AC Sweep

The AC Sweep is used for Bode plots, gain and phase plots, and phasor analysis. The circuit can be analyzed at a single frequency or at multiple frequencies. In this part we will illustrate its use at a single frequency for magnitude and phase results (phasors), and at multiple frequencies for Bode plots. 

It is important to realize the difference between the AC Sweep and the Transient Analysis discussed in Part 6. The AC Sweep is used to find the magnitude and phase of voltages and currents. The Transient Analysis is used to look at waveforms versus time. An example of a waveform versus time is  

This graph shows us a voltage versus time. If you want to look at the magnitude and phase of voltages and currents, use the AC Sweep. The magnitude of the above waveform is 5 V and the phase is zero degrees—in phasor notation, 5Z0°. The AC Sweep will give a result such as 5Z0°. 

The AC Sweep uses the sources VAC and IAC. These sources are functions of magnitude and frequency. Do not use the source VSIN for the AC Sweep. 

The source is an AC source with a magnitude of 10 volts and a phase of 37 degrees, 10Z370 in phasor notation. This magnitude can be interpreted as either peak or RMS. If you specify 10Z37° as the magnitude of the source, and the number 

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The node voltage analysis performed by PSpice is for DC node voltages only. This analysis solves for the DC voltage at each node of the circuit. If any AC or transient sources are present in the circuit, those sources are set to zero. Only sources with an attribute of the form DC=l/ff/i/ff are used in the analysis. If you wish to find AC node voltages, you will need to run the AC Sweep described in Part 5. The node voltage analysis assumes that all capacitors are open circuits and that all inductors are short circuits. 

In circuit design, we are sometimes concerned with how a circuit parameter affects performance. There are two ways to vary parameters in PSpice. The first is the DC Sweep, where we vary a parameter rather than a DC voltage. This method generates a single curve. The second is a Parametric Sweep that is run in conjunction with another analysis such as an AC Sweep, DC Sweep, or a Transient Analysis. The second method generates a family of curves. In this section we will demonstrate only the DC Parametric Sweep. Throughout this manual there will be examples using the Parametric Sweep in conjunction with the other analyses. 

This PRINT statement prints the phases of the specified voltages and currents. For example, VP(2,0) prints the phase of the voltage at node 2 relative to node 0 (ground), and IP(L L 1) prints the phase of the current through inductor L1. The C in the attribute tells the PRINT statement to print the results of the AC Sweep. When you change the attributes you should have the following schematic ... 

The AC Sweep setup is shown below. The results are stored in the output file ... 

We will now set up the AC Sweep for this circuit. Remember that the AC Sweep gives us the phase and magnitude of sinusoidal waveforms at specified frequencies. For example, if Vy = 5sin(1000t + 30), the result of the AC Sweep will be 5 for the magnitude and 30 for the phase. If you want to see Vy displayed as a function of time, you must run a Transient Analysis. 

Select the AC Sweep/Noise Analysis type and fill in the dialog box as shown below. 

At low frequencies the capacitor is an open circuit and V0 should equal Vi. At high frequencies the capacitor becomes a short, and the gain goes toward zero. The 3 dB frequency of the circuit is to = 1/RC = 1,000 rad/s = 159 Hz. We will set up an AC Sweep to sweep the frequency from 1 Hz to 10 kHz. Select PSplce and then New Simulation Profile from the Capture menus and then enter a name for the profile and click the Create button. Select the AC Sweep/Nolse Analysis type and fill in the parameters as shown in the AC Sweep dialog box below ... 

SOLUTIOn Set up an AC Sweep from 1 Hz to 1 MHz. Simulate the circuit and then run Probe ... 

Whenever you simulate an amplifier with transistors you should always check the bias point. (See Section 3.E, Transistor Bias Point Detail, on page 187.) If the bias is not correct, then the AC Sweep results are meaningless. Always check the bias point first. 

We must set up an AC Sweep. Select PSpice and then New Simulation Profile from the Capture menus and then enter a name for the profile and click the Create button. Select the AC Sweep/Nolse Analysis type and fill in the parameters as shown in the AC Sweep dialog box below ... 

The parameter we have defined (RF val) is a Global parameter. The sweep settings are similar to the DC and AC Sweeps discussed previously. We would like specific values for the parameter, so we will use the value list. Logically, the parametric sweep is executed outside the AC Sweep. First, RF Val will be set to Ilf and then the AC Sweep will be performed. Next, RF val will be set to lOh and then the AC Sweep will be performed. Then, RF val will be set to lOOH, and so on. Click the OK button to accept the settings to return to the schematic. 

As you watch the simulation s progress you will see that the AC Sweep runs four times, once for each value of the parameter RFya . In the window above, the current AC Sweep is for RF val = 100,000. 

Set up an AC Sweep (PSpice, New Simulation Profile, AC Sweep/Noise) to sweep frequencies from 1 Hz to 1 MHz at 100 points per decade. Run PSpice (PSpice, Run). When Probe runs, add the trace V(l) / I (I test). Remember that voltage divided by current is impedance. We are dividing the voltage between nodes 1 and 0 by the current flowing into and out of those nodes. This is the impedance between those nodes. You will see this trace ... 

The AC Sweep is used to generate plots of magnitude versus frequency, or plots of phase versus frequency. 

The AC Sweep and the dB command are used to generate Bode plots. 

The Transient Analysis uses the sinusoidal voltage waveform when it performs the simulation. This circuit would not work if an AC Sweep were used. To set up a Transient Analysis, select PSpice and then New Simulation Profile from the Capture menus ... 

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