* This regulator has a transistor in series to the Zener regulator and both in parallel to the load. The transistor works as a variable resistor, regulating its collector emitter voltage in order to maintain the output voltage constant. * With the input operating conditions, the current through the base of the transistor changes. This affects the voltage across the base emitter junction of the transistor VBE. The output voltage is maintained by the Zener voltage VZ which is constant. As both of them are maintained equal, any change in the input supply is indicated by the change in emitter base voltage VBE. * Hence the output voltage Vo can be understood as
* If the input voltage Vin is increased, the output voltage Vo also increases. * But this in turn makes the voltage across the emitter base junction VBE to decrease. If VBE decreases the base current and collector current decreases which in turn increases collector to emitter voltage VCE. Thus reducing the output voltage VO. * This decrease of output voltage compensates the initial increase in output voltage. Thus it acts as a regulator.