The AC Small Signal analysis 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 nonlinear 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.
All independent voltage and current sources that have AC values are inputs to the circuit. During AC analysis, the only independent sources that have nonzero amplitudes are those using AC specifications. The SIN specification does not count, as it is used only during transient analysis.
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To perform an AC analysis, the circuit must contain at least one independent source that has defined the parameters for the amplitude and phase of the AC signal. These parameters must be specified in the SPICE attributes of the symbol. |
Description
Enter a brief description of the type of analysis. The text entered is the title of the document in which the simulation results will be stored.
Analysis Range
In this section, you must specify the frequency range for which the analysis is to be performed.
Sweep Type
Defines how test points are determined within the specified frequency range.
Linear sweep. The frequency is swept linearly from Start frequency to Stop frequency for the total number of values indicated in Number of points.
Sweep by decades. The frequency is swept logarithmically by decades between the Start Frequency and Stop Frequency. For each interval, the number of values specified by Number of points is taken. The extremes of the intervals are such that extreme2=10*extreme1. For example, if you specify 1K for the initial value, 1Meg for the final value and 10 for number of points. The analysis interval is divided into three sub-intervals and the values are taken, on a logarithmic scale, as ten for each interval.
Sweep by octaves. The frequency is swept logarithmically by octaves between Start frequency and End frequency. For each interval, the number of values specified by Number of points is taken. The extremes of the intervals are such that extreme2=2*extreme1. For example, if you specify 1K for the initial value, 1Meg for the final value and 4 for number of points. The analysis interval is divided into ten sub-intervals and the values are taken, on a logarithmic scale, as four for each interval.
Start Frequency
The initial frequency for the sine wave generator (in Hz).
Stop Frequency
The final frequency for the sine wave generator (in Hz).
Number of Points
Specifies the number of points to be analysed. If the frequency varies linearly, this parameter specifies the total number of points otherwise it indicates the number of points to be analysed for each decade or octave.
Temperature
Specify the temperature at which the simulation will be performed.
Analysis Mode
Specify whether a single or multiple analysis should be performed. The multiple analysis performs several consecutive simulations, varying the value of some parameters at each step. When a multiple analysis is selected, the Setup button is activated. Click on the button to activate the dialog in which to set the specific parameters for the selected analysis mode.
The following analysis modes are supported, click a link to learn more about that analysis mode:
Collect Data For
Specifying the simulation data to be collected. Simulations, especially multiple simulations, can produce a large amount of data, causing excessive memory usage and lower simulation speed. This section allows you to specify which information resulting from the simulation should be stored. Select one of the following options:
Voltages, Currents, Probes: Saves data for the voltage at each node and the current in each supply in addition to the signals specified via the Probe objects included in the schematic.
Voltages, Currents: Saves data for the voltage at each node and the current in each supply;
Signals specified by Probe objects: Stores only the signals specified via the Probe objects included in the schematic.
The specified signals: To reduce the amount of data stored during a simulation it may be convenient to specify the names of only the vectors of interest, in this case only the data relating to the specified vectors will be stored. When this option is selected, the Setup button is activated; click on the button to activate the Vectors dialog box in which to specify the names of the vectors.
Options
Click on this button to activate the dialog box in which to set the options of the simulator. The options set are only valid for the current analysis and replace those defined in the general tab of the simulator options.
Plot
Click on this button to activate the dialog box in which to set the diagrams and the list of vectors to be included in the diagrams. See the Plot dialog box.