AC Small Signal Analysis

Description

An AC Small Signal analysis generates output that shows the frequency response of the circuit, calculating the small-signal AC output variables as a function of frequency.

It first performs an Operating Point analysis to determine the DC bias of the circuit, replaces the signal source with a fixed amplitude sine wave generator, then analyzes the circuit over the specified frequency range. The desired output of an AC Small Signal analysis is usually a transfer function (voltage gain, transimpedance, etc).

Setup

AC Small Signal analysis is set up on the AC Small Signal Analysis Setup page of the Analyses Setup dialog (after the dialog appears, click the AC Small Signal Analysis entry in the Analyses/Options list). An example setup for this analysis type is shown in the image below:

Parameters

• Start Frequency - the initial frequency for the sine wave generator (in Hz).
• Stop Frequency - the final frequency for the sine wave generator (in Hz).
• Sweep Type - defines how the total number of test points is determined from the initial value assigned to the Test Points parameter. The following three types are available:
  Linear - Total number of test points evenly spaced on a linear scale.
  Decade - Number of evenly spaced test points per decade of a log₁₀ scale.
  Octave - Number of evenly spaced test points per octave of a log₂ scale.
• Test Points - defines the incremental value for the sweep range, in conjunction with the chosen Sweep Type.
• Total Test Points (non-editable) - shows the total number of test points in the frequency sweep range, calculated from the initial value for Test Points and the chosen Sweep Type.

Notes

Before you can perform an AC Small Signal analysis, the circuit schematic must contain at least one signal source component with a value entered for the AC Magnitude parameter of its linked simulation model. It is this source that is replaced with a sine wave generator during the simulation.

The amplitude and phase of the swept sine wave are specified in the model parameters for the SIM model linked to the schematic component for the Source. To set these values, double-click on the source component in the schematic, to bring up the Component Properties dialog. In the Models region of the dialog, double-click on the entry for the associated simulation model to open the Sim Model dialog. When this dialog appears, select the Parameters tab to gain access to the AC Magnitude and AC Phase parameters. Enter the amplitude (in Volts) and the phase in (in Degrees). Units are not required. Set the AC Magnitude to 1 to have the output variables displayed relative to 0 dB.

Data is saved for all signals in the Available Signals list, on the General Setup page of the Analyses Setup dialog.

The simulation results are displayed on the AC Analysis tab of the Waveform Analysis window.

Examples

Consider the circuit in the image above, where an AC Small Signal analysis is defined with the following parameter values:

• Start Frequency = 1.000
• Stop Frequency = 1.000meg
• Sweep Type = Decade
• Test Points = 100
• Total Test Points = 601

The entry in the SPICE netlist will be:

*Selected Circuit Analyses:
.AC DEC 100 1 1E6

Running the simulation will yield the output waveforms shown in the image below: