bjt biasing

The circuit demonstrates the Vbe multiplier (the circuit formed by Q1, Rbe, and Rbc), a shunt voltage regulator that is often used to bias a push-pull output stage of a BJT amplifier. One interesting property of the Vbe multiplier is the convenience it offers when tracking the thermally induced variation in the required biasing voltage. The Vbe multiplier is popular in audio and general-purpose power amplifiers employing power BJTs. The LM741 operational amplifier did not adopt the biasing circuit employed in the uA741, the LM741 used a Vbe multiplier instead. Neglecting the current flowing to the base of the transistor, the output voltage of the Vbe multiplier is equal to (1 + Rbc/Rbe)Vbe. The scale factor does not have to be an integer, it is equal to 2.5 in the circuit shown. The AC source is included in the circuit to show the voltage regulating capability of the Vbe multiplier.