Kerala Plus Two Electronics (DHSE) key terms

A pure semiconductor crystal with no added impurities, in which the number of free electrons equals the number of holes. Its conductivity is low and rises with temperature because more electron-hole pairs are thermally generated.

A semiconductor deliberately doped with donor or acceptor impurities so that free electrons or holes dominate conduction. The added impurity atoms increase conductivity by many orders of magnitude compared with the pure crystal.

The controlled addition of a small amount of impurity atoms to a pure semiconductor to increase its conductivity. Pentavalent donors give n-type material and trivalent acceptors give p-type material.

A vacancy left in the covalent bond structure when an electron leaves its place. It behaves as a mobile positive charge carrier and drifts in the direction opposite to electron flow.

The charge carrier type present in the larger concentration in a doped semiconductor: electrons in n-type material and holes in p-type material. Majority carriers carry most of the current in a device.

The narrow zone at a p-n junction where mobile carriers have diffused away, leaving fixed ionised atoms and an internal electric field. This barrier must be overcome before appreciable current flows.

The built-in voltage across a forward-biased p-n junction that must be exceeded before the diode conducts strongly. It is about 0.7 V for silicon and 0.3 V for germanium devices.

A two-terminal device formed by joining p-type and n-type semiconductor regions. It conducts easily in forward bias and blocks current in reverse bias, so it acts as a one-way valve for current.

A heavily doped diode designed to operate safely in reverse breakdown at a specified voltage. It maintains an almost constant voltage across itself, so it is used for voltage regulation and reference levels.

A circuit that converts alternating current into pulsating direct current using diodes. It may be half-wave, full-wave centre-tap or bridge type, and is normally followed by a filter.

The ratio of the effective alternating component in a rectifier output to its average direct component. A smaller value means smoother output; capacitor filtering greatly reduces it.

A large capacitor connected across the rectifier output that charges at the peaks and discharges into the load between peaks. It smooths the pulsating output and reduces ripple voltage.

A circuit that holds its output voltage nearly constant despite changes in input voltage or load current. Common forms use a Zener diode with a series resistor or a three-terminal integrated regulator.

A full-wave rectifier using four diodes in a bridge arrangement. It gives full-wave output without a centre-tapped transformer and needs only two diodes conducting in each half cycle.

Setting the steady direct currents and voltages of a transistor so that it works in the desired region of its characteristics. Proper bias keeps the active device in the linear region and prevents distortion.

The no-signal direct-current values of collector current and collector-emitter voltage of an amplifier stage. It is usually called the Q-point and is plotted on the load line.

A straight line drawn on the transistor output characteristics showing all possible combinations of collector current and collector-emitter voltage for a given supply and load resistor. The Q-point lies on it.

A transistor amplifier in which the emitter is the common terminal for input and output. It gives voltage gain and current gain with the output inverted relative to the input.

The ratio of the output signal voltage to the input signal voltage of an amplifier. It is often expressed in decibels as twenty times the logarithm of the voltage ratio.

Returning a fraction of the output signal to the input of an amplifier. Negative feedback reduces gain but improves stability, bandwidth and linearity, while positive feedback can sustain oscillation.

The condition for sustained oscillation: the loop gain must be at least unity and the total phase shift around the loop must be zero or a multiple of 360 degrees at the oscillation frequency.

An electronic circuit that generates a continuous alternating waveform from a direct-current supply without an external input signal. It relies on positive feedback and a frequency-determining network.

An oscillator whose frequency is set by a quartz crystal vibrating at its mechanical resonance. It provides far better frequency stability than RC or LC networks and is used in clocks and radios.

A basic digital circuit with one or more inputs and one output that performs a fixed logical operation such as AND, OR, NOT, NAND or NOR. Its output depends on the input combination at that instant.

A table listing every possible combination of inputs to a logic circuit together with the resulting output for each combination. It completely defines the behaviour of a combinational logic function.

A gate such as NAND or NOR from which any other logic function can be built. Entire logic circuits can therefore be constructed using only one type of gate.

A system of mathematical logic using variables that take only the values 0 and 1, with operations AND, OR and NOT. It is used to simplify and analyse digital logic expressions.

A rectangular grid in which adjacent cells differ by only one variable, used to simplify Boolean expressions graphically. Grouping adjacent ones eliminates variables and yields a minimal expression.

A bistable digital circuit with two stable output states that stores one bit until it is changed by a clock or input. It forms the basic building block of registers and counters.

A chain of flip-flops in which each clock pulse moves stored bits one position along the chain. It is used for temporary storage and for serial-to-parallel or parallel-to-serial conversion.

The process of varying a parameter of a high-frequency carrier wave in accordance with a low-frequency information signal. It allows the information to be transmitted efficiently through free space or a cable.

A modulation scheme in which the amplitude of the carrier is varied in proportion to the instantaneous amplitude of the modulating signal, while its frequency stays fixed. It is simple but sensitive to noise.

A modulation scheme in which the instantaneous frequency of the carrier is varied in proportion to the modulating signal while the amplitude stays constant. It gives better noise immunity than amplitude modulation.

A number describing the depth of modulation. In amplitude modulation it is the ratio of modulating amplitude to carrier amplitude; values above one cause overmodulation and distortion.

A single integrated circuit containing a processor core, memory, timers and input/output peripherals, designed to control a dedicated application. It is programmed to read sensors and drive outputs.

A central processing unit built on one chip that fetches, decodes and executes instructions. It needs external memory and input/output devices to form a working computer system.

A dedicated computer system built into a larger device to perform a specific control function. It usually uses a microcontroller and runs fixed firmware rather than general-purpose software.

A circuit that converts a continuous analog voltage into a proportional digital number for processing. Its resolution is given in bits and its accuracy depends on the reference voltage.

A measuring instrument that displays a signal as a graph of voltage against time on a phosphor screen. Its time base and vertical amplifier scales allow amplitude, period and phase to be measured.

A general-purpose measuring instrument that can measure voltage, current and resistance, in analog or digital form. It is used for routine circuit testing and continuity checks.

A laboratory signal source that produces sine, square and triangular waves over a range of selectable frequencies and amplitudes. It is used to apply known test signals to circuits.

A logarithmic unit used to compare two power or voltage levels. A power ratio of ten to one equals ten decibels, and a voltage ratio of about 1.41 equals three decibels.

Joining metal conductors by melting a low-melting alloy onto heated surfaces so that it wets and bonds them. Good technique gives a shiny, smooth joint without excess solder or cold contact.

A reusable plug-in board with interconnected holes used to build and test circuits without soldering. Its rows and power rails let components be inserted and removed quickly during experiments.

The energy difference between the valence band and the conduction band of a material. It determines whether the material behaves as a conductor, semiconductor or insulator.