BJT Maximum and Minimum Collector Current

In amplifier design it is important to know how your bias will change with device tolerances. In this section we will find the minimum and maximum collector current of a BJT when we include variations in the transistor current gain, 0F, and resistor tolerances. The circuit above was previously simulated in the Transient Analysis and AC Sweep parts. We will use the same resistor values as before, but we will change the resistor models to include tolerance. The BJT is also changed to the model QBf. This model allows 0F to have a uniform distribution between 50 and 350. 

In the previous schematic, the plus sign on Rc is at the top of the resistor. This indicates that I(Rc) will be positive if it flows down. Remember, by definition, the collector current Ic is positive when it enters the collector terminal. Thus, I(Rc) - Ic. If the plus sign were at the bottom of Rc, then I(Rc) would equal -Ic. 

The bias collector current is a DC value, so we must run a DC Sweep to find its minimum or maximum value. Obtain the DC Sweep dialog box Select PSpice and then New Simulation Profile from the Capture menus, enter a name for the profile, and then click the Create button. Select the DC Sweep Analysis type. Fill in the parameters as shown  

The dialog box is set up to simulate the circuit at Vcc = 15 V. This was already specified in the circuit and appears to be redundant. It must be specified because the Worst Case analysis must run in conjunction with an AC Sweep, a DC Sweep, or a Transient Analysis. Since we are interested in the DC collector current we must set up a DC Sweep. 

Next we need to set up the Worst Case analysis. Click the LEFT mouse button on the O next to the text MDfltB Carto/Worst Casa to select the analysis. Fill in the dialog box as shown  

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