The output voltage of this circuit depends on the zener diode being approximately 1.2 V above its voltage. Thus, for the indicated circuit we have close to 6 V output. TIP series Darlington power transistors can be used, which will determine the maximum output current. Most modern operational amplifiers can be used. The maximum output voltage must be 2 V below the input voltage. The dissipation of R2 depends on the maximum current. For the used transistor this current is 10 A.
