DC Sweep

The question we will ask is How does the voltage at VO vary as VX is raised from 0 to 25 volts We will also view some of the currents through the components. Since this is a DC Sweep, all capacitors are assumed to be open circuits, and all inductors are assumed to be short circuits. We will now set up the DC Sweep. From the menu bar select PSpice and then New Simulation Profile ... 

A DC Sweep will be run to sweep V1 from -15 volts to +15 volts. Select PSpice and then New Simulation Profile from the Capture menus ... 

S0LUTI0I1 Use the DC Sweep to plot the current through the resistor versus the Zener voltage, VZ, Note that the plus sign on Rl indicates the positive voltage reference for the resistor. 

With the Secondary Sweep, we cannot easily identify which trace occurs with what temperature. To associate a trace with a specific temperature, we would need to run a single I-V characteristic at the specified temperature. If we ran a DC Sweep (sweep variable Vin) together with a Parametric Sweep (sweep variable temperature) we could then identify which trace was at what temperature. However, we cannot generate the I-V characteristic using the Parametric Sweep together with the DC Sweep. 

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. 

One of the more useful functions of the DC Sweep is to plot transfer curves. A transfer curve usually plots an input versus an output. A DC transfer curve plots an input versus an output, assuming all capacitors are open circuits and all inductors are short circuits. In a DC Sweep, all capacitors are replaced by open circuits and all inductors are replaced by short circuits. Thus the DC Sweep is ideal for DC transfer curves. The Transient Analysis can also be used for DC transfer curves, but you must run the analysis with low-frequency waveforms to eliminate the effects of capacitance and inductance. Usually a DC Sweep works better for a transfer curve. The one place where a transient analysis works better is plotting a hysteresis curve for a Schmitt Trigger. For a Schmitt Trigger, the input must go from positive to negative, and then from negative to positive to trace out the entire hysteresis loop. This is not possible with a DC Sweep. 

This circuit is also simulated in Section 6.F.I. We would like to sweep VS from -15 volts to +15 volts and plot the output. Select PSpice and then New Simulation Profile from the Capture menus, select a name for the profile, and then click the Create button. Select the DC Sweep Analysis type and fill in the dialog box as shown ... 

In the previous section, the question may arise as to how the Zener breakdown voltage affects the transfer curve. We will assume that you have followed the procedure of the previous section and have already set up the DC Sweep. 

Using a Secondary Sweep, we cannot have Probe identify which trace occurs at which value of the breakdown voltage. We could have generated this same family of curves using a DC Sweep together with a Parametric Sweep. An example is shown for an NMOS inverter in Section 4.D.5. Follow the procedure of Section 4.D.5 and set the DC Sweep variable to Vs and the Parametric Sweep to the Model parameter BV. You can then follow the procedure at the end of Section 4.D.5 to identify which trace is associated with which value of the breakdown voltage (BV). 

SOLUTIOn Set up the DC Sweep with the Primary and Secondary Sweeps as in the previous example. Simulate the circuit and Prty MTI F thC Pfimary SWeCP 311(1 the Sec°ndary Sweep are shown below ... 

Run a DC Sweep to sweep Vln from 0 to 5 V in 0.001 V increments. A small increment is required for a CMOS inverter ... 

A question we would like to ask is, how does the transfer curve of the circuit from the previous section change as we change the driver MOSFET width-to-length ratio We would like a family of curves that show the effect of changing the driver width. Families of transfer curves can be generated using the Parametric Sweep in conjunction with the DC Sweep or by using the Secondary Sweep. The Secondary Sweep was demonstrated in Section 4.D.2, so we will demonstrate the Parametric Sweep here. 

A Parametric Sweep allows us to sweep a parameter. We would like to sweep the width of the driver MOSFET in the circuit of the previous section. We are assuming that you have followed that procedure and have set up the MOSFET models and DC Sweep. We must first set up a parameter for the width of the driver. We will start with the completed circuit from the previous section. It is repeated here for convenience. 

We must now set up the Parametric Sweep. If you are continuing from the previous example, you will already have created a simulation profile for the DC Sweep. To open the profile, select PSpice and then Edit Simulation Profile from the menus. If you started this example as a new circuit, select PSpice and then New Simulation Profile, specify a name for the new profile, and then click the Create button. Set up a DC Sweep as shown below. Using either procedure, you should have the screen below ... 

The PSpice window indicates the current value of the parameter. Notice that for each value of the parameter, the input voltage is swept from 0 to 5 volts. Logically, the DC Sweep loop is executed inside the Parametric Sweep loop. When the simulation is complete, you will be asked which values of W VBl you would like to view. By default, all runs are selected ... 

The nested DC Sweep can be used to generate characteristic curves for transistors. We will illustrate generating these curves using a BJT. Wire the circuit below ... 

The dialog box lists the settings for the Primary DC Sweep. The Primary Sweep sweeps VCE from 0 volts to 15 volts in 0.01 volt increments. The Sweep type is linear, which means that the voltage points are equally spaced. 

For each value of VGS, we want to sweep VDS from 0 to 15 volts. Thus, we want VGS to be our nested sweep variable and VDS to be our main sweep variable. Note also that for jFETs we must sweep VGS from zero to a negative value. Fill in the DC Sweep and Secondary Sweep dialog boxes as shown ... 

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