This Low riple power supply circuit can be used in which a high current is needed with a low ripple voltage (for example in a high powered class AB amplifier when high quality reproduction is required).
Q1, Q2, and R2 may be considered as a power darlington transistor. ZD1 and R1 give a reference voltage at the base of Q1. ZD1 should be selected thus: ZDl = Vow-1.2. C2 can be picked for the degree of smoothness as its value is effectively multiplied by the combined gains of Q1/Q2, if 100 µF is chosen for C2, assuming minimum hfe for Q l and Q2, C = 100 x 15(Q1) x 25(Q2) = 37,000 uF.