U
ultra-linear amplification An audio-frequency push-pull output stage using tetrodes or pentodes in which the screen grids are fed from tappings on the primary winding of the output transformer. This arrangement feeds a fraction of the output voltage to the screen grids thus giving negative feedback and by suitable choice of the position of the tapping points odd harmonic distortion can be reduced to a minimum. Even harmonics are, of course, eliminated by the use of push-pull and thus an output stage of this type is capable of excellent linearity.
ultrasonics The study of acoustic waves in material media, the frequency being above that of the audible range. Such waves can be generated by magnetostriction and piezo-electric devices and have many practical applications. For example they are used to assist machining operations on brittle materials, to assist soldering, for echo-sounding purposes and in remote control of television receivers. The term is preferred to ‘supersonic’ which is now reserved for speeds greater than that of sound: supersonic persists, however, in the word superheterodyne.
ultraudion Early form of triode detector using positive feedback.
ultra-violet radiation Electromagnetic waves with a wavelength between approximately 10 and 380 nanometres, i.e. immediately adjacent to the visible spectrum. Although invisible to the human eye, such radiation can affect photographic plates and can produce ionisation.
unbalanced circuit See asymmetrical circuit.
uncommitted logic array (ULA) An array of logic elements fabricated without interconnections on a substrate. Such arrays can be mass produced by the planar process and can be used for a number of different purposes depending on the interconnections between the elements. Customers specify the interconnections they require and these are made in a subsequent metallising operation.
undercoupled circuits Two circuits resonant at the same frequency and between which the degree of coupling is intentionally less than the critical value. The effect of such coupling is to give a response with a single peak at the common resonance frequency and with an amplitude proportional to the degree of coupling but less than that for critical coupling.
undershoot Form of transient distortion of a step or pulse signal in which the response makes a temporary excursion before the main transition and in the opposite direction. See Figure U.1.
unijunction transistor A transistor consisting essentially of a filament of say n-type semiconductor with a p-junction near its centre, connections being brought out from the ends of the filament and from the p-region. The device is well suited for use in simple pulse-generating circuits. The graphical symbol for a unijunction transistor is given in Figure U.2.
unilateral conductivity Property of certain devices, notably electron tubes and semiconductor pn junctions, of conducting electricity easily in one direction and not at all or only slightly in the opposite direction. Such devices have obvious applications as rectifiers, detectors and switches.
unilateral impedance A mutual impedance through which power can be transmitted in one direction only. Active devices such as electron tubes and transistors are examples of unilateral impedances because they enable the current in one circuit to be controlled by the current in another circuit but the receiving-circuit current does not affect the sending-circuit current. Alternatively known as control impedances.
unilateralisation Neutralisation circuit used with bipolar transistors and in which the internal feedback is counteracted by an external series RC circuit, the constants of which are chosen to neutralise the internal collector-base resistance and capacitance. Such neutralisation, if properly adjusted, makes the input and output circuits of the transistor independent of each other so making stable RF amplification possible.
unipolar transistor A transistor which operates by virtue of only one type of charge carrier. An n-channel field-effect transistor is an example in which the charge carriers are electrons.
unitunnel diode A tunnel diode in which the current at the valley point is approximately equal to that at the peak point. See Figure T.16(a).
universal asynchronous receiver/transmitter (UART) In computers and data-processing equipment an interface unit between the central processor and a peripheral device.
universal receiver A receiver which operates from AC or DC mains. Such receivers were common when many parts of the country still had DC mains. The receivers had series-connected electron-tube heaters operating via a dropper resistor from the mains supply and HT was obtained via a half-wave rectifier which acted as a series resistor on DC mains. Also termed AC-DC receiver.
univoltage lens An electrostatic lens consisting of three apertured electrodes, the outer two with a fixed common bias relative to electron-gun cathode potential, focus being affected by variation of the voltage of the inner electrode. Such a lens has a very short focal length and is often used in electron microscopes.
UNIX A multitasking operating system available on a wide variety of computers. It allows multiple users to use the same computer and multiple applications may be run simultaneously.
unstable state Of a circuit a state which cannot persist, the circuit automatically reverting to an alternative state. When a circuit is placed in an unstable state, changes occur within it which automatically terminate the state after a certain time. Usually the change is the discharge of a capacitor through a resistor which, when the discharge has reached a certain point, precipitates the return to the alternative state. Thus the duration of the unstable state depends on the time constant of an RC combination and can be controlled by suitable choice of value for R or C. Monostable circuits have one unstable state, astable circuits have two.
upload To transmit data to another computer.
upward modulation Same as positive modulation.
user-friendly Simple and convenient to operate, needing no specialised skills or knowledge.
utility A piece of software that performs a useful function but which would normally be used in conjunction with other applications. It is not sufficiently comprehensive to be regarded as an application in its own right. For example, a program to load various bitmap formats would be regarded as a utility. It might be used in conjunction with graphics or desktop publishing software needing to display the bitmap.