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Project Notes

#459 OpAmp/TwinTeeOscillator

A twin-tee sine wave oscillator using an TL072 Op-Amp, running in the audio spectrum.

Build

Notes

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.

Design Guidelines

For oscillations to be sustained, x>=2, where:

x = C2/C1 = R1/R2

And the frequency is then given as:

frequency = 1/(2πRC)

Breadboard Construction

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.

Breadboard

Schematic

TwinTeeOscillator_bb_build

Breadboard Results

With a 9V supply and R1 = 4.7kΩ, I get oscillation at about 3.238kHz:

scope_bb_9v_4k7

Protoboard Construction

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)

protoboard_layout

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

scope_pb_5v_4k7

Some measurements with a selection of R1 values:

R1 Expected Frequency Actual Frequency
3.3kΩ 4.823kHz 4.503kHz
4.7kΩ 3.386kHz 3.084kHz
10kΩ 1.592kHz 1.445kHz

Build

Credits and References

About LEAP#459 OpAmpOscillators
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.