AENG001-4-2 Analogue Electronics
Experiment 2 – Voltage Divider Bias for BJTs
Introduction: Voltage divider bias is one of the best and most commonly used bias methods for stabilizing BJT circuits. In Section I of this experiment you will set up a voltage divider-biased circuit, predict circuit values, and make circuit measurements to evaluate the stability of the quiescent circuit values. Section II of this experiment deals with troubleshooting. In this section you will simulate three different faults and make measurements of the circuit for each fault. This will develop your troubleshooting knowledge by letting you observe the effects of a component’s failure and by giving you the measurement data of the fault.
Objectives: In this experiment you will: a. Add to your understanding of voltage divider bias b. Verify the stability of a voltage divider-biased circuit c. Determine circuit values for different component component failures
Equipment and Materials: DC power supply Digital multimeter Circuit protoboard NPN transistor [2], 2N3904 or equivalent Resistors: 270 Ω, 1.2 kΩ, 2.7 kΩ,12 kΩ, 2.2 MΩ
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AENG001-4-2 Analogue Electronics
Section 1 – Functional Experiment
Figure 2.1
1. Construct the circuit shown in Figure 2.1. Set the DC power supply to 12 V, and connect the power supply to your circuit. 2. Using the digital multimeter, measure and record the circuit parameters listed below. IE =
VB =
VC =
VCE =
VE =
3. Turn off the DC power supply and substitute a second 2N3904 transistor. Reapply circuit power, measure and record the circuit parameters again. IE =
VB =
VC =
VCE =
VE =
Changing transistors in the circuit gave two measurements with two different (although unknown) values of beta. To test the circuit stability further, we can use the fact that beta increases with temperature. The next set of measurements will be made while the transistor is being heated.
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AENG001-4-2 Analogue Electronics
Figure 2.2 4. (Optional) You can use a heat gun or soldering iron to heat the transistor. If you use a soldering iron, do not actually touch the transistor with the hot tip of the soldering iron – hold the soldering iron close to the transistor. Heat the transistor for about 1 minute, or until the case is very warm to the touch. Make the following measurements quickly following the heating of the transistor before it returns to ambient temperature: IE =
VB =
VC =
VCE =
VE =
An alternative method to heating the transistor is to cool it using component cooler spray. This will decrease beta. 5. Now you have three sets of data from your circuit. Solve the circuit for the same parameters as those measured. Calculate the Thevenin values of the voltage divider – RTH and V TH. Enter these values in the blanks provided below and in the beta box model in the Figure 2.2. Assume a beta of 150, and, using the beta box model, calculate emitter current, collector voltage, base voltage, and emitter voltage. Also enter these results in the appropriate blanks provided below. VTH =
RTH =
VB =
VE =
VC =
VCE =
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AENG001-4-2 Analogue Electronics
Section 2 – Troubleshooting Fault 1 – RB divider resistor fails When a resistor fails, it becomes a high resistance, or open circuit. In this procedure, you will simulate an open-resistor failure by substituting a large resistance value for resistor R B. 1. Modify your circuit of Figure 2.1 by replacing the 2.7 kΩ resistor with the 2.2 MΩ resistor. 2. Apply 12 V DC to the circuit. Using the digital multimeter, measure and record the circuit values listed below: VB =
VE =
VC =
VCE =
3. Compare the circuit values just obtained to those of a normal circuit (Section 1, step 2 or 3). Notice in particular the readings that had the large change.
Fault 2 – R1 divider resistor fails 1. Turn off the circuit power and replace R B with the 2.7 kΩ resistor. Exchange the 12 kΩ value of R 1 with the 2.2 MΩ resistor. 2. Reapply 12 V DC to the circuit. Using the multimeter, measure and record the circuit values listed below. VB =
VE =
VC =
VCE =
You should have found that the measured circuit values for this fault are quite different from those of a normal circuit. Also notice the differences from the values obtained for the R B fault.
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AENG001-4-2 Analogue Electronics
Fault 3 – Emitter resistor fails 1. Turn off the DC power to the circuit and replace resistor R 1 with the correct value of 12 kΩ. Exchange the 270 Ω emitter resistor for the 2.2 MΩ resistor. 2. Reapply 12 V DC to the circuit. Using the multimeter, measure and record the circuit values listed below. VB =
VE =
VC =
VCE =
Notice that each fault gave a different and unique set of circuit values, and that in each case, the transistor shifted to either a cut off or a saturation condition.
Discussion Section 1 1. Discuss, from the standpoint of the stable operating point, the voltage divider portion of a voltage divider-biased circuit. Consider the effects of large resistances in the divider, both firm (10% error) and stiff (1% error). 2. Discuss the stability of voltage divider-biased transistor based on the data you have taken. Consider such factors as the use of two different transistors, as well as the forced beta changes of heating or cooling the transistor, and the percentage difference in the values of emitter current and collector voltages. 3. If you used the quick method of your text for solving the firm or stiff divider for the circuit in Figure 2.1, do you believe this would give a satisfactory value for emitter current? Explain your answer.
Section 2 1. For each fault, describe the transistor state (cut off or saturation). Describe how you would use this knowledge in troubleshooting a circuit. 2. Based on your measured data of circuit failures, what circuit measurements would you need to make in order to identify each fault?
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