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The transistor is in series with the load resistance(RL).
The source DC voltage (input) is an un-regulated voltage, while the load voltage at the load resistor(output) is a regulated 12 volts.
This is an emitter in a follow up mode, so the transistor behaves like a variable resistance whose value is determined by the amount of the base the current.
Load voltage (VL) = Zener voltage (VZ) - Base Emitter Voltage (VBE)
Or; VBE = VZ - VL.....................(1)
VZ is always constant, if VL varies, VBE will varies too.
Example,
if RL increases, load current (IL) will decrease and VL will increases (VL = IL*RL) and VZ is fixed.
if VL increases, VBE will decreased (see equation1),as the level of conduction is reduced.
Slightly decreases the input current to compensate for the increase in value of RL so that VL is constant.
SUMMARY:
VL increases, VBE increases, IB decreases, VCE increase, IS decreases while VL decreases.
Output voltage is approximately equal to the Zener voltage(VZ) use for a larger load currents.
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