Electronics

G

Sensing Devices and Transducers

Measurements of mechanical, thermal, electrical, and chemical quantities are made by devices called sensors and transducers. The sensor is responsive to changes in the quantity to be measured, for example, temperature, position, or chemical concentration. The transducer converts such measurements into electrical signals, which can be fed to instruments for the readout, recording, or control of the measured quantities. Sensors and transducers can operate at locations remote from the observer and in environments unsuitable or impractical for human beings.

Some devices act as both sensor and transducer. A thermocouple has two junctions of wires of different metals; these generate a small electric voltage that depends on the temperature difference between the two junctions. A thermistor is a special resistor, the resistance of which varies with temperature. A variable resistor can convert mechanical movement into an electrical signal. Specially designed capacitors are used to measure distance, and photocells are used to detect light (see Photoelectric Cell). Other devices are used to measure velocity, acceleration, or fluid flow. In most instances, the electric signal is weak and must be amplified by an electronic circuit.

IV

Power-Supply Circuits

Most electronic equipment requires power for its operation to be supplied in the form of DC voltage and current. These can be provided by batteries or by internal power supplies that convert alternating current as available at the home electric outlet, into regulated DC voltages. The first element in an internal DC power supply is a transformer, which steps up or steps down the input AC voltage to a level suitable for the operation of the equipment. A secondary function of the transformer is to provide electrical ground insulation of the device from the power line to reduce potential shock hazards. The transformer is then followed by a rectifier, normally a diode. In the past, vacuum diodes and a wide variety of different materials such as germanium crystals or cadmium sulphide were employed in the low-power rectifiers used in electronic equipment. Today, silicon rectifiers are used almost exclusively because of their low cost and their high reliability.

Fluctuations and ripples superimposed on the rectified DC voltage (sometimes noticeable as a hum in a malfunctioning audio amplifier) can be filtered out by a capacitor; the larger the capacitor, the smaller the amount of ripple in the voltage. More precise control over voltage levels and ripples can be achieved by a voltage regulator, which also makes the internal voltages independent of fluctuations that may be encountered at an outlet. A simple, often-used voltage regulator is the zener diode. It consists of a solid-state p-n-junction diode, which acts as an insulator up to a predetermined voltage; above that voltage it becomes a conductor that bypasses excess voltages. More sophisticated voltage regulators are usually constructed as integrated circuits.

V

Amplifier Circuits

Electronic amplifiers are used mainly to increase the voltage, current, or power of a signal. An ideal linear amplifier would provide signal amplification with no distortion, so that the output was proportional to the input. In practice, however, some degree of distortion is always introduced. A non-linear amplifier may produce a considerable change in the waveform of the signal. Linear amplifiers are used for audio and video signals, whereas non-linear amplifiers find use in oscillators, power electronics, modulators, mixers, logic circuits, and other applications where an amplitude cut-off is desired. Although vacuum tubes played a major role in amplifiers in the past, today either discrete transistor circuits or integrated circuits are generally used.

A

Audio Amplifiers

Audio amplifiers, such as are found in radios, television sets, citizens’ band (CB) radios, and cassette recorders, are generally operated at frequencies below 20 kilohertz (1 kHz = 1,000 cycles/sec). They amplify the electrical signal, which is then converted to sound in a loudspeaker. Operational amplifiers (op-amps), built with integrated circuits and consisting of DC-coupled, multi-stage, linear amplifiers are popular for audio amplifiers. Some devices of this kind have high-power output stages capable of driving loudspeakers directly. Most, however, need to have their output further amplified before the signal is fed into the loudspeakers.

B

Video Amplifiers

Video amplifiers are used mainly for signals with a frequency spectrum range up to 6 megahertz (1 MHz = 1 million cycles/sec). The signal handled by the amplifier becomes the visual information presented on the television screen, with the signal amplitude regulating the brightness of the spots forming the image on the screen. To achieve its function, a video amplifier must operate over a wide frequency band and amplify all frequencies equally and with low distortion. See Video Recording.