18WuD2x

638 small-signal analysis • smoke detector
small-signal analysis Analysis of circuit or component
operation in which it is assumed that the
signals deviate from (fluctuate to either side of)
the steady bias levels by only a small amount.
Also see SMALL SIGNAL.
small-signal bandwidth The frequency at which
the output signal of an analog circuit decreases to
–3 dB, relative to the value for direct current. The
output voltage is generally set at 0.1 volt peak-topeak
for testing this value so that the circuit is
not overdriven.
small-signal component 1. A coefficient or parameter
(such as amplification, transconductance,
dynamic resistance, etc.) calculated or
measured under conditions of small-signal operation.
Also see SMALL SIGNAL and SMALL-
SIGNAL EQUIVALENT CIRCUIT. 2. A device
designed for operation at low signal levels.
small-signal diode See SIGNAL DIODE.
small-signal equivalent circuit For a given transistor
circuit, the equivalent circuit for low signal levels
(i.e., at amplitudes lower than saturation and
cutoff levels). Also see EQUIVALENT CIRCUIT.
small-signal operation Operation at low signal
amplitudes (i.e., at signal levels that do not extend
into the saturation or cutoff levels of a transistor,
diode, or other component).
small-signal transistor A transistor designed for
low-level applications, such as the amplification
of small voltages and currents and low-voltage
switching. Compare POWER TRANSISTOR.
SmallTalk A high-level computer programming
language that uses a graphical user interface
(GUI). It is used in complex design and research,
and in robotics.
smart home or business An electronically controlled
home or business, in which computers
and robots take care of cooking, dish washing,
floor scrubbing, waste removal, laundry, yard
maintenance, snow removal, and other mundane
chores. Intrusion detection and fire prevention
are constantly maintained. In some cases, intruders
can actually be identified or detained;
fires can be extinguished by controlled sprinkler
systems or remotely controlled robots. Some
tasks can be monitored and controlled directly by
the owner from remote locations, via telephone
lines or wireless.
smartness The ability of an electronic system, especially
a computer or control system, to perform
a complete series of operations, substituting alternative
steps, where necessary—all with a minimum
of instructions from, and supervision by,
human operators.
smearing In television or facsimile, a form of picture
distortion caused by an excessively narrow receiving
bandpass. The image appears fattened and
horizontally blurred. Contrast might also be lost.
smectic crystal A liquid crystal in which the
molecules are arranged in parallel layers and
cannot slide past each other.
S meter In a radio communications receiver, a meter
graduated in S units and/or decibels to indicate
the strength of a received signal.
1
3
5
S units
S units
7
9
S meter
Smith chart A curvilinear graph on which complex-number
impedance values can be plotted. It
is useful in evaluating the behavior of radiofrequency
circuits, transmission lines, and
antenna systems—especially with regard to
impedance mismatches and standing-wave ratio.
0 − j40
−50
SWR 10:1
−25
−100
Reactance
Resistance
50 + j0
50 − j50
SWR = ∞ :1
(Worst possible
mismatch)
0 Short
25
50
100
∞ Open
20
dB
dB
50 + j50
Smith chart
40
60
SWR = 4:1
+25
SWR = 2:1
+100
0 + 40
+50
smoke alarm A device that produces audible
and/or visible signals in the presence of smoke or
unusual gases in the air. Also see PHOTOELEC-
TRIC SMOKE ALARM.
smoke control See PHOTOELECTRIC SMOKE
CONTROL.
smoke detector Any circuit or device used to
sense the presence of smoke or noxious gases.