The series negative feedback ( the emitter degeneration ) makes the transistors act as voltage stabilizers; it forces them to adjust their V BE voltages ( base currents ) to pass the quiescent current through their collector-emitter junctions.
12.
It differs from the RS-232 interface, from which it is derived, in that the voltage levels were chosen to allow the bus also to supply power to bus participants, who can use a voltage stabilizer to derive an internal 5 V supply:
13.
In this bipolar junction transistor ( BJT ) implementation ( Figure 4 ) of the general idea above, a " Zener voltage stabilizer " ( R1 and DZ1 ) drives an " emitter follower " ( Q1 ) loaded by a " constant emitter resistor " ( R2 ) sensing the load current.
