Project 4: The Saturated Transistor

Objective

Normally, for a transistor, IC = β × IB. However, this relationship does not hold when a transistor is saturated. The objective of this project is to determine the relationship of the collector current to the base current when a transistor is saturated.

General Instructions

Using the same breadboarded circuit you built in Project 3, set the base current to several values, starting at 90 μA and increasing the base current. Record measurements of the collector current and collector voltage at each value of the base current.

Step-by-Step Instructions

Follow these steps and record your measurements in the blank table following the steps.

1. Set up the circuit you built in Project 3.1.
2. Adjust the potentiometer to set IB at approximately 90 μA.
3. Measure and record IB.
4. Measure and record IC.
5. Measure and record VC.
6. Adjust the potentiometer slightly to lower its resistance, which sets a larger value of IB.
7. Measure and record the new values for IB, IC, and VC.
8. Repeat steps 6 and 7 until you reach the lower limit of the potentiometer, which is also the highest value of    IB.

Expected Results

Figure 3.25 shows the test setup for this project with the potentiometer set at its lower limit, providing the highest value of IB. The test setup is the same as that used in Project 3.1; however, the value of IB is considerably higher than in Project 3.1.

Fig 3.25

 Compare your measurements to the ones shown in the following table.

Your data will probably have slightly different values but should indicate that IC stays constant for values of VC of 0.2 and below, whereas IB continues to rise. In this region, the transistor is fully ON (saturated) and IC can't increase further. This agrees with the data sheet published by Fairchild Semiconductor for the 2N3904 transistor, which indicates that the transistor saturates at VC = 0.2 volts.

Now, compare the circuits shown in Figure 3.26.

Fig 3.26

Because the base circuit is broken (that is, it is not complete), there is no base current flowing.