Important Point about Non-inverting Amplifier

Important Point about Non-inverting Amplifier

Consider a non-inverting amplifier shown in the Fig. 2.23.1 (a). In this case,

V_o = (1 + R_f/R₁)V_in

But consider a non-inverting amplifier shown in the Fig. 2.23.1 (b). In this case Vin is not directly applied to the non-inverting terminal.

Key Point Remember that the non-inverting amplifier always amplifies voltage at its non-inverting terminal i.e. V_B by (l + R_f /R₁) times.

In second case the output voltage V_o is given by,

Then analyzing circuit at non-inverting terminal, VB can be obtained interms of Vin. In the circuit shown in the Fig. 2.23.1 (b), R₂ -R₃ forms potential divider and as op-amp input current is zero,

This fact helps to analyze complex op-amp circuits in which non-inverting amplifier is a subpart.

Example 2.23.1 Find V_o for the circuit shown in the Fig. 2.23.2.

Solution : This is non-inverting amplifier but it will amplify V_B by 1 + R_f / R₁ and not V_in = 10 V.

The op-amp input current is zero hence part of the circuit is as shown in the Fig. 2.23.2 (a).

Example 2.23.2 Find V_o for the following circuit shown in Fig. 2.23.3.

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Solution : Use Superposition principle. When 4 V is acting, 3 V is grounded then it behaves as inverting amplifier.

V_o1 = -4K/2K × 4 = -8V

When 3 V is acting , 4 V is grounded then it acts as noninverting amplifier.

Example 2.23.3 Find V_o  of  the following circuit.

Solution : Use Superposition principle.

Case 1:1V acting, all other sources are grounded. The node B is grounded hence node A is at virtual ground. Hence 25 kΩ resistor does not carry any current.

The circuit behaves as an inverting amplifier.

V_o1 = -50 / 40 × 1 = - 1.25 V

Case 2 : 2V acting, all other sources are grounded. It behaves as an inverting amplifier.

V_o2 = -50 / 25 × 2 = - 4 V

Case 3: 3V acting all other sources are grounded. It behaves as a noninverting amplifier with,

Case 4: 4 V acting, all other sources are grounded. It acts as a noninverting amplifier with,

Example 2.23.4 Obtain the closed loop voltage V_o/V_i of the circuit shown below

Solution : The various branch currents are shown in the Fig. 2.23.5 (a).