Basic electronics: Class 10 Physics

Key points

Electronics deals with the control of electrons in devices such as diodes, transistors and integrated circuits. Radio, television, computers and mobile phones all depend on it. Most modern devices use semiconductors, which are small, cheap, reliable and use little power.

Thermionic emission is the release of electrons from a hot metal surface. When a metal is heated strongly, some free electrons gain enough energy to leave the surface. A heated filament inside an evacuated tube releases electrons. A positive plate (anode) nearby attracts them, producing a beam. This idea was used in vacuum tubes and is still used in the electron gun of a cathode ray tube.

The cathode ray oscilloscope (CRO) displays how a voltage changes with time. Its main parts are the electron gun (heated cathode and anodes that form and speed up the beam), the deflection system (X plates and Y plates) and the fluorescent screen. The electron beam makes a bright spot where it hits the screen. The Y plates deflect the spot up and down according to the input voltage. The X plates are connected to a time base that sweeps the spot across the screen at a steady rate. Together they draw a graph of voltage against time.

Uses and readings of a CRO:

• Measuring voltage: peak voltage = number of vertical divisions × the Y gain setting (volt per division).
• Measuring time period: T = number of horizontal divisions for one cycle × the time base setting (s per division). Then f = 1/T.
• Displaying waveforms of sound and a.c. signals, and checking faults in electronic circuits.

Materials are classified as conductors, insulators and semiconductors. Silicon and germanium are semiconductors. Their resistance falls as temperature rises, the opposite of metals. Pure semiconductors are intrinsic and conduct poorly. Their conductivity can be increased greatly by doping, which means adding a very small amount of a different element. The result is an extrinsic semiconductor.

Two types of extrinsic semiconductor:

• n-type: silicon doped with a pentavalent atom (five outer electrons, such as phosphorus or arsenic). Each impurity atom gives one extra free electron. The majority carriers are electrons.
• p-type: silicon doped with a trivalent atom (three outer electrons, such as boron or aluminium). Each impurity atom leaves a gap called a hole. The majority carriers are holes, which behave as positive charges.

A p-n junction is formed when p-type and n-type materials are joined. Near the boundary electrons from the n side fill holes on the p side. This leaves a thin region without free carriers called the depletion layer, with a built-in potential barrier (about 0.7 V for silicon). A p-n junction is called a diode. In forward bias the p side is joined to the positive terminal. The barrier is reduced, the depletion layer narrows and the diode conducts. In reverse bias the p side is joined to the negative terminal. The depletion layer widens and almost no current flows. So a diode passes current in one direction only.

Rectification is the conversion of a.c. into d.c. using diodes. In half wave rectification one diode passes only one half of each cycle, and the output frequency equals the input frequency (50 Hz). In full wave rectification a bridge of four diodes uses both halves, and the output pulses at twice the input frequency (100 Hz for a 50 Hz supply). The output is pulsating d.c. A capacitor connected across the output smooths it. A light emitting diode (LED) is a diode that gives off light when forward biased. It uses little power and lasts long, so it is widely used in lamps, displays and indicators. A resistor is placed in series with an LED to limit the current.

An analogue signal changes smoothly and continuously, for example the voltage from a microphone. A digital signal has only two levels, high (1) and low (0). Digital signals are less affected by noise, easy to store and process by computers, and can be copied without loss. A digital system works on the binary number system, using only 0 and 1.

A logic gate is a circuit that takes one or more digital inputs and gives one digital output according to a rule. The basic gates are AND, OR and NOT. NAND is AND followed by NOT. NOR is OR followed by NOT.

Truth tables (inputs A, B and output Y):

• AND: Y = 1 only when both A and B are 1. (0,0)→0, (0,1)→0, (1,0)→0, (1,1)→1.
• OR: Y = 1 when at least one input is 1. (0,0)→0, (0,1)→1, (1,0)→1, (1,1)→1.
• NOT: one input, output is the opposite. 0→1 and 1→0.
• NAND: opposite of AND. (0,0)→1, (0,1)→1, (1,0)→1, (1,1)→0.
• NOR: opposite of OR. (0,0)→1, (0,1)→0, (1,0)→0, (1,1)→0.

NAND and NOR gates are called universal gates because any other gate can be built from them. Logic gates are the building blocks of computers, calculators, alarms and digital watches. Many gates together are packed on a silicon chip called an integrated circuit.

Study tip: Learn all five truth tables as a single chart and check each by describing the rule in words, for example NOR is true only when both inputs are false. Practise a neat labelled diagram of the CRO and a forward and reverse biased diode. For CRO numericals, list the setting values first, then multiply by the number of divisions.

Definitions and flashcards

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Formula sheet

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Short questions with model answers

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Numericals with worked steps

Common mistakes

• Joining a diode in reverse bias and expecting current. The p side must go to the positive terminal for forward bias.
• Saying n-type semiconductors are negatively charged. They are neutral. The name refers to the majority carriers.
• Mixing up AND and OR truth tables, or giving NAND the same output as AND.
• Reading the CRO time base in the wrong unit, for example using ms as s.
• Forgetting the series resistor with an LED, which would let too much current flow.

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