Op-amp Ideal Non-inverting Amplifier

Ideal Non-inverting Amplifier

An amplifier which amplifies the input without producing any phase shift between input and output is called non-inverting amplifier. The basic circuit diagram of a non-inverting amplifier using op-amp is shown in the Fig. 2.22.1 (a). The input is applied to the non-inverting input terminal of the op-amp.

Derivation of closed loop gain :

The node B is at potential V_in, hence the potential of point A is same as B which is V_in, from the concept of virtual share.

V_A = V_B = V_in … (2.22.1)

From the output side we can write,

I = V_o – V_A / r_f 

I = V_o – V_in / r_f  … (2.22.2)

At the inverting terminal,

I = V_A – 0 / R₁  i,e. I = V_in / R₁  … (2.22.3)

Entire current passes through R₁ as input current of op-amp is zero.

Equating equations (2.22.2) and (2.22.3),

The positive sign indicates that there is no phase shift between input and output.

The input and output waveforms are shown in the Fig. 2.22.1 (b).

1. Comparison

The Table 2.22.1 provides the comparison of the ideal inverting and non-inverting amplifier op-amp circuits.

Observations

1. The voltage gain is always greater than one.

2. The voltage gain is positive indicating that for a.c. input, the output and input are in phase while for d.c. input, the output polarity is same as that of input.

3. The voltage gain is independent of open loop gain of op-amp, but depends only on the two resistance values.

4. The desired voltage gain can be obtained by selecting proper values of R_f and R₁ 

Example 2.22.1 Determine the output voltage for the following circuits.

May-04, 17, Marks 6.5

Solution : From the above Fig., it is non-inverting amplifier with R_f = 5 k Ω, R₁ = 1 k Ω

 Example 2.22.2 For a non-inverting amplifier using an op-amp assume R₁ = 470 R₂ = 4.7 kohm. Calculate the closed loop voltage gain of the amplifier.

Solution: The circuit is shown in the Fig. 2.22.2

Gain = 1+ R₂ / R₁

= 1+ 4.7×10³ / 470 = 11

Review Questions

1. Draw the circuit of noninverting amplifier using op-amp and derive expression for its gain May-04, Marks 4

2. For the op-amp configuration shown in the Fig. 2.22.3, determine the R_f  if the gain required is 61.