Theory & Procedure, Transistor Characteristics | Physics Class 12 - NEET PDF Download

Our Objective:

To study the static characteristic of a transistor (Common Emitter Configuration)

The Theory:

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal.

Bipolar Junction Transistors

Bipolar Junction Transistors are transistors which are made up of 3 regions, the base, the collector, and the emitter. A small current entering in the base region of the transistor causes a much larger current flow from the emitter to the collector region. Bipolar junction transistors come in two main types, npn and pnp. A npn transistor is one in which the majority current carrier are electrons. Electron flowing from the emitter to the collector forms the base of the majority of current flow through the transistor. The other type of charge,holes, are a minority. pnp transistors are the opposite. In pnp transistors, the majority current carriers are holes.

The circuit diagram for npn transistor is shown below.

Transistor characteristics:

1) Input characteristics

Keeping the collector- emitter (V_CE) voltage constant, the base- emitter (V_BE) voltage is increased from 0 and the corresponding base current (I_B) values are noted. This is repeated for increasing values of V_CE. The family of curve obtained by plotting I_B against V_BE for each V_CE value is called input characteristics.

2) Output Characteristics

By keeping the base current (I_B) constant, collector- emitter (V_CE) voltage is varied and the corresponding I_C values are obtained. This is repeated for increasing values of I_B. The family of curves obtained by plotting I_C against V_CE for each value of I_B is called output characteristics.

Learning Outcomes:

Students understand the following terms:

• Transistor
• pnp and npn transistors
• Transistor characteristics

Students will be able to do the experiment, once they understand the procedure.

Material required:

• Rheostat
• Voltmeter
• Ammeter
• Battery
• One way key
• Transistor
• Bread board

Real Lab Procedure:

• Connections are made as shown in the circuit diagram.

• The rheostat Rh₁ is used to vary base voltage (input voltage) V_BE and it is read from voltmeter V1. The base current (input current) I_B is measured using a microammeter (µA). The collector voltage (output voltage) V_CE is varied using the rheostat Rh₂ and readings are noted from voltmeter V₂. The collector current (output current) I_C is measured by the milliammeter (mA).

Input Characteristics

• The collector voltage V_CE is kept constant (eg. 1V) by adjusting the rheostat Rh₂.
• The base voltage V_BE is varied from zero by adjusting the rheostat Rh₁.
• The base current I_B is noted in each step.
• A graph is drawn with V_BE along X-axis and I_B along Y-axis.
• The experiment is repeated with V_CE kept constant say 2V, 3V, 4V etc. and corresponding graphs are plotted. 

Output characteristics

• The base current I_B is kept constant (eg. 20µA) by adjusting the rheostat Rh₁.
• Now the collector voltage is increased by adjusting the rheostat Rh₂.
• The corresponding collector current I_C is noted.
• A graph is drawn with V_CE along X-axis and I_C along Y-axis.
• The experiment is repeated with keeping I_B constant, say 40µA, 60µA, 80µA etc and similar graphs are plotted.

Observations:

Input characteristics

Output characteristics

Results:

The graphs shows the input and output characteristics of a transistor.