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15.4: Procedure

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  • 15.4.1: A Quick Check

    1. A quick and easy way to determine if a transistor is damaged is through the use of the resistance (or diode) function of a multimeter. The multimeter will produce a small current in order to determine the connected resistance value. This current is sufficient to partially forward or reverse bias a PN junction. Thus, for an NPN device, placing the red lead on the base and the black lead on the emitter and collector in turn will produce forward bias on the junctions and the meter will show a low resistance. Reversing the leads will create reverse bias and a high resistance will be indicated. If the leads are connected from collector to emitter, one of the two junctions will be reverse biased regardless of lead polarity, and thus, a high resistance is always indicated. Before proceeding to the next step, check the three transistors using this method to ensure that they are functioning. (Note: some multimeters include a “beta checker” function. This may also be used to determine if the devices are good but the beta value produced should not be considered precise as the measurement current and voltage are most likely different from the circuit in which the transistor will be used.)

    15.4.2: Base Bias

    2. Consider the circuit of Figure 15.3.1 with Vbb = 11V, Vcc = 15V, Rb = 330 k\(\Omega\) and Rc = 1.2 k\(\Omega\). Assume \(V_{BE}\) = 0.7 volts. Further, assume that beta is 150 (a typical value for this device in this application). Calculate the expected values of \(I_B\), \(I_C\) and \(I_E\), and record them in the “Theory” columns of Table 15.5.1. Note that the theoretical values will be the same for all three transistors.

    3. Based on the expected value of \(I_C\), determine the theoretical value of \(V_{CE}\) and record it in Table 15.5.2. Also, fill in Table 15.5.2 with the typical (theoretical) beta value of 150.

    4. Build the circuit of Figure 15.3.1 with Vbb = 11V, Vcc = 15V, Rb = 330 k\(\Omega\) and Rc = 1.2 k\(\Omega\). Measure and record the base, collector and emitter currents, and record them in the first row of Table 15.5.1. Find the deviations between the theoretical and experimental currents, and record these in Table 15.5.1.

    5. Measure the base-emitter and collector-emitter voltages and record in the first row of Table 15.5.2. Based on the measured values of base and collector current from Table 15.5.1, calculate and record the experimental betas in Table 15.5.2. Finally, compute and record the deviations for the voltages and for the current gain in Table 15.5.2.

    6. Remove the first transistor and replace it with the second unit. Repeat steps four and five using the second row of Tables 15.5.1 and 15.5.2.

    7. Remove the second transistor and replace it with the third unit. Repeat steps four and five using the third row of Tables 15.5.1 and 15.5.2.

    15.4.3: Design

    8. One way of improving the circuit of Figure 15.3.1 is to redesign it so that a single power supply may be used. As noted previously, the base current is largely dependent on the value of \(V_{BB}\) and \(R_B\). If the supply is changed, the resistance can be changed by a similar factor in order to keep the base current constant. This is just an application of Ohm’s law. Based on this, determine a new value for \(R_B\) that will produce the original \(I_B\) if \(V_{BB}\) is increased to the \(V_{CC}\) value (i.e., a single power supply is used). Record this value in Table 15.5.3.

    9. Rewire the circuit so that the original \(R_B\) is replaced by the new calculated value (the nearest standard value will suffice). Also, the \(V_{BB}\) supply should be removed and the left side of \(R_B\) connected to the \(V_{CC}\) supply. Measure the new base current and record it in Table 15.5.3. Also determine and record the deviation between the measured and target base current values.

    15.4.4: Computer Simulation

    10. Build the original circuit in a simulator. Run a single simulation and record the \(I_B\), \(I_C\), \(I_E\) and \(V_{CE}\) values in Table 15.5.4.

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