What is the use of transistor in electronics?
- A transistor is an electronic device which has three terminals labelled as base, collector and emitter.
- It is a solid state electronic device that has many functions such as being a switch, amplifier, voltage stabiliser and signal modulator.
- Transistors are made by joining the n-type semiconductors and p-type semiconductors together.
Transistor Types and Symbols
- There are two types of transistors, the npn transistor and pnp transistor.
- An npn transistor consists of a tljin layer.of p-type semiconductor sandwiched by two layers of n-type semiconductors. In an npn transistor, both the collector and emitter are n-type semiconductors while the base is a p-type semiconductor.
- A pnp transistor consists of a thin layer of n-type semiconductor sandwiched by two layers of p-type semiconductors. In a pnp transistor, both the collector and emitter are p-type semiconductors while the base is an n-type semiconductor.
- Figure shows the symbols for the two types of transistors, the npn transistor and the pnp transistor.
- The arrow in each of the symbol represents the direction of conventional current flow. In the npn transistor, current flows from C to E, whereas in pnp transistor, current flows from E to C.
What is the basic function of a transistor?
- A transistor cannot function if only the collector and the emitter terminals are connected to a battery. The transistor is said to be switched off. Figure shows that when the transistor is switched off, current cannot flow through it, therefore the bulb does not light up.
- If a small battery is connected to the base as shown in Figure, to forward bias the base-emitter junction, a small current can flow through the base. This switches on the transistor. The transistor conducts and allows a larger collector current to flow through it from the collector to the emitter. Hence, the bulb lights up.
- The behaviour of the transistor shown in above Figure can be summarised as follows:
(a) If there is a small current in the base-emitter circuit, the transistor is switched on and conducts electricity.
(b) A small current in the base-emitter circuit allows a larger current to flow in the collector-emitter circuit.
- These two characteristics show that a transistor can be used as a current amplifier and a control switch.
How does a transistor works as an amplifier?
Transistor as a Current Amplifier:
- The electronic equipment shown in Figure consists of amplifier circuits that are made up of transistors. The function of an amplifier circuit is to amplify a small direct current or a.c. voltage (signal).
- A transistor functions as a current amplifier by allowing a small current to control a larger current. The size of the collector current, IC, is primarily determined by the base current, IB.
- Figure shows a simple amplifier circuit used to show current amplification by measuring IB and IC.
- A variable resistor R is used to control the base emitter current, IB. The output collector current, IC is observed using a milliammeter.
- A typical graph of IC versus IB as shown in Figure can be used to derive the current gain for the transistor used in the circuit. Note that a small change in the base current will result in a big change in the collector current.
- The current amplification or the current gain can be calculated from the graph as:
Transistor Works as an Amplifier Experiment