Transistor Buffer for Zener Diode Voltage Regulator
Buffering for the zener is provided by the impedance matching and current amplifying characteristics of the emitter follower and draw less current from it. To a Zener regulator, it’s a useful adjunct. The capacitor on the right circuit is used to reduce ripple.
Other variation of capacitor placement for noise reduction, you can just add the capacitor in parallel with zener diode in the first circuit, or you can swap the position of zener diode and capacitor in the second circuit. This last option improve the second circuit to suppress the noise coming from zener diode regulation mechanism.
Depending on the characteristic of both load impedance and source voltage, whether static or dynamic, or how much is the dynamic impedance range, you can select the optimum value of collector resistor value to minimize the transistor’s power dissipation without loosing the regulation capability. In the extreme example, at high dynamic source voltage or load impedance, the collector resistor can be omitted (shorted). At the other opposite extreme condition, when both source voltage and load impedance is static, the transistor and other components can be omitted, leaving only the collector resistor in series with the load to produce a voltage divider.