SINUSOIDAL OSCILLATORS

In this lab we will use positive feedback to produce two sinusoidal oscillators - the Wein bridge oscillator and the phase-shift oscillator, both described in Section 13.2 of Sedra and Smith.

PRE LAB -- Find the R and C need to make the Wein oscillate at 1 and 10 kHz. Find the R and C and gain need for the phase-shift oscillator to oscillate at 10 kHz.

A. Construct the non-inverting amplifier (part of circuit in Figure 5.42A in Horowitz and Hill). Leave off the RC positive feedback network for now.

• Since your lamp is a different one than shown in the figure, measure its resistance and choose a feedback resistor (use a pot so you can adjust it) to give a gain of 3. Since these resistors are small, the output will current limit at several volts. Keep the output small enough so this does not happen.
• Measure and set the gain to 3 at 0.1 Vpp in.
• Don't change the gain. Measure the gain for an input of 0.01 and 1.0 Vpp. They should be different than 3.

B. Add the Wein bridge positive feedback for a frequency of 1 kHz.

• Test the circuit operation (Figure 5.42A). (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?
• The purity of the sine wave is affected by the severity of the limiting; demonstrate this by increasing the gain so that the op-amp saturates.

C. Modify your oscillator to give an oscillation frequency of 10 kHz.

D. Construct and test a phase-shift oscillator of 10 kHz frequency (Sedra and Smith, Figure 13.8). Set the amplifier gain to compensate for losses in phase-shift network. Explain the circuit operation.