A transistor is a type of semiconductor device that can be used to both conduct and insulate electric current or voltage. It is a three-terminal device having two PN junctions. The three terminals have different sizes and different doping concentrations.

Emitter is of moderate size and heavily doped. Collector is moderately doped and larger as compared to the emitter. The base is very thin and lightly doped.

A junction transistor is formed by sandwiching a thin layer of p-type semiconductors with two n-type semiconductors or sandwiching a thin layer of n-type semiconductors with two p-type semiconductors. Based on this, there are two types of transistors:

a) n-p-n transistor
b) p-n-p transistor

When a transistor is to be connected in a circuit, one of the terminals has to be common between input and output. Hence, there are three configurations as follows:

• Common emitter (CE) configuration
• Common base (CB) configuration
• Common collector (CC) configuration

CE configuration:

When a transistor is used in CE configuration, the input is fed between the base and emitter terminals and the output is derived between the collector–emitter terminals.

a) Input characteristics

The variation of the input current 𝐼_𝐵 with input voltage 𝑉_𝐵𝐸 keeping output voltage 𝑉_𝐶𝐸 constant is known as input characteristics. The input resistance is the ratio of base – emitter voltage to the change in base current. $$r_i=\left(\frac{\Delta \mathrm{V_{BE}}}{\Delta \mathrm{I_{B}}}\right){\mathrm{V}_{\mathrm{CE}}=\mathrm{constan} \mathrm{t}}$$

b) Output characteristics

The variation in output collector current with output voltage for different values of input current is known as output characteristics. The output resistance is the ratio of change in collector – emitter voltage to the change in collector current at constant base current. $$r_o=\left(\frac{\Delta \mathrm{V_{CE}}}{\Delta \mathrm{I_{C}}}\right){\mathrm{I}_{\mathrm{B}}=\mathrm{constant}}$$

c) Transfer characteristics

The variation in output collector current with base current at constant output voltage is known as the transfer characteristics. The current gain is the ratio of change in collector current to the change in base current. $$\beta=\left(\frac{\Delta \mathrm{I_C}}{\Delta \mathrm{I} _{\mathrm{ B}}}\right){\mathrm{V}_{\mathrm{CE}}=\text { constant }}$$

The voltage gain, $$A_V=\frac{\Delta V_o}{\Delta V_i}=\frac{\Delta \mathrm{I_C}}{\Delta \mathrm{I_B}} \cdot \frac{r_o}{r_i}$$ $$\Rightarrow A_V=\beta \frac{r_o}{r_i}$$

Requirements for this Science Experiment

⦁ NPN Transistor ⦁ Variable Power Supply ⦁ Plug Key ⦁ Ammeter ⦁ Voltmeter ⦁ Rheostat