Question

(a) Describe briefly the functions of the three segments of n-p-n transistor. 

(b) Draw the circuit arrangement for studying the output characteristics of n-p-n transistor in CE configuration. Explain how the output characteristics is obtained.

Answer 1

(a) The three segments of n-p-n transistor are emitter, base and collector.

(i) Emitter: The function of emitter is to emit the majority carriers which are electrons in the case of n-p-n transistor. It is moderate in size and heavily doped.

(ii) Base: The function of base is to provide proper interaction between emitter and collector. It is then and lightly doped 

(iii) Collector: the function of collector is to collect the majority carries. It is large in size and moderately doped.

(b) Circuit diagram of an n-p-n transistor in CE configuration: The CE configuration of transistor is when grounded emitter is kept as a common terminal. Base is input terminal and collector is output terminal.

Here, I_s is base current; V_BE is base–emitter voltage; I_e is emitter current; V_CE is collector emitter voltage, and I_C is the collector current. Here, B, E and C corresponds to base, emitter and collector, respectively. The output characteristics of C_E configuration represents variation of collector current I_C with respect to the collector–emitter voltage V_CE keeping base current Ib fixed.

To get the output characteristics of an n-p-n transistor in CE configuration, we need to carry out the following steps:

• Adjust a suitable constant value of base current, I_b. 

• Apply various values of collector–emitter voltage V_CE and note that the corresponding values of collector current I_C. 

• Repeat the above step for various constant base currents. 

• Plot a graph between collector–emitter voltage V_CE and collector current I_C. 

• The current obtained are called output characteristics.