24.4: Procedure

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1. Consider the circuit of Figure 24.3.1 using Vcc = 5 volts, Vee = −12 volts, Rs = 22 k\(\Omega\), Rb = 470 k\(\Omega\), Re = 1 k\(\Omega\), Rload = 220 \(\Omega\), Cin = 10 \(\mu\)F and Ce = 470 \(\mu\)F. Using the approximation of a negligible DC base voltage, determine the DC collector current and \(r’_e\), and record these in Table 24.5.1. Using the \(r’_e\), calculate the expected \(Z_{in}\), \(Z_{in(base)}\), \(Z_{out}\) and \(A_v\). Record these in Table 24.5.2. If a transistor curve tracer or beta checker is not available to get an approximate value of beta for the transistors, estimate the pair at 10,000.

2. Build the circuit of Figure 24.3.1 using Vcc = 5 volts, Vee = −12 volts, Rs = 22 k\(\Omega\), Rb = 470 k\(\Omega\), Re=1k\(\Omega\), Rload = 220 \(\Omega\), Cin = 10 \(\mu\)F and Ce = 470 \(\mu\)F. Disconnect the signal source and check the DC transistor voltages to ensure that the circuit is biased correctly. (Note: The base should be close to zero while the emitter will be two \(V_{BE}\) drops less, or about −1.4 VDC.)

3. Using a 1 kHz sine wave setting, apply the signal source to the amplifier and adjust it to achieve a source voltage of 2 volts peak-peak (i.e., to the left of Rs).

4. Measure the AC peak-peak voltages at the source, the base, and the load, and record these in Table 24.5.3. Also note the phase of the load voltage compared to the source. If distortion asymmetry is observed between the positive and negative peaks, make a note of it. Also, capture images of the oscilloscope displays (\(V_s\) with \(V_b\) and \(V_b\) with \(V_{load}\)).

5. Set the distortion analyzer to 1 kHz and % total harmonic distortion (% THD). Apply it across the load and record the resulting reading in Table 24.5.3.

6. Finally, unhook (i.e., open) the load and measure the resulting load voltage. Record this in the final column of Table 24.5.3.

7. Using the measured base and load voltages from Table 24.5.3, determine the experimental gain for the circuit. Using the measured source and base voltages along with the source resistance, determine the effective input impedance via Ohm’s law or the voltage divider rule. In similar fashion, using the loaded and unloaded load voltages along with the load resistance, determine the effective output impedance. Record these values in Table 24.5.4. Also determine and record the percent deviations.

24.4.1: Troubleshooting

8. Return the load resistor to the circuit. Consider each of the individual faults listed in Table 24.5.5 and estimate the resulting AC load voltage. Introduce each of the individual faults in turn and measure and record the load voltage in Table 24.5.5.

24.4.2: Computer Simulation

9. Build the circuit in a simulator and run a Transient Analysis. Use a 1 kHz 1 volt peak sine for the source. Inspect the voltages at the source, base and load. Record these values in Table 24.5.6. Add the Distortion Analyzer instrument at the load and record the resulting value.