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Description
The two circuits below demonstrate the production of low-frequency sine waves through the manipulation of the phase of the signal within an RC circuit, resulting in oscillation when the total phase shift reaches 360 degrees. The transistor circuit on the right generates a satisfactory sine wave at the collector of the 3904, which is amplified by the JFET to provide a low-impedance output. Circuit gain is paramount for minimizing distortion and adjustment of the 500-ohm resistor may be required to attain a steady waveform with minimal distortion. This transistor circuit is generally not advised for real-world applications due to the sensitive calibration needed.
Circuit diagram
The op-amp based phase shift oscillator exhibits significantly greater stability compared to the single transistor version, as its gain can be elevated above what is necessary to maintain oscillation. The output signal is extracted from the RC network, effectively filtering out most harmonic distortions. A second op-amp then buffers and restores the amplitude of the sine wave, providing a gain of approximately 28dB. The frequency of the circuit is approximately 600 Hz, determined by the RC values utilized (7.5K and 0.1uF), and can be reduced proportionally by increasing the network resistors (7.5K). The 7.5K value at pin 2 of the op-amp governs the oscillator circuit's gain and is chosen to cause slight clipping at the positive and negative peaks of the output waveform. The sine wave output at pin 7 is approximately 5 volts p-p, utilizing a 12-volt power supply, and presents as a very clean signal on an oscilloscope, as the RC network effectively eliminates most distortion originating at pin 1.
More about components
The 3904 is a general-purpose operational amplifier commonly employed in electronic circuits for amplifying signals. It’s a three-terminal device, featuring an inverting input, a non-inverting input, and an output. This op-amp is known for its versatility and is often used in oscillator circuits and amplifier designs. Its key specifications include a common-mode input range, a differential input impedance, and a slew rate, all of which contribute to its performance in various applications. Typically, the 3904 is selected for its low noise characteristics and stable performance across a range of operating voltages.