The simple series voltage regulator consists of 2 parts:
(a) a zener regulator consisting of the resistor R_S and a zener diode
(b) a power transistor T1 connected as an emitter follower providing almost unity gain.

The output voltage from the regulator will be approximately the zener voltage - 0.7 (V_be of T₁).

 

The output of this regulator can be adjusted by the proper choice for R₁ and R₂.  V_out is given by V_Z*R₁/(R₁+R₂).

R₁ and R₂ must be chosen such that the current drawn by them is much smaller than the current through the load.  To achieve this, we need a high value for R₁ and R₂.  However, the resistance values must be smaller than the input impedance of the opamp.  This is done so that the resistaors does not load the opamp.

The DC supply of the opamp is obtained from the unregulated supply, V_in.