BUGARIN_BJT Transistor Amplifier

The circuit is a current-limited linear voltage regulator with floating load. Load voltage regulation is based on a Zener reference boosted by a series pass NPN transistor. The current limiter is formed by the sense resistor Rsense and transistor Q2. At low values of current the voltage drop across the sense resistor is low and the collector current of Q2 is also low, the load voltage is maintained at about 5 V. Sliding the potentiometer wiper arm down increases the load current, load voltage regulation will continue to work until the load current reaches about 100 mA. At this value of current the voltage across Rsense causes a collector current of Q2 that pulls the cathode of Zener diode down. Further decrease in load resistance will not cause a significant increase in load current since the voltage in Rsense is now clamped by the base to emitter voltage (Vbe) of transistor Q2. The load voltage falls with further decrease in load resistance.