Transient Response

Transient Analysis Plot of the voltages or currents as a function of time. – Output plot mimics the oscilloscope display. If simulating the operation of a circuit that has a sinusoidal source, you must select Vsin or Isin. – Do NOT use Vac as the sinusoidal voltage source or Iac as the sinusoidal current source. These are sinusoidal sources that should be used only when running an AC Sweep simulation, where calculations are performed as a function of the frequency of operation of the voltage or current source, not as a function of time. 2

PSpice Schematic with LM324 Note (a) the orientation of the negative and positive input terminals, (b) the polarity of the voltage sources tied to the V+ and V- inputs on the op amp, and (c) the placement of the voltage markers.

Polarity of Voltage Sources These two sources are the same.

Setting Attributes for Vsin 5 Double click on the Vsin symbol highlighted in red on the schematic. Set: VOFF (a d.c. offset voltage) to 0V since there is no V dc term in the equation shown to the left. VAMPL (the amplitude of the sine wave) to Vm, which is 5V (or 5) in this example, FREQ (frequency, f, in Hertz – this is not angular frequency) to  Hz. 1kHz, 1000, or 1k.

Transient Analysis Set-up Final Time should be 3-5 times the period (T) of V1. T = 1/f where f is the frequency of the sinusoidal signal. Click on the Transient button located at the lower right of the Analysis Setup pop-up window.

Click on to open the file that contains the plot. You may have seen a count up of the time step (lower right) as the program go through the calculations as a function of time. When done, Simulation complete should appear in the lower left. A plot will be generated where the number of curves will be equal the number of voltage and current markers on the schematic. Note that the curve displayed in this example is not quite sinusoidal.

To obtain a smoother sinusoid In the Transient pop-up window, set a Step Ceiling that is a small fraction of the period of the sinusoid that you are plotting.

Place voltage markers at Vsin and the output pin of the operational amplifier.

Plot of the input voltage and output voltage as a function of time Note that this plot is different from the ones that you will generate as the amplitude of the input signal is higher and the op amp signal didn’t saturate.

Reading Data Off of a PSpice Graph

Cursors Click on the plot to activate the cursors and move them along the plot that has been selected. – The voltage and time of the point on the curve where the cursor is located is displayed in the Probe Cursor pop-up window. Click on the symbol next to the plot name to change which plot the cursors are associated with.