DC Operating Point Analysis

DC analysis determines the quiescent DC operating point of the circuit with inductors shorted and capacitors opened. A DC analysis, known as the “Initial Transient Solution,” is automatically performed prior to a transient analysis to determine the transient initial conditions. A DC analysis, referred to as the “Small Signal Bias Solution,” is performed prior to an AC small-signal analysis to determine the linearized small-signal models for all nonlinear devices. It should be noted that these two operating point calculations may be different, depending on the DC and transient stimulus used.

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DC Sweep Analysis

The .DC function is a special subset of the DC analysis feature. It is used to perform a series of DC operating points by sweeping voltage and/or current sources, and performing a DC operating point calculation at each step value of the source(s). At each step, the DC voltages, currents, and computed device/model parameters can be recorded. The .DC line defines which sources will be swept, and in what increments. One or two sources can be involved in the DC sweep. If two are involved, the first source will be swept over its range for each value of the second source. This option is useful for obtaining semiconductor device output characteristics or calculating load lines.

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AC Analysis

AC analysis computes the small signal response of the circuit. Output variables are recorded as a function of frequency. Before the AC analysis is performed, IsSpice4 first computes the DC operating point of the circuit. It then determines the linearized, small-signal models for all of the nonlinear devices in the circuit, based on this operating point. The resultant linear circuit is then analyzed over the specified range of frequencies. Therefore, it is important to establish the proper DC circuit biasing in order for the AC analysis to produce useful data. For example, biasing an op-amp in its linear range will give different AC results than if the op-amp is saturated.

DC Bias Note: It should be noted that the small-signal bias point is determined by the DC values on the independent source rather than the initial transient signal generator values.

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 (normal case), then that input is traditionally set to unity magnitude and zero phase. By doing so, the output variables have the same value as the transfer function. For example, if the input is a voltage source with magnitude 1, then the output node voltages would equal gain: Gain = Vout/Vin which equals Vout, with Vin = 1.

Although the AC analysis performs a sinusoidal steady-state analysis, it should not be confused with a transient (time domain) analysis using a large signal SINE wave. The AC analysis is a small-signal analysis where all nonlinearities are linearized. For instance, if the DC biasing of a transistor gain stage produces a gain of ten, then the gain will remain ten no matter what the input. If 1 is the input, then 10 is the output. If 100 is the input, then 1000 is the output. The gain is linearized. Under nonlinear conditions, however, the gain of the transistor will roll off as the input is increased. The “VName 1 0 SIN.....” stimulus is only used for time-domain analyses, and should not be confused with the “Vname 1 0 AC 1” AC stimulus.

Frequency Mixing Note: The AC analysis is a single frequency analysis. Only one frequency is analyzed at a time. Therefore, circuits performing signal mixing will not benefit from the AC analysis. In order to see frequency mixing, you will have to run a transient analysis and convert the output waveforms into the frequency domain using a Fourier transform.

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Pole-Zero Analysis

Pole-zero analysis computes the poles and/or zeros of a small-signal AC transfer function. The program first computes the DC operating point and, like the AC analysis, determines the linearized small-signal models for all of the nonlinear devices in the circuit. The circuit is then analyzed to find the poles and zeros. The pole-zero analysis works with resistors, capacitors, inductors, linear-controlled sources, independent sources, BJTs, MOSFETs, JFETs, MESFETs, and diodes. Transmission lines are not supported.

Two types of transfer functions are allowed, VOL and CUR: VOL represents (output voltage)/(input voltage), while CUR represents (output voltage)/(input current). These two types of transfer functions cover all cases. For each transfer function, you can find the poles, zeros or both. The primary emphasis of this feature is if there is non-convergence in finding poles or zeros, then at least the other entity can be found. The input and output ports are specified as two pairs of nodes. Thus, there is complete freedom regarding the output and input ports and the type of transfer function. The results of the pole-zero analysis may be found in the output file.

The method used in the analysis is a suboptimal numerical approach. For large circuits, it may take a long time or fail to find all of the poles and zeros. For some circuits, particularly those with active devices and op-amp macro models, the method may become lost and find an excessive number of poles and zeros.

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Transient Analysis

Transient analysis computes a circuit response as a function of time over any time interval. During a transient analysis, any number of independent sources may produce active time-varying stimulus signals.

The initial conditions are normally determined by a DC analysis called the initial transient solution. The UIC (Use Initial Condition) option may be used to allow the simulation to begin from a user-specified state.

Dynamic time-step is controlled by a trapezoid integration technique used by the IsSpice4 simulator by default. You can also use GEAR integration or Intusoft’s exclusive VSECTOL to control the time-step.

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Fourier Analysis

Fourier analysis produces the magnitude and phase versus frequency response for the DC and first 9 harmonic frequencies of a design simulation, plus total harmonic distortion. Further, the normalized frequency and phase are printed, along with the total harmonic distortion. Several output variables may be listed for the analysis. Numerical accuracy limits the value of this analysis to rather high values of distortion, usually greater than .1%, which is the default computational accuracy. The results will be found in the output file.

Care must be taken when performing this analysis, because IsSpice4 is actually performing a Discrete Fourier Transform (DFT). Care must be taken when performing a DFT on a non-periodic waveform. A more flexible version of the Fourier analysis is available through the use of the ICL Fourier function. This version allows a variable number of harmonics and complex expressions instead of just node voltages.

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