Non-inverting amplifiers are the second closed-loop configuration of an op-amp. In the figure below, the input signal VIN is applied directly to the positive input terminal of the op-amp. R1 applies negative feedback back to terminal 1 (inverting terminal); one terminal of R2 is connected to ground.
Similarly, whatever voltage is at terminal 2 will automatically appear at terminal 1 because of the infinite open-loop gain A.
The gain of the non-inverting configuration is positive, hence the name non-inverting. The input impedance of this closed-loop amplifier is ideally infinite, since no current flows into the positive input terminal of the op-amp. The output of the non-inverting amplifier is taken at the terminals of the ideal voltage source A(VIN+-VIN-), which is independent of the current that may be drawn from the output terminal into a load impedance. This means the output resistance of the non-inverting configuration is zero.
Putting these properties together, we arrived at the equivalent circuit model of the non-inverting amplifier configuration, shown in the equivalent circuit model on the right. This model is obtained under the assumption that the op-amp is ideal.