Low dissipation voltage regulator

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the schematic diagram of the voltage regulator shown in FIGURE, input terminals 10 and 11 are connected to a source of unregulated supply voltage.
Such a voltage source is characterized by some nominal direct voltage, V.
sub.
IN, normally having some amount of level variation.
The regulator circuit shown in FIGURE provides at output terminals 12 and 13, a constant level of direct output voltage, V.
sub.
OUT, where V.
sub.
OUT < V.
sub.
IN.
Terminals 11 and 13 are commonly connected to some reference ground potential as shown in FIGURE by terminal 14.
The regulator circuit is biased as follows.
Assuming some nominal input voltage is across terminals 10 and 11 with the polarity as indicated, a bias current flows through resistors R1 and R2 and zener diode D1 to the ground potential at terminal 14.
This bias current is sufficient to reverse bias the PN junction of diode D1 into a linear avalanche region of operation.
As a result of the zener action, a fixed bias voltage is applied to the base of transistors Q1 and Q2.
Assuming a nominal load is connected across terminals 12 and 13, both transistors Q1 and Q2 are biased into an active region, and are conducting current to the load at the regulated output voltage, V.
sub.
OUT.
The extent to which transistor Q2 shares the output current load with transistor Q1 depends upon the voltage level at input terminals 10 and 11.
The reason for this is as follows.
With the emitter-base junction of transistor Q2 forward biased the emitter electrode will be held at 0.
7V (for silicon transistors) above the reference voltage of zener diode D1.
Since one end of resistor R2 is connected to the emitter electrode, the voltage drop across resistor R2 will vary with changes in input voltage, V.
sub.
IN.
A change in voltage across resistor R2 will cause a corresponding change in current with must be conducted through transistor Q2 (less the bias current through resistor R1).
Since resistor R1 is connected across the emitter-base junction of Q2, the voltage drop will be constant causing the current through the resistor to remain fixed