# 4.6: Procedure

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1. The voltage gain of the non-inverting amplifier can be determined accurately from the feedback resistors \(R_i\) and \(R_f\). Calculate the voltage gains for the amplifier of Figure 4.5.1 for the \(R_f\) values specified, and record them in Table 4.7.1.

2. Assemble the circuit of Figure 4.5.1 using the 4k7 \(\Omega\) resistor.

3. Set the generator to a 1 kHz sine wave, 100 millivolts peak.

4. Apply the generator to the amplifier. Measure and record the output voltage in Table 4.7.1. Also, compute the resulting experimental voltage gain and gain deviation.

5. Repeat step 4 for the remaining \(R_f\) values in Table 4.7.1.

6. For any given \(R_i\), \(R_f\) combination, the voltage gain should be stable regardless of the precise op amp used, even if it is of an entirely different model. To verify this, first set \(R_f\) to 22k \(\Omega\).

7. Set the generator to a 1 kHz sine wave, 100 millivolts peak.

8. Apply the generator to the amplifier. Measure and record the output voltage in Table 4.7.2. Also, compute the resulting experimental voltage gain and gain deviation.

9. Repeat step 8 for two other op amps.

10. It is not practical to use an ohmmeter to determine the input impedance of an active circuit. Instead, input impedance can be found by utilizing the voltage divider effect. Modify the circuit by adding the extra input resistor as shown in Figure 4.5.2.

11. Set \(R_f\) to 4k7 \(\Omega\).

12. Set the generator to a 200 Hz sine wave, 1 volt peak.

13. Apply the generator to the amplifier. Use a DMM to measure and record the AC potential from \(V_{in}\) to point X (i.e.,\(V_A\), the voltage across the 100k) in Table 4.7.3. Using KVL, determine the voltage from point X to ground \((V_B)\) and record in Table 4.7.3 (don’t forget to compensate for peak versus RMS readings). Finally, compute the resulting input impedance by using the voltage divider rule. Note: If the DMM is not sensitive enough and registers 0 volts for \(V_A\), it is safe to assume that \(Z_{in}\) is considerably larger than the 100k \(\Omega\) sensing resistor.

## 4.6.1: Distortion Measurement

14. Return to the amplifier of Figure 4.5.1 with \(R_f\) set to 10k \(\Omega\). If available, the LF351 is a good choice for the op amp in this portion. Replace the general purpose generator with the low distortion sine source set to 1 kHz. Adjust its output level so that the output of the op amp is approximately 0 dBV.

15. Apply the distortion analyzer to the output of the op amp, read the resulting THD percentage and record it in Table 4.7.4.

16. Repeat steps 14 and 15 using the remaining \(R_f\) values in Table 4.7.4.

## 4.6.2: Troubleshooting

17. Continuing with the amplifier of Figure 4.5.1, reset \(R_f\) to 4k7 \(\Omega\). Estimate and then measure the results for each individual error presented in Table 4.7.5.