Low-drop voltage regulator

EE 82 July/August 1986 58 Integrated 3-pin voltage regulators are not suitable for use where the in- put and output voltages are nearly equal. In fact, with most such regulators, the input voltage is typically 3 V higher than the output potential. To cater for situations where the two voltages are nearly equal, it is necessary to use discrete components. The series transistor is then connected in a common emitter circuit, so that the output voltage is lower than the input voltage only by the saturation voltage of the transis- tor. However, it is then difficult to pro- vide short-circuit protection as is the case in integrated regulators. But, where there is a will, there is a way. In Fig. 1, the series transistor obtains its base current from T2, which together with Ti forms a differential amplifier. This arrangement ensures that the junction of voltage divider R4-R5 has the same potential as the cathode of zener D2. The crux of the circuit is that Ta has a certain current amplifi...