Non Inverting Amplifier
An op amp can be successfully used as main element of a non inverting amplifier.
The op amp has two input terminals (pins). One is inverting which is denoted with (-) sign and other is non-inverting and this is denoted with (+) sign. When any signal is applied to the non-inverting input of an op amp, it does not change its polarity when it is amplified at the output terminal. So, in that case, the gain of the amplifier is always positive. Let us explain the thing by constructing the op amp circuit with the feedback loop as shown below,
Here, in the above circuit, an external resistance R1 and feedback resistance Rf are connected to the op amp at inverting input of the op amp. Now, applying Kirchhoff Current Law at node 1, we get, Now, if we assume that the op amp in the circuit is ideal op amp, then,
Again, Therefore, equation (i) can be rewritten as,
The closed loop gain of the circuit is, This term does not contain any negative part. Hence, it is proved that, the input signal to the circuit will be amplified without changing its polarity at output. From the expression of voltage gain of an non inverting op amp , it is clear that, the gain will be unity when Rf=0 or R1→ ∝. So, if we short circuit the feedback path and/or open the external resistance of the inverting pin, the gain of the circuit becomes 1. This circuit is called voltage follower or unity gain amplifier. This is used to isolate two cascaded circuits, because of its infinitely large impedance, at op amp inputs.