Page 278 - Op Amps Design, Applications, and Troubleshooting

260 POWER SUPPLY CIRCUITS

The transistor power dissipation can be found by applying Equation (6.6).

The minimum current gain (h FE or ft) for the transistor can be found by applying
our basic transistor formula for current gain:

With a circuit like that shown in Figure 6.8, base current is provided by the output
of the op amp. We will establish the maximum current to be provided by the op
amp as one-fourth of the short-circuit output current rating of the op amp, which,
in the case of a 741, is listed in the data sheet as 20 milliamperes. Therefore, we
shall limit the output current (and therefore the base current of Qj) to one-fourth
of 20 milliamperes, or 5 milliamperes. The minimum current gain for Qi can now
be determined as shown:

The minimum collector-to-emitter voltage breakdown rating for Qi is found by
determining the maximum voltage across Qj. That is,

In our particular case, the collector-to-emitter voltage rating is computed as

There are many transistors that will satisfy the requirements for Qj. Let us select
an MJE1103 (refer to Appendix 2) for this application. The calculations presented
in Appendix 10 indicate that the transistor will require a heat sink for safe oper-
ation.