Page 289 - Op Amps Design, Applications, and Troubleshooting

Shunt Voltage Regulation 271

In our particular case, the required value for R 3 is found as follows:

The power rating for R 3 is determined by using the basic power formula under
worst-case conditions:

Select Transistor Q|. There are several transistor characteristics that must be
considered when selecting a particular device for Q^

1. Maximum collector current
2. Current gain (h FE or /3)
3. Breakdown voltages
4. Power dissipation

The maximum collector current that the transistor will be expected to carry can be
computed with Equation (6.15).

We estimate the highest current that should be supplied by the op amp as 25 per-
cent of the short-circuit output current of the op amp. The short-circuit output
current for a 741 is listed in the manufacturer's data sheet as 20 milliamperes, so
we will restrict the output current of the op amp to 25 percent of 20 milliamperes,
or 5 milliamperes. We can now apply the basic current gain equation for transis-
tors to determine the required transistor current gain.

This is a fairly high-current gain for a power transistor and may necessitate the
use of a Darlington pair for Qj.
The power dissipation in Qi can be found by applying Equation (6.16) under
conditions of maximum output voltage.