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:
(R4 + Rec) / Rec (expressed in dB) = U1A's open-loop gain (expressed in dB) - the magnitude of the notch depth (expressed in dB)
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 capacitor in U1B's feedback path was chosen to provide adequate AGC-loop stability for the selected oscillation frequency. The 10K resistor in parallel with Q1 provides start-up.
The ratios of the resistors and capacitors in the twin-T network must be accurate to ensure oscillation, and at the desired frequency. For that reason, ±1% components are indicated. R1, R2, and R3 should be metal film, and C1, C2 and C3 should be C0G ceramic or mica (best), or polystyrene (good). Polyester capacitors may be used for economy if large C1, C2, and C3 values, such as those exceeding 10nF, are required.
A capacitor ratio of 2:1 is most conveniently obtained with the standard
values that have that nominal relationship, e.g., 750pF and 1500pF (as shown),
1000pF and 2000pF, 1100pF and 2200pF, 1500pF and 3000pF, and 1800 and 3600pF.
Alternatively, four equivalent capacitors may be used, with two of them
paralleled to make C1. The same technique is also applicable for the three
resistors, especially when an exact 2:1 ratio of resistance values is not
Interestingly, Q1's emitter and collector may be interchanged without any significant effect on the circuit's performance, although convention dictates that the emitter is biased negatively with respect to the base. If the two diodes are reversed, then Q1 may be a PNP transistor such as type 2N3906.
The oscillator's amplitude, as well as frequency, is essentially independent of the power supply voltage. An output amplitude of 1.6 volts peak-to-peak was chosen to accommodate power supply voltages as low as 6 volts. Replacing the TL072CP with a rail-to-rail dual operational amplifier such as the Analog Devices type AD823AN will permit operation from even lower supply voltages, such as 4 volts. The oscillator will operate conveniently from a single 9-volt battery, and consumes less than 5 milliamperes of current when using the TL072CP. The 470 ohm resistor in the output prevents instabilities from capacitive loads such as long cables, and the 1uF capacitor removes the DC offset at U1A. The output's 100K resistor discharges the 1uF capacitor to prevent voltage transients when connecting a load.
The author gratefully acknowledges Rick Hansen for providing an analysis
of this circuit's operation.
April 12, 2001
Text and images ©2001 by Arthur Harrison
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