Example - 4

R1 = 100Ω, R_f = 8.2 k, R_C = 10 k. Assume that the amplifier is nulled at 25°C. If V_in is 20 mV peak sine wave at 100 Hz. Calculate E_r, and V_o values at 45°C for the circuit shown in fig. 2.

Fig. 2

Solution:

The change in temperature ΔT = 45 - 25 = 20°C.

Error voltage = 51.44 mV

Output voltage is 1640 mV peak ac signal which rides either on a +51.44 mV or -51.44 mV dc level.

Example - 5

Design an input offset voltage compensating network for the operational amplifier µA 715 for the circuit shown in fig. 3. Draw the complete circuit diagram.

Fig. 3

Solution:

From data sheet we get v_in = 5 mV for the operational amplifier µA 715.

V = | V_CC | = | - V_EE | = 15 V

Now,

If we select R_C = 10Ω, the value of R_b should be
                  R_b = (3000) R_C = 30000Ω = 304Ω

Since R > R_max, let R_S = 10 R_max where R_max = R_a / 4. Therefore,

If a 124Ω potentiometer is not available, we may prefer to use to the next lower value avilable, such as 104Ω, so that the value of R_a will be larger than R_b by a factor of 10. If we select a 10 kΩ potentiometer a s the R_a value, R_b is 12 times larger than R_a, Thus

R_a = 10 kΩ potentiometer
 R_b = 30 kΩ
R_c = 10Ω.

The final circuit, which also includes the pin connections for the µA 715, shown in fig. 4.

Fig. 4

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