Exercise 2: Collector Current Versus Base Current EXERCISE OBJECTIVE When you have completed this exercise, you will be able to demonstrate the relationship of collector current to base current by using a transistor circuit. You will verify your results with a multimeter. DISCUSSION The transistor circuit shown is the one used in this exercise. Two voltage supplies are used: one to bias the base and the other to supply the collector current. To vary the base current, adjust potentiometer R2. The base-emitter junction is forward biased because the base is more positive than the emitter. Festo Didactic 91564-P0 255
Because the base is more negative (less positive) than the collector, the base-collector junction is reverse biased. The emitter current (I E ) equals the sum of the base (I B ) and collector (I C ) currents: I E = I B + I C. a. base-collector junction is reverse biased. b. base-emitter junction is forward biased. junction is forward biased. 256 Festo Didactic 91564-P0
is forward biased. biased, is that the base region is thin and lightly doped compared to the collector and emitter regions. Because the base region is thin and lightly doped, the electrons from the emitter pass more easily to the heavily doped collector region than out the base terminal. than 5 percent is base current. Festo Didactic 91564-P0 257
The emitter current of a transistor is 10 ma. The collector current would be about a. 0.5 ma and the base current would be about 9.5 ma. b. 9.5 ma and the base current would be about 0.5 ma. Gain For any given transistor, the ratio of collector-base currents (I C B ) is fairly constant when the baseemitter junction is forward biased and the base-collector junction is reverse biased (operating in the linear region). The ratio of dc collector current and base current (I C B ) is the dc current gain. The symbols DC (beta) or h FE represent the dc current gain. DC = h FE = I C B Beta ( DC ) can be between 10 and 500 and increases with the operating temperature of the transistor; the Current gain ( DC ) is an important transistor property because a small base current (I B ) can control a much larger collector current (I C ). Because of gain ( DC ), transistors can amplify a small signal into a large signal. The gain ( DC ) of a transistor is 200. The base current (I B ) is 0.02 ma. Calculate the collector current (I C ). I C = DC x I B I C = ma (Recall Value 1) 258 Festo Didactic 91564-P0
PROCEDURE Locate the TRANSISTOR LOAD LINES AND GAIN circuit block, and connect the circuit shown. NOTE: For the remainder of this procedure, select the R B value (R3, R4, or R5) that provides the easiest base current adjustment. Select the desired resistor by using a two-post connector. Most likely, R4 (47 k ) is the best resistor value for the initial procedure steps. The need for base resistors with different values Adjust the positive variable supply connected to the collector resistor (R9) to 5.0 Vdc, with reference to the Q1 emitter. Festo Didactic 91564-P0 259
In the following procedure steps, you will determine the relationship between collector current (I C ) and base current (I B ). You will set I C by adjusting the voltage drop across 100 collector resistor R9. You will determine I B by measuring the voltage drop across 1 k base resistor R6 and by using Ohm s law to calculate I B. Connect the multimeter across collector resistor R9 to measure the voltage drop (V R9 ). Adjust the knob on potentiometer R2 to set V R9 at 0.20 Vdc. This adjustment sets I C at 2.0 ma. 260 Festo Didactic 91564-P0
I C is 2.0 ma. Measure the voltage drop across base resistor R6 (1 k ). V R6 = Vdc (Recall Value 1) Connect the multimeter across collector resistor R9 to measure the voltage drop (V R9 ). Adjust the knob on R2 to set V R9 at 0.60 Vdc. This adjustment sets I C at 6.0 ma. Festo Didactic 91564-P0 261
I C is 6.00 ma. Measure the voltage drop across R6 (1 k ). V R6 = Vdc (Recall Value 2) Connect the multimeter across collector resistor R9 to measure the voltage drop (V R9 ). Adjust the knob on R2 to set V R9 at 1.00 Vdc. This adjustment sets I C at 10.0 ma. 262 Festo Didactic 91564-P0
I C is 10.00 ma. Measure the voltage drop across R6 (1 k ). V R6 = Vdc (Recall Value 3) Complete the following table to show the collector current (I C ) versus base current (I B ) data that you just obtained. The graph below plots I C versus I B for transistors with gains of 300 and 50. V R9 I C = (V R9 /100 ) x 10 3 V R6 I B = (V R6 /1 k ) x 10 6 0.20 Vdc 2.0 ma Vdc (Step 5, Recall Value 1) A 0.60 Vdc 6.00 ma Vdc (Step 7, Recall Value 2) A 1.00 Vdc 10.00 ma Vdc (Step 9, Recall Value 3) A Festo Didactic 91564-P0 263
The I C versus I B data for the transistor on your circuit can have a wide variation because the gain ( DC the I C versus I B plots for gains of 50 and 300. Does your I C versus I B data fall between the plots for gains of 300 and 50? a. yes b. no From your data, calculate the I B change required to increase I C from 2 ma to 10 ma. Change in I B = µa (Recall Value 4) Calculate the gain ( DC ) of Q1 from the 8.0 ma change in I C. (Recall Value 4 B for an DC = I C B (Recall Value 4 DC = (Recall Value 5) Does your calculated gain ( DC ) from collector and base current measurements fall within a. yes b. no CONCLUSION A large transistor collector current is controlled by a small base current. The ratio of collector current to base current is the current gain of a transistor. The gain of your transistor (Q1) was determined to be around 100. The transistor emitter current equals the sum of the collector and base currents. The base current is generally less than 5 percent of the emitter current. 264 Festo Didactic 91564-P0
REVIEW QUESTIONS 1. The current gain property of a transistor permits a. a small collector current (I C ) to control a large base current (I B ). b. a small base current (I B ) to control a large collector current (I C ). c. a small emitter current (I E ) to control a large collector current (I C ). d. collector current (I C 2. DC ) is expressed by which relationship? a. DC = I C B b. DC = I B C c. DC = I B E d. DC = I C E 3. DC ) of the transistor used in this exercise was between a. 5 and 30. b. 50 and 300. c. 300 and 500. d. 30 and 50. 4. Suppose a collector current (I C DC ) of 200. Calculate the base current (I B DC = I C B ). a. 0.02 ma b. 0.02 µa c. 0.05 µa d. 0.05 ma 5. The base current (I B ) of a transistor is usually a. 95 percent of the emitter current (I E ). b. 50 percent of the emitter current (I E ). c. 50 percent of the collector current (I C ). d. less than 5 percent of the emitter current (I E ). Festo Didactic 91564-P0 265