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26.4: Procedure

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  • 26.4.1: Resistor versus Diode Bias and Crossover Distortion

    1. Consider the circuit of Figure 26.3.1 using Vcc = 6 volts, R1 = R2 = 2.2 k\(\Omega\), R3 = R4 = 220 \(\Omega\), Rload = 100 \(\Omega\), C1 = C2 = 10 \(\mu\)F and C3 = 100 \(\mu\)F. Ideally this circuit will produce a compliance of just under 6 volts peak-peak.

    2. Build the circuit of Figure 26.3.1 using Vcc = 6 volts, R1 = R2 = 2.2 k\(\Omega\), R3 = R4 = 220 \(\Omega\), Rload = 100\(\Omega\), C1 = C2 = 10 \(\mu\)F and C3 = 100 \(\mu\)F. Disconnect the signal source and insert an ammeter into the collector of Q1. Record \(I_{CQ}\) in Table 26.5.1.

    3. Connect the signal source and apply a 1 kHz sine at 2 volts peak. Look at the load voltage and capture the oscilloscope image. There should be considerable notch or crossover distortion.

    4. Cycle through the remaining supply voltages in Table 26.5.1, repeating steps 2 and 3. Only images of the first and last trials need be captured. As the bias current increases, the notch distortion should decrease.

    5. Replace R3 and R4 with switching diodes, as shown in Figure 26.3.2. Repeat steps 2 through 4 using this circuit and Table 26.5.2. Overall, the superior matching of the diodes to the transistors should result in decreased notch distortion.

    26.4.2: Dual Supply and Power Analysis

    6. Add the negative power supply so that the circuit now appears as Figure 26.3.3. Set the power supplies to +/−6 volts DC. This should produce similar bias and amplification results to the single 12 volt supply circuit of Figure 26.3.2. Although the output coupling capacitor is no longer needed (one advantage of the dual supply topology), leave it in for safety sake.

    7. Based on the \(I_{CQ}\) recorded for the 12 volt supply in Table 26.5.2, determine the theoretical \(P_{DQ}\). Also determine the expected compliance, \(P_{Load(max)}\), \(I_{supplied}\), \(P_{supplied}\) and efficiency. Record these values in the Theoretical column of Table 26.5.3.

    8. Apply the signal source to the amplifier and adjust it to achieve a load voltage that just begins to clip. Reduce the amplitude slightly to produce a clean, unclipped wave. Record this level as the experimental compliance in Table 26.5.3. From this, determine and record the experimental maximum load power. Also, capture an image of the oscilloscope display.

    9. Insert an ammeter in the collector and measure the resulting current with the signal still set for maximum unclipped output. Record this in Table 26.5.3 as \(I_{supplied}\) (Experimental). Remove the ammeter.

    10. Using the data already recorded, determine and record the experimental \(P_{DQ}\), \(P_{Supplied}\), and \(\eta\). Finally, determine the deviations for Table 26.5.3.

    26.4.3: Distortion

    11. Unlike class A distortion which gets worse as the signal increases, notch distortion is relatively fixed. Therefore, it represents a smaller percentage of the overall output signal as the signal increases. To see this effect, adjust the signal level to achieve a load voltage of 8 volts peak-peak. There should be no clipping. Set the distortion analyzer to 1 kHz and % total harmonic distortion (% THD). Apply it across the load and record the resulting reading in Table 26.5.4 (8 Vpp). Decrease the generator to achieve a load voltage of 1 volt peak-peak and record the resulting THD.

    26.4.4: Computer Simulation

    12. Build the circuit in a simulator and run a Transient Analysis. Use a 1 kHz 7 volt peak sine for the source. Inspect the voltage at the load. Record the peak clip points in Table 26.5.5. Reduce the input signal so that clipping disappears. Add the Distortion Analyzer instrument at the load and record the resulting value.

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