User Guide

Chapter 6 Analog behavioral modeling
186
Cautions and recommendations
for simulation and analysis
Instantaneous device modeling
During AC analysis, nonlinear transfer functions are
handled the same way as other nonlinear parts: each
function is linearized around the bias point and the
resulting small-signal equivalent is used.
Consider the voltage multiplier (mixer) shown in
Figure 54. This circuit has the following characteristics:
Vin1: DC=0v AC=1v
Vin2: DC=0v AC=1v
where the output on net 3 is V(1)*V(2).
During AC analysis, V(3) = 0 due to the 0 volts bias point
voltage on nets 1, 2, and 3. The small-signal equivalent
therefore has 0 gain (the derivative of V(1)*V(2) with
respect to both V(1) and V(2) is 0 when V(1)=V(2)=0). So,
the output of the mixer during AC analysis will be 0
regardless of the AC values of V(1) and V(2).
Another way of looking at this is that a mixer is a
nonlinear device and AC analysis is a linear analysis. The
output of the mixer has 0 amplitude at the fundamental.
(Output is nonzero at DC and twice the input frequency,
but these are not included in a linear analysis.)
If you need to analyze nonlinear functions, such as a
mixer, use transient analysis. Transient analysis solves the
full, nonlinear circuit equations. It also allows you to use
input waveforms with different frequencies (for example,
VIN1 could be 90 MHz and VIN2 could be 89.8 MHz). AC
analysis does not have this flexibility, but in return it uses
much less computer time.
Figure 54
Voltage multiplier circuit
(mixer).
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