Experiment 8,   EE 3401 Electronics I Laboratory
Characteristics of Bipolar Junction Transistors  

Objectives:

1. To determine the operating characteristics of a Bipolar Junction Transistor (BJT).

2. To know the operating difference between the two basic types of BJT's : The NPN and PNP transistor.

3. To learn the relationship between the transistor collector current I_C, and the transistor base current I_B.

4. To identify the cutoff and saturation regions over the operating range of a BJT.

Parts and Equipment:

BJT's: NPN - 2N5088  (Beta > 350)
NPN - 2N3904  (50 < Beta < 150)
PNP - 2N3906  (100 < Beta < 300)
Resistors: 47 kW, 18 kW, 1 kW, 2 each
2 DC power supplies
Digital multimeter
Breadboard and wire

Textbook Reference:

Hambley, Allan R., "Electronics," 2^nd Ed. Prentice Hall, 2000, pp.211-234

Serda and Smith, "Microelectronic Circuits," Oxford University Press, New York, NY, 1991, pp. 191-220, or Savant, Roden, and Carpenter

"Electronic Design," The Benjamin/Cummings Publishing Company, Inc., Redwood City, CA, 1991, pp. 69-84

Material Review:

See Text:

Procedure 1: NPN Characteristics

1. Construct the following circuit (Fig. 1) treating each transistor section as an individual circuit to be analyzed separately.

R_C = 1 kW, R_B = 47 kW

2. Set V_CC at +15 VDC, and adjust V_BB to 0.5 VDC. Measure the actual value of each.

3. Use a Digital Multimeter and measure V_BE, and V_CE for each section of the circuit. Later you can calculate the voltages across the base and collector resistor from the voltage measurements and then calculate the currents.

4. Increase V_BB to 1.0 VDC, and repeat measurements as in step 3. Continue recording all measurements of both sections while incrementing V_BB in steps of 0.5 V until you reach 3.0 V, then by 1.0 V increments to 6.0 V, Then replace the two 47 kohm resistors with 18 kohm resistors and steps from 3.0 V to 6 V in steps of 1V.

Procedure 2: PNP Characteristics.

For this procedure combine groups so that each group can have two dc power supplies, one for V_CC and one for V_BB.

1. First disconnect the +V_CC and +V_BB voltages from the circuit and remove the two NPN transistors. Then replace one of the transistors with the PNP transistor. The pin out for the PNP transistor is the same as the previous transistors (see fig. 2).

2. Set V_CC at -15 VDC, and adjust V_BB to - 0.5 VDC.  You will need the second power supply to get the second negative voltage.

3. Use a Digital Multimeter and measure V_BE, and V_CE for the circuit.

4. Change V_BB to -1.0 VDC, and repeat measurements as in step 3. Continue recording all measurements while changing V_BB in steps of -0.5 V until you reach -3.0 V, then by -1.0 V increments to -6.0 V, and finally in steps of -3.0 V to V_BB = -15 V.

CALCULATIONS/GRAPHS:

1. Using Ohms Law and Kirchoff's voltage law, calculate I_B, I_C, and ß for each case for all three transistors.

2. Using a spreadsheet program plot the following graphs: 

a. I_C as a function of V_CE

b. I_C as a function of I_B

c. ß as a function of I_C

d. I_B as a function of V_BE

e. I_C as a function of V_BE

Each graph will contain 3 curves, one for each transistor. Since the values of the voltages and currents are negative for the PNP transistor the curves would plot in the 3rd quadrant instead of the first quadrant as the NPN transistors. Therefore you should use the absolute value of the voltage and current values for plotting the curve for the PNP transistor. Note: Since the x values on the graphs are not the same for each of the transistors, you must arrange your data to insure that each set of y values gets plotted with its correct set of the x values.

3. On each graph, identify both the cutoff region (cutoff occurs when I_C = 0) and saturation region (saturation occurs when V_CE < 0.3 V) of each curve.  Since V_CE is not one of the variables plotted on some of the graphs you must check your data to see which values correspond to this condition.

Questions:

1. Does the I_B vs. V_BE graph give any information on the material from which the transistors are made?

2. Why is the I_C vs. V_CE graph the same for all three transistors? (Write KVL around the loop through the power supply, R_C, and the transistor (V_CE). Then solve for I_C.

3. Explain any difference in the V_BE relationship between the NPN and PNP transistors.

Conclusions:

What can you say about the characteristics of bipolar junction transistors based on your experimental observations?

This page last updated 02/06/2006