Question

Explain the working of the n-p-n transistor in a common base configuration.

Answer 1

The common base configuration for n-p-n transistor

In a Common Emitter configuration, the emitter of the transistor is common to both the input and the output.

Working:

Biasing of n-p-n transistor

1. The majority of charge carriers in the emitter of the n-p-n transistor are electrons.
2. In the above figure, the emitter-base junction is forward-biased while the collector-base junction is reverse-biased.
3. At the instant when the EB junction is forward biased, electrons in the emitter region have not entered the base region as shown in the below figure.
   Injection of majority carriers into base
4. These electrons can either flow through the base circuit and constitute the base current (I_B), or they can also flow through the collector circuit and contribute towards the collector current (I_C).
5. The base is thin and lightly doped, the base current is only 5% of I_E.
6. Electrons injected from the emitter into the base diffuse into the collector-base depletion region due to the thin base region. When the electrons enter the collector-base depletion regions, they are pushed into the collector region by the electric field at the collector-base depletion region. collector current (I_C) flows through the external circuit as shown in the below figure. The collector's current I_C is about 95% of I_E.
   Electron flow through a transistor
7. From the above figure, we can conclude that I_E = I_B + I_C. Since the base current I_B is very small we can write I_C ≈ I_E.