# 16.4: Procedure

## 16.4.1: Determining $$V_{LED}$$

1. The forward potential of an LED depends on its design and the current flowing through it. The other two circuits in this exercise are designed to produce LED currents of approximately 10 mA so a determination of the forward potential of this particular diode at 10 mA is desired. Assemble the circuit of Figure 16.3.1 using R = 470 $$\Omega$$, and E = 5 volts. Insert an ammeter in line with the LED. Increase E until 10 mA is reached (the LED should be reasonably bright). Record the resulting LED voltage in Table 16.5.1.

## 16.4.2: Saturating Switch

2. Consider the saturating switch of Figure 16.3.2 using Vcc = 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 16.5.2 (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 16.5.2 as well.

3. Build the saturating switch of Figure 16.3.2 using Vcc = 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 16.5.2 (experimental). Also compute and record the deviations between theory and experimental results.

4. 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 16.5.2. Measure these parameters, record them in Table 16.5.2, and also compute and record the resulting deviations.

5. 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 16.5.2.

6. 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 16.5.2.

## 16.4.3: Non-saturating Current Source

7. Consider the non-saturating current source of Figure 16.3.3 using Vcc = 10 volts, Vbb = 5 volts and Re = 470 $$\Omega$$. Using a typical beta of 150, calculate the base and collector currents, and the collector-emitter voltage and record them in the first row of Table 16.5.3 (theory).

8. Build the non-saturating current source of Figure 16.3.3 using Vcc = 10 volts, Vbb = 5 volts and Re = 470 $$\Omega$$. Measure and record the base and collector currents, and record the collector-emitter voltage in the first row of Table 16.5.3 (experimental). Also compute and record the deviations between theory and experimental results.

9. Remove Vbb and connect the base terminal to ground. Without the base source potential, the base current will be zero. Determine the theoretical base and collector currents along with the collector-emitter voltage and record them in the second row of Table 16.5.3. Measure these parameters, record them in Table 16.5.3, and also compute and record the resulting deviations.

10. Reconnect the Vbb supply to the base and swap in the second transistor. Repeat steps 8 and 9 using the next two rows of Table 16.5.3.

11. Reconnect the base resistor to the Vbb supply and swap in the third transistor. Repeat steps 8 and 9 using the final two rows of Table 16.5.3.

## 16.4.4: Design

12. As seen in steps 7 through 11, the LED current of Figure 16.3.3 is a function of the base supply and the emitter resistor. Determine a new value for the emitter resistance that will yield an LED current of 15 mA. Record this value in Table 16.5.4. Obtain a new resistor close in value to the calculated result and swap it into the circuit. Measure the resulting LED current and record in Table 16.5.4.

## 16.4.5: Computer Simulation

13. Simulate the circuit of Figure 16.3.2 and record the currents and voltage in Table 16.5.5.

14. Simulate the circuit of Figure 16.3.3 and record the currents and voltage in Table 16.5.6.