of a neuron to a smaller set of values. This
set is usually
[0, 1]. Typical functions are the
sigmoid function or singularity functions like
the step or ramp.
active contour
a deformable template
matching method that, by minimizing the
energy function associated with a specific
model (i.e., a specific characterization of the
shape of an object), deforms the model in
conformation to salient image features.
active device
a device that can convert
energy from a DC bias source to a signal at
an RF frequency. Active devices are required
in oscillators and amplifiers.
active filter
(1) a filter that has an en-
ergy gain greater than one, that is, a filter that
outputs more energy than it absorbs.
(2) a form of power electronic converter
designed to effectively cancel harmonic cur-
rents by injecting currents that are equal and
opposite to, or 180
◦
out of phase with, the tar-
get harmonics. Active filters allow the out-
put current to be controlled and provide sta-
ble operation against AC source impedance
variations without interfering with the system
impedance.
The main type of active filter is the series
type in which a voltage is added in series with
an existing bus voltage. The other type is the
parallel type in which a current is injected
into the bus and cancels the line current har-
monics.
active impedance
the impedance at the
input of a single antenna element of an ar-
ray with all the other elements of the array
excited.
active layer
See
active region
.
active learning
a form of machine learn-
ing where the learning system is able to in-
teract with its environment so as to affect the
generation of training data.
active load
a transistor connected so as to
replace a function that would conventionally
be performed by a passive component such
as a resistor, capacitor, or inductor.
active load-pull measurement
a mea-
surement method where transfer characteris-
tics of a device can be measured by electri-
cally changing the load impedance seen from
the device. In an active load-pull measure-
ment, the load impedance is defined by using
an output signal from the device and an in-
jected signal from the output of the device.
active logic
a digital logic that operates
all of the time in the active, dissipative region
of the electronic amplifiers from which it is
constructed. The output of such a gate is
determined primarily by the gate and not by
the load.
active magnetic bearing
a magnetic
bearing that requires input energy for stable
support during operation. Generally imple-
mented with one or more electromagnets and
controllers.
active mixer
a mixer that uses three termi-
nal devices such as FET rather than diodes as
nonlinear element. One advantage of active
mixers is that they can provide conversion
gain.
active network
an electrical network
that contains some solid state devices such as
bipolar junction transistors (BJTs) or metal-
oxide-silicon field effect transistors (FETs)
operating in their active region of the volt-
age vs. current characteristic. To ensure that
these devices are operating in the active re-
gion, they must be supplied with proper DC
biasing.
active neuron
a neuron with a non-zero
output.
Most neurons have an activation
threshold. The output of such a neuron has
zero output until this threshold is reached.
active power
See
real power
.
c
2000 by CRC Press LLC