Lecture - 29: Voltage Regulator

Example –2

Determine R1 and extreme values of I1 in the circuit of Example-1, if Q 1 is replaced with the compound connection of fig. 2. Assume both transistors have a minimum hFE of 15 and negligible cutoff currents.

Solution:

The current through R1 should be selected about 50 percent higher than the maximum current required by the base of Q12. For worst-case design, the minimum hFE of 15 is used, yielding a composite hFE ≈ 15 (15) = 225 for both Q11 and Q12. This yields

Fig. 2

We now select I2≈ 0.65 mA; R1 is calculated as follows:

where,

The extreme values of I1 occurs when Vi is at its extremes. The minimum I1 is  0.65mA (as indicated above), while the maximum is

The variation in I1 is 1.5 - 0.65 = 0.85mA.

Example - 3

In the regulator shown in fig. 3 R1 = 50KΩ, R2 = 43.75KΩ and VZ = 6.3V. If the 15 V output drops 0.1 V, find the change in VBE2 that results.

Fig. 3

Solution:

When VO = 15 V,

Therefore, VBE2 = V2 - VZ = 7 V - 6.3 V = 0.7 V. When VO = 15 V - 0.1 V = 14.9 V, V2 becomes

Therefore,

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