Electronic Devices and Circuits – EET2222

Lab 4 - Transistor Characteristics and Beta

Reading:	Floyd, Electronic Devices, Section 4-3.
Objectives:	After completing this experiment, you will be able to: 1. Measure and graph the collector characteristic curves for a bipolar junction transistor. 2. Use the characteristic curves to determine the β of the transistor at a given point.
Procedure: Procedure: (cont’d)	1. Measure and record the resistance of the resistors for the circuit in Figure 1. 2. Connect the common-emitter configuration shown in Figure 1 (see Figure 2 for the pin-outs of a 2N2219A transistor). Start with both power supplies set to 0 V. R1 limits the base current to safe levels and allows indirect measurement of the base current. R2 is part of the biasing circuit and also allow indirect measurement of the collector current. 3. Calculate what VR1 will be when IB is 50 μA using Ohm’s Law and the measured value of R1. Then, slowly increase VBB until VR1 is that calculated value. 4. Without changing VBB, slowly increase VCC until the voltage across the transistor’s collector and emitter is +2.0 V (this is VCE). Measure and record VR2 is the appropriate location in Table 1. 5. Using Ohm’s Law and the measured value of R2, calculate the collector current, IC. Enter the computed collector current in Table 1. 6. Repeat steps 4 & 5 for each of the values of VCE listed in Table 1. 7. Calculate the value of VR1 needed to IB to be 100 μA and 150 μA. 8. Reset VCC to 0V and adjust VBB until VR1 is at the calculated value for IB = 100 μA. Then repeat steps 4 & 5 for each of the values of VCE listed in Table 1. 9. Reset VCC to 0V and adjust VBB until VR1 is at the calculated value for IB = 150 μA. Then repeat steps 4 & 5 for each of the values of VCE listed in Table 1. 10. Plot the three collector characteristic curves using the data from Table 1. The collector characteristic curve is a graph of VCE (x-axis) versus IC (y-axis) for a constant base current. Choose a scale for both variables that allow the largest current to fit on the graph. Label each curve with the base current it represents. Properly label each axis with suitable units and values. 11. Using the collector characteristic curve, determine the current gain, β, for the transistor at a VCE values of 3.0 V and 5.0 V for base currents of 50 μA, 100 μA, and 150 μA. Tabulate your results in Table 2. 12. Use the curve tracer to view the collector characteristic curves of at least three different 2N2219A transistors. Determine the value of β for each at a VCE value of 5 V and a base current of 100 μA. Tabulate your results in Table 3. 13. Your lab report is due at the end of the period. Please use the green engineering paper.
Questions:	1. Does the experimental data indicate that β is constant at all points? Does this have any effect on the linearity of the transistor? 2. What is the maximum power dissipated in the transistor for the data taken in the experiment? 3. Compute alpha (α) for the transistor at VCE = 4.0 V and IB = 100 μA. 4. What value of VCE would you expect if the base terminal of a transistor were open? Explain your answer. 5. Does the experimental data indicate that β is constant for different transistors of the same model? What kind of effect might this have on the design of a transistor circuit?
Figure 1:
Figure 2

Table 1

VCE (measured)	Base Current = 50 μA		Base Current = 100 μA		Base Current = 150 μA
	VR2 (measured)	IC (computed)	VR2 (measured)	IC (computed)	VR2 (measured)	IC (computed)
2.0 V
4.0 V
6.0 V
8.0 V

Table 2

	Current Gain, β
VCE	IB = 50 μA	IB = 100 μA	IB = 150 μA
3.0 V
5.0 V

Table 3

Transistor	Current Gain, β
1
2
3