AC Analysis

The AC small-signal analysis portion of DR. SPICE computes the AC output variables as a function of frequency. The program first computes the DC operating point of the circuit and determines linearized, small-signal models for all of the non-linear devices in the circuit. The resultant linear circuit is then analyzed over a user-specified range of frequencies. The desired output of an AC small-signal analysis is usually a transfer function (voltage gain, transimpedance, etc.). If the circuit has only one AC input, it is convenient to set that input to unity and zero phase, so that output variables have the same value as the transfer function of the output variable with respect to the input.

Entry	Description
Start Frequency	The minimum frequency for the analysis.
Stop Frequency	The maximum frequency for the analysis.
Number of Points	Specifies the number of points per decade, octave or across the whole linear range.
Step Type	Dictates how DR. SPICE calculates the interval between the start and stop frequencies. The choices are Decade, Octave and Linear.
Title	Tile which will appear within the Simulation menu.

The AC Analysis dialog box is analogous to the .AC analysis command found in the circuit file. The syntax is:

.AC DEC ND FSTART FSTOP
.AC OCT NO FSTART FSTOP
.AC LIN NP FSTART FSTOP

Examples:

.AC DEC 10 1 10K
.AC DEC 10 1K 100MEG
.AC LIN 100 1 100HZ

The following list describes these abbreviations:

DEC = decade variation
ND = number of points per decade
OCT = octave variation
NO = number of points per octave
LIN = linear variation
NP = number of points of linear variation
FSTART = starting frequency
FSTOP = final frequency

Example AC Circuit Analysis: Butterworth Filter, Order 7

This is one of the example circuits provided with DR. SPICE. You can run the simulation and view the results. In the following example, you can look at the roll-off characteristics of a seventh-order Butterworth filter. See the following circuit:

AC Analysis Example Circuit

The SPICE file created from the schematic is available in the examples directory as but7.cir. The file looks like this:

7TH ORDER BUTTERWORTH LPF 1KHZ ISRC INPUT 0 DC 0 AC 1 CP1 INPUT 0 4.96UF LS1 INPUT A1 14.3MH CP2 A1 0 5.28UF LS2 A1 A2 11.1MH CP3 A2 0 3.36UF LS3 A2 OUTPUT 5.2MH CP4 OUTPUT 0 707NF RLOAD OUTPUT 0 50 .AC DEC 10 1 1MEG .END

If you open the SPICE file and run it now, the output should look like this:

AC Analysis Output File

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