User`s guide

40 Analo

Behavioral Modelin

Cautions and

Recommendations for

Simulation and Anal

sis

Instantaneous Device Modelin

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 6-20.

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.

ure 6-20

Volta

e Multiplier

Circuit (Mixer)