# BJT Base curret

1. Aug 21, 2011

### likephysics

What is the BJT base current if the base is connected to a zener diode like in the circuit here - http://www.satcure-focus.com/tutor/page5.htm
With a voltage divider it's thevenin's equivalent. But with zener, should I consider it as a voltage source?

2. Aug 21, 2011

### yungman

Base current depends on the emitter current you are drawing $I_E=\beta I_B\;$, it does not depend on the zener. Think of the zener as a shunt regulator. The zener draw enough current to keep the zener voltage at the base. Whether the zener can hold 5.6V depend on the R1 and emitter current and the beta of the transistor. If you draw too much current at the emitter and the base current is high enough that the R1 drop more voltage, then the output voltage will go down.

The zener is a shunt regulator.