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power-amplifier device • power grid 549
Input
power amplifier
power-amplifier device A high-current tube or
transistor designed especially for high power
output. Such a device does not always provide
significant voltage amplification, but always provides
power amplification. Compare VOLTAGE-
AMPLIFIER DEVICE.
power at peak torque Symbol, Pp. For a torque
motor, the input power in watts needed for peak
torque at stall at a winding temperature of 25 degrees
Celsius.
power attenuation 1. A reduction of power level.
2. See POWER LOSS.
power bandwidth For a high-fidelity audio amplifier,
the difference between the maximum and
minimum frequencies at which the amplifier can
produce at least 50 percent of its maximum
power output, with less than a certain amount of
total harmonic distortion (usually 10 percent).
power blackout A situation in which all electric
power is lost to utility subscribers in a defined region.
power consumption 1. For a direct-current device,
the normal operating voltage multiplied by
the normal drawn current. 2. For an alternatingcurrent
circuit, the root-mean-square voltage
multiplied by the root-mean-square current.
power control The adjustment of the output voltage
of a power supply, usually by means of a variable
autotransformer, silicon-controlled rectifier,
thyratron, or similar device.
power cutoff frequency Symbol, f co . The frequency
at which the power gain of a transistor drops 3 dB
below its low-frequency value.
power derating For a temperature higher than the
specified ambient temperature, a deliberate reduction
of the power dissipated by a component
or device. This is done to prevent failure of the
component or device. Also see DERATING, DER-
ATING CURVE, and DERATING FACTOR.
power difference An expression of the power lost
in a circuit when power is absorbed by a dielectric
material.
power diode A heavy-duty diode that is usually
used in power-supply service. Also called rectifier
diode.
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Output
power dissipation Abbreviation, PD. The power
consumed by a device during normal operation.
This power is not available in the electrical output
of the device. An example is the direct-current
power dissipated in the collector circuit of a highfidelity
audio amplifier.
power divider A circuit that distributes power, in a
predetermined manner, among various loads.
power drain The amount of power drawn by a device.
It can be operating power or standby power.
power dump See DUMP, 2.
power equations Variations of the basic power
equation: P = EI = E 2 /R = I 2 R, where P is the
power in watts, E is the voltage in volts, I is the
current in amperes, and R is the resistance in
ohms.
power factor Abbreviation, PF. In an alternatingcurrent
circuit, the ratio (expressed either as a
decimal or a percentage) of true power (power actually
consumed) to apparent power (simple
product of voltage and current). The power factor
is equal to the cosine of the phase angle. Also see
AC POWER.
power-factor balance In a capacitance bridge, a
separate null adjustment for the internal resistance
component of a capacitor under measurement.
The dial of the variable component for this
adjustment reads directly in percent power factor
in some bridges.
power-factor correction To raise the power factor
of an inductive circuit by inserting a parallel capacitance.
In power circuits, this affords improved
economy of operation because the current
drain is brought more in line with that of a resistive
circuit.
power-factor meter An instrument that gives direct
readings of power factor (lead or lag). One
such meter uses a dynamometer-type movement
(see ELECTRODYNAMOMETER) in which the rotating
element consists of two coils fastened together
at right angles.
power-factor regulator A device that regulates the
power factor of an alternating-current line.
power-factor relay An alternating-current relay
actuated by a rise or fall in power factor, with respect
to a predetermined value.
power frequency 1. See POWER-LINE FRE-
QUENCY. 2. The frequency of an alternatingcurrent
generator. 3. The output frequency of a
power inverter (see INVERTER, 1).
power-frequency meter An instrument for measuring
power-line frequency. It can use electromechanical
devices, or can directly count the
number of alternations per second.
power gain Abbreviations, PG or PG dB . The extent
to which power is increased by a power amplifier.
It can be expressed as the ratio of power output to
power input as PG = P o /P i , or in decibels as
PG dB = 10 log 10 (P o /P i ).
power grid An aggregation of power-generating
stations, transmission lines, and associated