(Vs - Vz - Vbe)-Ref. (Constant) Current Source

"Constant" current source using the difference between the power supply voltage (Vs) and the sum of Zener voltage (Vz) and base-to-emitter voltage (Vbe) as a voltage reference. This circuit is used when smaller reference/base voltage (Vref/Vb) is desired (e.g., when the collector voltage needs a large part of the power supply voltage as headroom). The Zener voltage is just subtracted from the power supply voltage, the Zener diode does not help in regulating the output current. In fact, the Zener diode causes the base voltage to draw the power supply ripple which results to wide variation in output current. The circuit is highly sensitive to supply voltage fluctuation so when smaller variation in output current is desired the power supply should be adequately regulated. With the saved settings of the circuit, the output current is about 10 mA when the AC ripple is at 0 V. As the ripple goes to maximum the output current grows, exceeding 15 mA (150%), and as the ripple goes to minimum, the output current falls to less than 5 mA (50%).