Page 212 - Electrical Engineering Dictionary

tween its input voltages, while rejecting any differential inclusion a multivalued dif-
signal common to both inputs. ferential equation,
The differential amplifier is designed such
˙ x ∈ F(t, x),
that the difference between the two inputs is
amplified (high differential gain), while the where F(t, x) is a nonempty set of velocity
signals appearing at either individual input vectors at x ∈ R for each time t on some
n
(referenced to ground potential) sees a very time interval. The set F(t, x) can be viewed
low gain (low common-mode gain, usually as the set of all possible “velocities” ˙ x(t) of a
loss). The differential amplifier is usually dynamical system modeled by the multival-
used as the first component at the receiving ued, or multifunction, differential equation.
end of a communications link using twisted A solution x(t) is an absolutely continuous
pair cable (either shielded or unshielded) as function on some time interval whose veloc-
the transmission medium. This provides a ity vector ˙ x lies in the set F(t, x) for almost
method to reject any common-mode noise all t. See also Filippov method.
induced onto the twisted pair transmission
line, including common-mode noise falling differential kinematics equation v =
within the useful bandwidth of the commu- J(q)˙q can be interpreted as the differential
nications link. The figure of merit for the kinematics mapping relating the n compo-
differential amplifier is its common mode re- nents of the joint velocity vector to the r ≤ m
jection ratio (CMRR), computed by dividing components of the velocity vector v of con-
the differential-mode gain by the common- cern for the specific task. Here n denotes
mode gain. number of degrees of mobility of the struc-
ture, m is the number of operational space
differential coding a coding scheme that variables, and r is the number of operational
codes the differences between samples. See space variables necessary to specify a given
predictive coding. task. See also geometric Jacobian.

differential entropy the entropy of a con- differential length vector the vector sum
tinuous random variable. For a random vari- of the differential length changes in each of
able X, with probability density function the three coordinate directions along a given
f(x) on the support set S, the differential curve.
entropy h(X) is defined as
differential mode gain for a differential
Z
amplifier, the ratio of the output signal ampli-
h(X) =− f(x) log f(x)
tude to the amplitude of the difference signal
S
between the amplifier input terminals.
provided the integral exists. Also writ-
ten h(f ), emphasizing the sole dependence differential pair a two-transistor BJT
upon the density. See also entropy, relative (FET) amplifier in which a differential in-
entropy, mutual information. put signal is applied to the base (gate) ter-
minals of the two transistors, the output is
differential gain the amplification factor taken differentially from the collector (drain)
of a circuit that is proportional to the differ- terminals, and the emitter (source) terminals
ence of two input signals. The differential are connected together to a constant current
gain may be expressed in percentage form source. Also known as an emitter-coupled
by multiplying the above amplification fac- pair(BJT)orsource-coupledpair(FET).This
tor by 100, or in decibels by multiplying the configuration is often used as the basis of the
common logarithm of the differential gain by differential input stage in voltage feedback
20. op-amps.

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2000 by CRC Press LLC