Extending the frequency range of a Wien-bridge oscillator using composite operational amplifiers

The author describes the use of composite operational amplifiers to extend the useful operating frequency range of an operational amplifier-based Wien-bridge oscillator. The effect of the finite gain-bandwidth product (GB) of the operational amplifier (OA) is analyzed. This analysis shows that for a certain GB the frequency of oscillation is less than the theoretical value and that the error increases with frequency. A composite OA is then used to decrease the errors due to GB. This is verified theoretically and experimentally. Although the composite OA improves the frequency performance of the oscillator, the amplitude of the output decreases with frequency. An automatic-gain control circuit is incorporated to keep an output amplitude constant over the useful frequency range. Distortion measurements are presented.>