Twin-T Audio Sine Wave Oscillator

The circuit illustrated produces a clean sine wave signal ideal for audio testing, or wherever a good-quality sine voltage source is required. It uses one integrated circuit which is a widely-available dual operational amplifier, the Texas Instruments TL072CP. The values shown provide an output frequency of about 1000 Hertz. This frequency may be

varied over a wide range, using the equation shown in the schematic. Frequency is almost exclusively determined by the twin-T network in U1A`s feedback loop, provided that the gain-bandwidth product and slew rate specifications for the op amp are sufficiently high. For frequencies exceeding 10kHz, an faster op-amp in the U1A position is recommended. The oscillator`s frequency-selective twin-T network, C1, C2, C3, R1, R2, and R3, is in the negative feedback path of the operational amplifier. U1A`s positive-input to output transfer function is a notch filter with a frequency and depth determined by those components` values, with zero degrees of phase shift at the notch frequency. The twin-T network characteristics are defined by these equations: The positive feedback required for oscillation is provided by R4. Transistor Q1 is part of an automatic gain control (AGC) servo that regulates U1A`s positive feedback to keep the sine wave output at a consistent amplitude. The circuit will oscillate at the notch frequency when Q1`s emitter-to-collector resistance (Rec) is: U1B and its associated components complete the servo. The series-connected diodes provide negative peak-level detection of the sine wave voltage and also determine the amplitude of the output. Q1 is operated close to maximum conduction, so the small voltage from its emitter to collector contributes negligible distortion to the sine wave. The 0. 1uF integrating...

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