AC Analysis is used to calculate the small-signal response of a circuit. In AC Analysis, the DC operating point is first calculated to obtain linear, small-signal models for all nonlinear components. Then, the equivalent circuit is analyzed from a start to a stop frequency. The result of an AC Analysis is displayed in two parts: gain versus frequency and phase versus frequency.
Multisim performs AC Analysis using the following process:
Assumptions: The analysis is applied to an analog circuit, small-signal. Digital components are treated as large resistances to ground.
Consider the circuit shown in Figure 1. This is a fourth-order Butterworth low-pass filter with a cutoff frequency of 500 Hz and a passband gain of 10 (20 dB); this circuit was taken from . You will use AC Analysis to determine its frequency response.
Figure 1. Butterworth low-pass filter.
Complete the following steps to configure and run an AC Analysis:
If you want to perform the analysis with specific values for magnitude and phase, double-click the input source, Vin, go to the Value tab and enter values for AC Analysis Magnitude and AC Analysis Phase. In this exercise you will use the default values, 1V and 0°, respectively. The additional settings in the Value tab are used for other analyses or for simulating with the instruments.
Table 1. Parameters used in AC Analysis.
Start frequency (FSTART)
Stop frequency (FSTOP)
Indicates how the analysis frequency is swept. There are three options:
Number of points per decade
Note: In SPICE, the command that performs an AC Analysis has the following form:
.AC <F_SWEEP> <POINTS> <F_START> <F_STOP>
Where .AC initializes an AC Analysis; <F_SWEEP> is the sweep type; <POINTS> is the number of points in the sweep; <F_START> and <F_STOP> are the start and stop frequencies respectively. Note that these are the same parameters that were defined in Table 1, however, in Multisim you do not have to worry about the complex SPICE syntax.
Figure 2. Frequency parameters for the AC Analysis.
The parameters shown in Figure 2 will perform an AC Analysis with frequency sweep from 1 to 10,000 Hz with four subintervals: 1 to 10, 10 to 100, 100 to 1,000, and 1,000 to 10,000. Each subinterval will have 10 points. The greater the number of points calculated, the more accurate the results will be.
Figure 3. Output variable for the AC Analysis.
Figure 4. AC Analysis results.
An additional example file, low_pass_filters.ms11, can be found in the Downloads section. This file contains two subcircuits: a fourth-order low-pass filter in cascade and the Butterworth filter used in the previous exercise. Run AC Analysis to compare their magnitude responses.
 Electric Circuits, James W. Nilsson, Pearson Prentice Hall, 2005, ISBN 0-13-146592-9.
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