Voltage Transfer Curve of Op - amp

Voltage Transfer Curve of Op - amp

The graph of output voltage Vo plotted against the differential input voltage V_d ; assuming gain constant is called voltage transfer curve or characteristics of op-amp.

1. Idea Voltage Transfer Curve

Ideally open loop gain of op-amp is ∞

AOL = V_o / V_d = ∞

Vd = V_o / ∞ = 0

Thus for zero input, the output of op-amp is always at saturation level ± V_sat , due to infinite gain. Thus voltage transfer curve for ideal op-amp is a vertical line as shown in the Fig. 2.16.1.

Thus ideally range of input for linear operation of the op-amp is zero.

2. Practical Voltage Transfer Curve

Practically A_OL is finite for the op-amp. For op-amp 741C, it is 2 ×10⁵.

V_o = A_OLV_d

±V_sat = 2 × 10⁵ V_d

The saturation voltages are almost ± 15 V.

V_d  = ±15 / 2 ×10⁵ = ± 75 µV

Hence practically till V_d is between - 75 µV and + 75 µV, the output will vary linearity with input. But once V_d exceeds ± 75 µV, the output is saturated.

Thus the practical voltage transfer curve is as shown in the Fig. 2.16.2.

Thus,

i) If V_d is greater than corresponding to b, the output attains + V_sat.

ii) If V_d is less than corresponding to a, the output attains - V_sat .

iii) Thus range a-b is input range for which output varies linearity with the input. But as A_OL is very high, practically this range is very small.

Review Question

1. What is voltage transfer curve of an op-amp ? Explain.