17.6: Procedure


1. Determine the theoretical output frequency for the circuit of Figure 17.5.1. Also, estimate the output amplitude. Record these in Table 17.7.1.

2. Construct the circuit of Figure 17.5.1 using the medium speed op amp.

3. Record the output frequency and amplitude of the sine wave in Table 17.7.1 and determine the deviation between the theoretical and experimental results.

4. Save a copy of the oscilloscope display of the output wave as Graph 1.

5. Replace the capacitors with the 10n F units. Determine the theoretical output frequency using this new value and record the result in Table 17.7.2.

6. Record the output frequency and amplitude of the wave in Table 17.7.2 and determine the deviation between the theoretical and experimental results.

7. Save a copy of the oscilloscope display of the output wave as Graph 2.

8. Replace the capacitors with the 1n F units. Determine the theoretical output frequency using this new value and record the result in Table 17.7.2.

9. Record the output frequency and amplitude of the wave in Table 17.7.2 and determine the deviation between the theoretical and experimental results.

10. Save a copy of the oscilloscope display of the output wave as Graph 3.

11. Replace the medium speed op amp with the low speed op amp and examine the output.

12. Save a copy of the oscilloscope display of the output wave as Graph 4. Swap out the capacitors with the 10n F and then the 100 nF units. Are the wave shapes identical to those created with the faster op amp?

13. In order for this circuit to oscillate, the forward gain of the op amp must compensate for the loss through the Wien bridge, in this case, a gain of 3 is required. To verify this, open the 5k6 resistor. This will drop the gain to about 1 and oscillation should cease.

17.6.1: Computer Simulation

14. One of the trickier parts of this oscillator is getting the op amp gain adjusted properly. If it is too high, the signal will be distorted. In this circuit, the initial gain is greater than 3 but as the signal grows, the two diodes turn on, partially shunting the 2k7 and reducing the effective value of $$R_f$$, and hence, the voltage gain. To demonstrate this, build the circuit in the simulator with a 741 op amp and 10n F capacitors. First perform a Transient Analysis and inspect the output wave shape. Replace the 2k7 with larger values and note the effect on the wave shape. Finally, return the resistor to 2k7, delete the two diodes and observe the new wave shape.

This page titled 17.6: Procedure is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by James M. Fiore via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.