In this article, let’s understand the non-inverting op-amp configuration , and let’s see, how it can be used as a buffer.
Fig. 1 shows the non-inverting configuration of the op-amp. In this configuration, the input is applied at the non-inverting terminal of the op-amp.
Fig. 1 Non-Inverting op-amp Configuration
In this configuration, there is a negative feedback from the output to the input side. As shown in Fig.2, the fraction of output voltage is given as feedback to the input side. Due to this negative feedback, the op-amp operates in the linear region. In this configuration, the output of the op-amp can be as Vo = ( 1 + Rf / R1) Vin
Fig.2. Negative Voltage Feedback in the Non-Inverting op-amp configuration
Derivation of Closed Loop Voltage Gain of the non-inverting op-amp Configuration
Here, it has been assumed that the op-amp is ideal op-amp, and no current is flowing into the op-amp terminals. As shown in figure 2, the fraction of output voltage (Vx) is given as feedback to the input.
Vx = R1x Vo / (R1 + Rf)
Since, op-amp is operating in the linear region, the concept of virtual ground / virtual short is valid.That means the voltage at the inverting and the non-inverting input terminals will be the same. (V+ = V-)
Here, V+ = Vin and V- = Vx. That means Vin = Vx = R1 x Vo / (R1 + Rf)
Vo = ( 1 + Rf/ R1) x Vin
As, per the equation, in the case of the non-inverting op-amp, the output signal is in phase with the input signal. Moreover, the input impedance of the non-inverting op-amp is very high compared to inverting op-amp. (Ideally, it is infinite, because, for the ideal op-amp, no current is flowing into the op-amp terminal) And because of the high input impedance, the op-amp can be used as a buffer in many applications.
Op-amp as a Buffer
As shown in figure 3, the op-amp is used as a buffer. In the non-inverting configuration, If Rf = 0 and R1 = ∞ then it will act as a buffer.
Fig.3 Op-Amp as Buffer
When op-amp is used as a buffer, then it provides very high input impedance and low output impedance. Also, since op-amp is used with the negative feedback ( in the linear region), the voltage at the inverting and non-inverting terminal will be same. (V+ = V-)
And for the buffer circuit, Vo = Vin. That means the output of the op-amp follows the input signal. In this configuration, the gain of the op-amp is unity. And that’s why it is also known as unity follower.
Because of the high input impedance and low output impedance, the buffer can isolate the two stages of the circuit and at the same time, it can provide the output of the one circuit as an input to other circuit. (As shown in the figure 4). It is particular useful, when there is a impedance mismatch between the two stages.
Figure 4. Op-Amp as a buffer can be used to isolate the two circuits