(1) 30 Volt, 0.4 Ampere Stabiliser: A practical series stabiliser circuit for 30 volts, 400 mA output is shown in Fig 21.7. A 6.2 volt reference diode (Df is used and the control element consists of a 2N1049 power transistor (actually a Darlington connected pair with a 2N656 transistor, this pair of transistors acts as a 2N1049 with very high gain). The circuit around Q4 and diode Q2 is called a pre-regulator circuit, and is included to provide a constant current to the collector of Q3 and the base of Q2. This circuit prevents any ripple existing on the input voltage from appearing at the base of Q2 and being amplified by the series transistor. The capacitor C is included to prevent high-frequency instability

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