Bipolar Junction Transistor (BJT) Currents

BIPOLAR JUNCTION TRANSISTOR CURRENTS

The direction of a conventional current is always opposite to the electron current in any electronic device.

Therefore the conventional currents in a NPN transistor are shown in fig.3.43(a). However, the direction of a conventional current is the same as that of a hole current in a PNP transistor and is as shown in fig.3.43(b). In this figure, IE denotes the emitter current, IB is the base current and IC is the collector current respectively.

The emitter current is the sum of the collector and base currents.

IE = IB + IC

Since the base current is very small, therefore

IE ≈ IC

Example: 3

In a common-base connection, a certain transistor has an emitter current of 10 mA and a collector current of 9.6 mA. Calculate the value of the base current.

Solution:

IE = 10 mA, IC = = 9.6 mA

IE = IC + IB

10 mA = 9.6 mA + IB

IB = 10 - 9.6

IB = 0.4 mA

Example: 4

The base current of a transistor is 30 μA and the emitter current is 7.2 mA. What is the value of collector current?

Solution:

IE = 7.2 × 10-3

IB = 30 × 10-6

IE =IC + IB

IC = 7.2 × 10-3- 30 × 10-6

IC = 7.17 mA

Example: 5

Following current readings are obtained for transistor circuit IE = 2 mA and IB = 20 μA. Compute the value of IC.

Solution:

IC = IE - IB

= 2 × 10-3 - 20 × 10-6

IC = 1.98 mA