The Non-inverting Amplifier

The non-inverting amplifier is another type of signal conditioner that instead of turning the signal upside-down simply and ideally only changes the signal amplitude and possibly shifts the DC average level of the signal (but we will not discuss level shifting at this point.)

Looking at Figure 1 you note that instead of putting the input signal into R1 as we did for the inverting amplifier, we insert it into R3 and the positive input of the amplifier (A) so the signal does not invert as it is “passed through” the amplifier. But the amplifier gain is still determined by the resistors connected to the negative terminal as before. This time the gain is positive and is

G = (Vout / Vin) = (R1 + R2) / R1

R3 and R4 are optional resistors. R3 can be zero and R4 can be open (or infinite value – not in the circuit.) R4 can also be leakage resistance to ground. If R3 and R4 are used then there is a voltage divider effect on the signal so that the actual positive terminal input signal is

Vin’ = Vin * R4 / (R3 + R4)

Then we have an additional gain factor of

G’ = R4 / (R3 + R4)

and the overall gain will be

Gtotal = G’*G

If R3 and R4 are used they can be any convenient values but if you set R3 = R1 and R4 = R2, then you have a balanced impedance input on each terminal of the op-amp. We will discuss this further later. The power supply connections are not shown for the sake of simplicity.

As with the inverting op-amp circuit, the current into the summing point (the negative input) is again zero by definition. Also the current into the positive terminal is zero by definition of the ideal op-amp. This requires the following equations to be true:

Vout = I * (R1 + R2)

where

I = Iin = Ifb

and Iin is the “input current” through R1 and Ifb is the “feedback current” through R2, and

Iin = Vin / R1

So we now have the formula for the output voltage of the op-amp as


Vout = (Vin / R1 ) * (R1 + R2)

which is the same as

Vout = G * Vin