20.4: Procedure

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20.4.1: PNP Voltage Divider

1. Consider the circuit of Figure 20.3.1 using Vee = 10 volts, R1 = 10 k\(\Omega\), R2 = 3.3 k\(\Omega\), Re = 4.7 k\(\Omega\) and Rc = 5.6 k\(\Omega\). Using the approximation of a lightly loaded “stiff” voltage divider, determine the theoretical base, emitter and collector voltages, and record them in Table 20.5.1 (Theory).

2. Build the circuit of Figure 20.3.1 using Vee = 10 volts, R1 = 10 k\(\Omega\), R2 = 3.3 k\(\Omega\), Re = 4.7 k\(\Omega\) and Rc = 5.6 k\(\Omega\). Measure the base, emitter and collector voltages and record them in the first row of Table 20.5.1 (Experimental).

3. Swap the transistor with the second transistor and repeat steps 1 and 2 using the second row of the table.

4. Swap the transistor with the third transistor and repeat steps 1 and 2 using the third row of the table.

20.4.2: Troubleshooting

5. Consider each of the individual faults listed in Table 20.5.2 and estimate the resulting base, emitter and collector voltages. Introduce each of the individual faults in turn and measure and record the transistor voltages in Table 20.5.2.

20.4.3: PNP LED Driver

6. Consider the PNP saturating switch of Figure 20.3.2 using Vee = Vbb = 5 volts, Rb = 4.7 k\(\Omega\) and Rc = 220 \(\Omega\). Calculate the base and collector currents and record them in the first row of Table 20.5.3 (Theory). As the circuit is in saturation, the theoretical \(V_{CE}\) is close to zero and may be found on the transistor data sheet via the \(V_{CE}/I_C\) saturation graph. Record this value in the first row of Table 20.5.3 as well.

7. Build the saturating switch of Figure 20.3.2 using Vee = Vbb = 5 volts, Rb = 4.7 k\(\Omega\) and Rc = 220 \(\Omega\). Measure and record the base and collector currents, and record the collector-emitter voltage in the first row of Table 20.5.3 (Experimental). Also compute and record the deviations between theory and experimental results.

8. Remove the base resistor from Vbb and connect it to ground. Without a base source potential, the circuit will be in cutoff. Determine the theoretical base and collector currents along with the collector-emitter voltage and record them in the second row of Table 20.5.3. Measure these parameters, record them in Table 20.5.3, and also compute and record the resulting deviations.

9. Reconnect the base resistor to the Vbb supply and swap in the second transistor. Repeat steps 3 and 4 using the next two rows of Table 20.5.3.

10. Reconnect the base resistor to the Vbb supply and swap in the third transistor. Repeat steps 3 and 4 using the final two rows of Table 20.5.3.

20.4.4: Design

11. A simple way to program the LED current in the driver is by altering the collector resistor. First, measure the LED potential while it is lit. Assuming that the collector-emitter saturation voltage is negligible, all of the power supply voltage will drop across the collector resistor when the LED is lit, with the exception of the LED voltage. Ohm's law can then be used to determine a resistance value for a desired target current. Compute the required value of resistance to achieve an LED current of 8 mA. Replace the collector resistor with the nearest value available and measure the resulting current. Record the appropriate values in Table 20.5.4.