Day 4 (6th June 2011)

Built a constant current source using a PNP transistor  with constant base current. Since the collector current is a constant multiple of the base current, the current is independent of the load(provided the transistor is in active region).

In this case a Zener diode of breakdown voltage 6.2 V was used to provide a constant base voltage of 23.8V. The drop across the base emitter is a constant 0.7V. Therefore the emitter current is given by

Ie=Vee-Vz+0.7/(Rpot)

Thus by choosing an appropriate resistance using the potentiometer the required current can be obtained. Since Ic is almost equal to Ie , a constant collector current is maintained.

For this working the Zener diode has to be maintained in breakdown. To ensure this a minimum amount of current should flow through the diode. This was done by choosing R1 such that the required current flows through the Zener diode.