# What are the Characteristics of a Transistor?

The characteristics of a junction transistor can be obtained by:

• Common emitter transistor
•  Common base transistor
•  Common collector transistor

We will discuss only the common emitter characteristics of a junction transistor. The common emitter characteristics can be obtained by studying the graph between voltage and electric current. The base acts as an input terminal, collector acts as an output terminal and the emitter is earth. The base- emitter battery is forward biased by the base-emitter junction and the collector-emitter battery is reversed biased by the base-collector junction. The base current ($$I_B$$) and the collector current ($$I_C$$) is measured by the ammeter and the voltage ($$V_{BE}$$) and ($$V_{CE}$$) is measured by the voltmeter.

What are the input characteristics of a common emitter transistor?

• At different constant values of collector voltages ($$V_{CE}$$), there are graphs plotted between base voltage ($$V_{BE}$$) and the base current ($$I_B$$).

• The current increases slowly and the input characteristics of a transistor are obtained.

What are the output characteristics of a common emitter transistor?

• Output characteristics have a constant value of base current ($$I_B$$) and it is represented by the graph plotted between the collector-emitter voltage ($$V_{CE}$$) and collect current ($$I_C$$).

• The output characteristics give ($$I_B$$) at 50µA, the value of ($$I_B$$) gradually increases. It is used in the circuits to get undisturbed output.

Transistor as an Amplifier
We will discuss the common base amplifier where the base of the transistor is common to emitter and collector. The characteristic of a common base amplifier circuit is different using an n-p-n and p-n-p transistor.

How n-p-n does transistor act as an amplifier circuit?

•  In n-p-n transistor, base is common to emitter and collector. The base-emitter battery ($$V_{BE}$$) uses an emitter as forward biased, which results in small input resistance ($$R_{in}$$). The base-collector battery ($$V_{BC}$$) uses collector as a reverse biased, which results in large output resistance (R out). When a low voltage signal is applied across the input terminal, it results as an amplified signal across the base and collector circuit.
• When we apply Kirchhoff’s law in the circuit the circuit is passed through the circuit and it is represented as:
$$I_E$$ = $$I_B + I_C$$
A voltage drop will occur across the collector-base circuit when current IC will flow through the circuit. Thus, voltage across collector and base will be:
$$V_{BC}$$ = $$V_{CE} – I_C R_L$$
• Emitter current will change when there is a change in emitter voltage which in turn will change collector current ($$I_C$$).Now, the collector current will change accordingly.
• When the input signal is applied, due to the change in the collector voltage an amplified output can be obtained.

The circuit of the p-n-p transistor is same as an n-p-n transistor with only the polarities reversed. You can find out more about transistor circuit and explore more on physics formulas at BYJU’S

#### Practise This Question

Two lenses with powers; 2D & -4D are kept together. What is the effective focal length of the combination?