In this lab we will use positive feedback to produce two sinusoidal oscillators - the Wein bridge oscillator and the quadrature oscillator, both described in Section 12.2 of Sedra and Smith. In both cases, amplitude limiting will be used to limit the signal amplitude and to provide unity gain, as described in Section 12.1 of Sedra and Smith.

A. Construct the amplitude-limiting amplifier circuit that will be used in the Wein bridge oscillator (Figure 12.3). Measure the gain. Test and document the operation of the circuit as an inverting limiter and explain why the slopes and breakpoints (Figure 12.3b) have the values that they do (the operation is explained in Section 12.1).

B. Add the Wein bridge positive feedback and test the circuit operation (Figure 12.5). (What would be the problem in the event that the circuit did not oscillate?) Compare the frequency of oscillation with theory. What determines the amplitude of the oscillations? What happens if the amplitude-limiting circuit is disconnected? The purity of the sine wave is affected both by the loop gain and by the severity of the limiting; can you demonstrate this?

C. How would your oscillator be modified to give an oscillation frequency of 10 kHz? Try it.

D. Construct and test a quadrature oscillator of 10 kHz frequency (Figure 12.9 of Sedra and Smith). Use the same limiting circuitry as before. How does the limiter affect the signal amplitude? Demonstrate that the two sinusoidal outputs are in quadrature with each other (i.e. 90 degrees apart in phase). Explain the circuit operation. If you have time, modify the limiter circuit to give 20 V p-p output amplitude.