Electrical Engineering Dictionary - part 124

offset voltage

for an ideal differential pair

or op-amp, zero output corresponds to zero
differential input. In reality, some nonzero
input called the “offset voltage” is required
for the output to be zero. This means that in
the presence of an offset voltage error, a zero
input will produce a nonzero output. Offset
voltage is caused primarily by mismatch in
the two transistors of a differential pair. See
also

common centroid

Ohm’s Law

a fundamental law which

states that the voltage across a resistance is
directly proportional to the current flowing
through it. The constant of proportionality is
known as the resistance.

This concept can be generalized to include

the relationship between the voltage and cur-
rent in all situations, including alternating
voltages and currents. In this case, all the
quantities are measured as complex numbers,
known as phasors, that are functions of fre-
quency. This broadens the basic definition
of resistance, which is a real number mea-
sured in ohms, to that of impedance, which
is a complex number with magnitude mea-
sured in ohms and phase angle in degrees.
The real part of the complex number repre-
senting impedance is the resistance while the
imaginary part is the reactance. Ohm’s Law
is a central concept to most electrical engi-
neering theories.

Ohm, Georg Simon

(1789–1854) Born:

Erlangen, Germany

Ohm is best known for his discovery of

what we now call Ohm’s Law. Ohm held a
variety of teaching posts at secondary schools
as well as universities. In 1827 he published
his greatest work, Die Galvanische Kette.
Along with Andre Ampere, Ohm was the first
to publish rigorously mathematical and the-
oretical work on electricity. Ohm’s famous
law states that current in a resistor is pro-
portional to the applied voltage and inversely
proportional to the resistance. Ohm’s work
was initially scorned because it lacked the
experimental evidence. Worldwide acclaim
changed Ohm’s fortunes several years later.

He is honored by having his name used as
the unit of resistance, the ohm, and the unit
of conductivity, the mho.

ohmic contact

a heavily doped and/or low

barrier height metal to semiconductor inter-
face or contact that has a very low resistance
relative to the remainder of the device, such
that the device performance is not signifi-
cantly degraded. At lower doping levels, the
ohmic contact is described by Ohm’s Law,
while at higher doping levels, tunneling dom-
inates.

ohmic loss

a term used to describe the

power dissipated due to the finite conductiv-
ity of the metallic structure of an antenna,
waveguide, transmission line, etc.

ohmic medium

a medium in which con-

ductivity is independent of the applied field.

ohmic region

the voltage-controlled re-

sistance region of operation of a transistor.
Also referred to as the triode region, non-
saturated region, and pinch-off region. This
region is in effect up to the point that the chan-
nel of the transistor is completely depleted of
charge carriers.

oil circuit breaker

a power circuit breaker

that uses oil as an insulating and arc-clearing
medium.

oil-filled transformer

a transformer in

which the magnetic core and the windings are
submerged in an insulating oil. In addition to
serving as an insulator, the oil provides a heat
exchange medium to cool the transformer.

oil-paper insulation

an insulation scheme

used in transformers and cables in which con-
ductors are insulated with heavy paper im-
pregnated with a dielectric oil.

OLAP

See

optical linear algebraic

processor

OLE

See

optical logical etalon

2000 by CRC Press LLC

OMCVD

See

metal-organic chemical

vapor deposition

omnidirectional antenna

an antenna that

radiates power equally in all directions in
at least one plane through its radiation cen-
ter, but whose radiation pattern may vary for
other such planes.

on-line error detection

a real-time detec-

tion capability that is performed concurrently
with useful work (e.g., parity checkers, com-
parators in duplicated systems).

on-line memory

memory that is attached

to a computer system.

on-line optimization interval

time inter-

val over which optimization of the decisions
required to be made by the considered deci-
sion mechanism at a particular time instant
during the control system operation is per-
formed; setting of the on-line optimization
interval is one of the essential decisions dur-
ing design of a controller with the model-
based predictive control mechanism.

on-line testing

concurrent testing to de-

tect errors while circuit is in operation.

on–off keying (OOK)

a binary form of

amplitude modulation in which one of the
states of the modulated wave is the absence
of energy in the keying interval (the “off”
state). The “on” state is represented by the
presence of energy in the modulated wave.

on-wafer measurements

electrical mea-

surements made using a wafer probe station
by directly contacting the device under test
using special test probes.

one decibel desensitization point

a re-

duction in a device output signal power by
one decibel due to one or more additional
signals that compress the device output.

one’s complement

(1) a representation of

integer numbers in which a data word is orga-

nized such that negative numbers all contain
a binary “one” in the leftmost bit while posi-
tive numbers contain a “zero” in the leftmost
bit, and in which the negative numbers are the
bit-by-bit inverse of their positive equivalent.

(2) the operation of inverting a data word

so that all ones become zeros and vice versa.

one-cycle control (OCC)

the switched

signal is sensed to control the pulse timing
such that the error between the control ref-
erence and the average value of the switched
signal is zero in each cycle. In a constant fre-
quency implementation, the switch is turned
on by a constant frequency clock. The output
of the switch is integrated and compared to
a reference; when the reference is reached,
the switch is turned off and the integrator is
reset. One-cycle control achieves zero error
within each cycle under steady-state or tran-
sient conditions. In addition, this method ef-
fectively rejects input perturbations, corrects
switching errors, and provides high linearity.

one-dimensional coding scheme

a scheme

in which a run of consecutive pixels of the
same gray scale is combined together and
represented by a single code word for trans-
mission.

Scan lines are coded as “white” or “black,”

which alternate along the line. The scans are
assumed to begin with white and are padded
with white if this is not the case. Generally,
run-length coding is a one-dimensional cod-
ing scheme.

one-dimensional correlator

a correlator

where both input signals are one dimensional,
such as temporal signals.

one-dimensional space integrating corre-
lator

a correlator where a single lagged

product of two input signals is performed at
a given instant and integrated spatially, such
as with a lens onto a single photodetector. A
series of such operations are performed for
each lag value to give a temporal sequence
that is the correlation function.

2000 by CRC Press LLC

one-gun color display

a color CRT in

which a single electron gun produces one
electron beam that is controlled to produce
the proper excitation of each of the three color
phosphors.

one-level memory

an arrangement of

different (in terms of speed, capacity, and
medium) types of memory such that the pro-
grammer has a view of a single flat memory
space. See also

hierarchical memory

virtual

memory

one-level storage

See

one-level memory

one-line diagram

an abbreviated sche-

matic representation of a power system in
which three-phase transmission lines are
shown as single lines between principal cir-
cuit components and from which circuit pa-
rameters are often omitted.

one-out-of-N coding

a method of train-

ing neural networks in which the input vec-
tor and/or the output vector have only one
nonzero element (usually equal to unity) for
each training example.

one-port device

an electrical network in

which only one external terminal is available
for analysis. Antennas can be modeled as
one-port devices.

one-port oscillator

a device in a class

of circuits in which the amplifying device
and its nonlinearity are lumped into a sin-
gle, reactance-free, controlled source. The
oscillations in this approach are most suc-
cessfully found using the harmonic balance
method (especially when the nonlinearity can
be approximated by a polynomial) or by the
phase plane methods (for second-order sys-
tems).

one-shot multivibrator

a circuit that

is obtained from a closed-loop regenerative
bistable system including two similar am-
plifiers connected by coupling circuits. In
one-shot multivibrator, one of the circuits is

purely resistive, another, which is frequently
called “toggling” circuit, includes a reactance
(capacitor or coil). The circuit is normally
in its stable state; by an external pulse it is
transferred in its quasi-stable state, and the
toggling circuit helps to preserve the quasi-
stable condition for the “timing-out” time.
Then the circuit returns to its normal state.

one-step-ahead control

the control

method to drive the value of the

d-step-ahead

output, where

d is the inherent time delay of

the system, to its desired level in one step.

ontogenic network

a network that adapts

its topology during training through the ad-
dition or deletion, as appropriate, of connec-
tions and neurons, until the problem of inter-
est is satisfactorily accommodated. Learning
can be either supervised or unsupervised.

OOK

See

on–off keying

op-amp

See

operational amplifier

OPC

See

optical proximity correction

opcode

a part of an assembly language

instruction that represents an operation to be
performed by the processor.

Opcode was

formed from the contraction of “operational”
and “code.”

open circuit impedance

the impedance

into an N-port device when the remaining
ports are terminated in open circuits.

open drip-proof (ODP)

pertaining to a

ventilated machine whose openings are con-
structed to prevent drops of liquid or solid
particles falling on the machines at an angle
less than 15

◦

from the vertical from entering

the machine either directly or by rolling along
a horizontal or inwardly inclined surface of
the machine.

open kinematic chain

a chain that con-

sists of one sequence of links connecting two
ends of the chain.

2000 by CRC Press LLC

open loop gain

See

open-loop gain

open system architecture

a layered archi-

tectural design that allows subsystems and/or
components to be readily replaced or modi-
fied; it is achieved by adherence to standard-
ized interfaces between layers.

open systems interconnection (OST) model

a framework for organizing networking tech-

nology developed by the International Stan-
dards Organization (ISO). Generally called
the OSI model.

open waveguide

a type of waveguides

whose cross section is not bounded by per-
fect electric conductor, i.e., a waveguide for
which the boundary value problem is on an
infinite domain. A few examples are opti-
cal fiber, microstrips, coplanar waveguides,
dielectric waveguides.

open-circuit test

a transformer test con-

ducted by applying nominal voltage on the
low voltage side while keeping the high volt-
age side open. By measuring the power in,
current, and voltage, the magnetizing reac-
tance of the transformer equivalent circuit
can be determined.

open-circuit quarter-wave transmission
line

transmission line of ninety degrees

electrical length where one end of the line
is terminated in an open-circuit impedance.
The properties of the transmission line re-
sult in the non-open circuited end to exhibit
a short-circuit impedance.

open-delta transformer

a connection

similar to a delta–delta connection, except
that one single-phase transformer is removed.
It is used to deliver three-phase power using
only two single-phase transformers. The nor-
mal capacity of the open-delta transformer is
reduced to 57.7% of its delta rating.

open-loop control system

a control sys-

tem in which the system outputs are con-

trolled by the system inputs only, and no ac-
count is taken of the actual system output.

open-loop gain

the gain of an operational

amplifier with no feedback applied (with the
negative feedback loop “open”).

open-loop-feedback control

predictive

control policy with repetitively used decision
mechanism, that at a given time instant, con-
siders current state of the controlled process
— or the current estimate of this state — and
computes the values of the control inputs for
the next intervention instant or interval by
performing an open-loop optimization of the
process operation over specified prediction
(optimization) interval. The open-loop op-
timization can be defined as an open-loop
stochastic optimization problem or as a de-
terministic optimization problem using a sin-
gle forecast of the free inputs over specified
prediction interval; other forms of open-loop
optimization problems can also be defined.
Model-based-predictive control, widely used
as an industrial standard, is usually realized
in the form of the open-loop-feedback con-
trol.