User`s guide

10
-
8 AC Analyses
Usin
g
a DC source
Consider the circuit shown here. At the DC bias point, PSpice
A/D calculates the partial derivatives which determine the linear
response of the multiplier as follows:
where the terms in bold are calculated at the DC bias point.
For this circuit, this equation reduces to:
This means that the multiplier acts as an amplifier of the AC
input with a gain that is set by the DC input.
Caution: Multipl
y
in
g
AC sources
Suppose that you replace the 2 volt DC source in the above
example with an AC source with amplitude 1 and no DC value
(DC=0). When PSpice A/D computes the bias point, there are no
DC sources in the circuit, so all nodes are at 0 volts at the bias
point. Now the linear equivalent of the multiplier block is a
block with gain 0, which means that there is no output voltage at
the fundamental frequency.
VOut()VIn
1
()
VOut()∂
VIn
1
()∂
---------------------
⋅ VIn
2
()
VOut()∂
VIn
2
()∂
---------------------
⋅
+=
VIn
1
()
=
VIn
2
()⋅ VIn
2
()VIn
1
()⋅
+
VOut()VIn
1
()
2
VIn
2
()
0
⋅
+
⋅
=
This is exactly how a double-
balanced mixer behaves, which
is in practice, a simple multiplier.
Note
A double-balanced
mixer with inputs at the same
frequency would produce
outputs at DC and at twice the
input frequency, but these
terms cannot be seen with a
linear, small-si
g
nal analysis.