The actual supply should get -20 V coming from rectifier/filter that is given to the collector of Darlington PNP-type pass transistor, a TIP 105. The base drive for the TIP105 is supplied via resistor R5. The base of the TIP is driven from Vz terminal at pin 9, this is anode of 6.2 V zener diode which links with the emitter of uA 723 output control transistor. The way of providing the positive feedback needed for feedback action is shown.
This method introduces positive feedback by raised current flow via resistors R1 and R2 under short-circuit conditions. This forward biases the base-emmiter junction of the 2N2907 sensing transistor. which will lowers base drive to the TIP 105. ( Texas Instrument,Linear and Interface Circuits Applications, Vol I )
