Sinusoidal Oscillators


Posted on Feb 6, 2014

If a linear feedback amplifier contains reactive elements, and at some frequency the loop gain Ab = -1, sinusoidal oscillations will be obtained. The sine wave oscillators investigated in this experiment consist essentially of two separate but connected parts; the amplifier part and the frequency determining part. Probably the simplest type of feedback oscillator is shown in figure 1


Sinusoidal Oscillators
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. Here the amplifier function is the ratio, V2 / V1 = - R2 / R and through the phase shift circuit, the ratio V1/V2 is a function of frequency. In other words the amplifier yields a phase inversion of 180 ° and a 180-degree phase-shift-circuit is used to produce a zero loop phase shift. An oscillator circuit in which a balanced bridge is used as the feedback network is the Wien-bridge oscillator shown in figure 3. The "bridge" is clearly indicated in figure 4. The four arms of the bridge are Z1, Z2, R3 and R4. The input to the bridge is the output Vo of the op. amp. , and the output of the bridge between nodes 1 and 2 supplies the differential input to the op. amp. There are two feedback paths in figure 3, positive feedback through Z1 and Z2, whose components determine the frequency of oscillation, and negative feedback through R3 and R4, whose elements effects the amplitude of oscillation. The loop gain is given by -b A where: The maximum frequency of oscillation is limited by the slew rate of the amplifier. Continuous variation of frequency is accomplished by varying simultaneously the two capacitors. Changes in frequency range are accomplished by switching in different values for the two identical resistors R1 and R2. a. Condition For Oscillation: With an ohmmeter, adjust the potentiometer of R4 to the calculated value in preliminary question 3, and turn the circuit on. Vary the potentiometer setting and note the effect...




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