How transistor shunt regulator is work?

The above figure shows the circuit diagram of transistor shunt regulator where the transistor is connected in shunt (parallel) with the load hence the circuit is called transistor shunt regulator. The zener voltage V_Z of zener diode is reverse biased by the unregulated DC voltage. To limit the zener current a resistor R₁ is connected in series with a zener diode. A transistor Q1 is connected in shunt with the load resistor R_L is used as controlling device and its function is to shunt part of the supply current from the load. The transistor must pass the current all the time it is operating, for a shunt regulator to perform a efficiently. It may be noted from the circuit that the load voltage V_Out is equal to the sum of zener voltage and the base to emitter voltage V_BE of a transistor.

V_Out = V_Z + V_BE          OR         V_BE = V_Out -V_Z

There is the voltage for given diode is fixed, therefore any increase or decrease in load voltage will have a corresponding effect on the voltage V_BE.