Page 270 - Op Amps Design, Applications, and Troubleshooting

252 POWER SUPPLY CIRCUITS

voltage. All of this closed-loop action occurs almost instantly, so the actual load
voltage never really changes significantly. Although the circuit compensates for
changes in load current and in input voltage, it may still drift as a result of
changes in temperature. This latter effect can be essentially eliminated by select-
ing a zener diode with a temperature coefficient that is opposite (+2 mV/°C) from
that in the op amp. For our purposes, we will ignore the effects of temperature
changes.
The output or reference voltage for the circuit shown in Figure 6.5 can be
approximated with Kirchhoff's Voltage Law. That is,

Transistor Qi is a simple current booster (as discussed in Chapter 2). The output
current of the op amp is limited to about 5 milliamperes, but required zener cur-
rents may be substantially higher than this. Assuming that the junction break-
down voltages are adequate, there are only three critical parameters for the
selection of Q^

1. Current gain (/3 or h FE)
2. Power dissipation (P D)
3. Collector current

The minimum required current gain for Q! can be determined from the basic tran-
sistor equation for current gain,

where 5 milliamps is the maximum recommended output current for the MC3401
op amp.
The power dissipation for Qi can be determined from the basic power equa-
tion.

Resistor RI simply establishes the desired zener current. Ohm's Law gives us an
approximate value.