 1

The inverting input is not connected to ground- rather it is connected to a voltage divider between the op-amp output and ground. The output voltage is thus quite easy to find, given the voltage at the inverting input. But you don't know that voltage yet.

When the op-amp is balanced, the voltage at the two inputs is equal. Ideally, the inputs don't affect the voltages. Ideally the output voltage is driven to whatever voltage balances the op-amp, assuming negative feedback and infinite gain.

The voltage at the non-inverting input is just the mid-point of another voltage divider and the instructor has made it easier on you by setting one end to zero.

Answer 2

What you have is a summing amplifier, which will get the signals coming from v1 and v2 in order to sum them(and multiply by a constant and such is defined by resistors values in the circuit).

And yes the voltage in the non-inverting input is going to be equal to the inverting input. In your image there´s a resistor in the feedback network connected between ground and v_, so v2 is not zero.

With this information you might now be able to figure out what the voltage in the input should be.