A twin-tee sine wave oscillator using an TL072 Op-Amp, running in the audio spectrum.
A twin-tee filter in the feedback loop of an op-amp can be used to generate a sine wave.
- one tee comprises an R-C-R low-pass filter
- the other is a C-R-C high-pass filter.
Together, these circuits form a notch filter which is tuned at the desired frequency of oscillation.
For oscillations to be sustained, x>=2, where:
x = C2/C1 = R1/R2
And the frequency is then given as:
frequency = 1/(2πRC)
I’m using a TL072:
- one op-amp unit is used to establish a buffered AC ground at VCC/2
- the other is for the oscillator
R2 is a pot so that it can be adjusted to be ~ R1/2 and thus allow the circuit to oscillate depending on the R1 values used.
With a 9V supply and R1 = 4.7kΩ, I get oscillation at about 3.238kHz:
For some more reliable measurements with various resistor values, I put the circuit on some protoboard. Here’s the basic layout I used. It includes:
- connections for substituting R1 values
- pins for output and capacitively coupled output (via 100nF capacitor)
- pins for ground and AC ground
- some capacitors for power supply smoothing (100nF and 100µF across the power supply)
With a 5V supply and R1 = 4.7kΩ, I get oscillation at about 3.084kHz. The wave is not a perfect sine wave, with two main distortions present:
- noticable cross-over distortion in both directions
- clipping at the top of the wave if R2 not adjusted correctly
Some measurements with a selection of R1 values:
|R1||Expected Frequency||Actual Frequency|