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Generating a clean sine wave

Other Parts Discussed in Thread: OPA743

Hey team,

I need to generate a clean Sine wave from 15kHz to 100kHz.  The amplitude does not have to be very great, it can be +5V to -5V.  I've tried to put together a Wein-bridge oscillator circuit, but the waveform starts to deform into a triangle wave at these frequencies.

How would you recommend generating this Sine wave?

Regards,

Kareem

  • Hi Kareem,

    Triangulation of a large-signal sine wave is often an indicator of op amp slew rate limiting. The output amplitude is actually quite high in this application. The minimum slew rate required for a 100 kHz, 5 V peak, sine output is 3.2 V/us (SR = 2 pi ∙ fp∙ VPK). Make sure the op amp you are using has a slew rate is somewhat greater than 3.2 V/us. I would use an op amp having a 5 to 10 V/us typical slew rate.

    Regards, Thomas

    PA - Linear Applications Engineering

  • Hi Tom,

    I am using the OPA743, whose datasheet claims to have 10V/us.  However, I am still seeing triangulation of the output, even when decreasing the rails to smaller voltages (like +1V to -1V).  I will attach some screenshots I have gotten along with the TINA file I have put together:

    The above waveform was when I ran the device at a +-2.5V supply instead of +-5V.  In addition to the triangulation, the output signal is only moving at about 71kHz from what I can measure, when the values I entered should have yielded a 100kHz signal.

    Could you please explain what I am missing?

    Sine Wave Generator.TSC

  • Hi Kareem,

    I suspect that you are loading the OPA743 output too heavily with the component values you have selected for the Wein-bridge elements. The output of the OPA743 sees the two 17.5 Ohm resistors in series to ground, and also the capacitor reactances. You will note that the original circuit used 3.4 kilohm resistors, which is a much more reasonable load for the OPA743.

    Try selecting resistors in the kilohms for R1 and R2, and recalculate C1 and C2 for the oscillation frequency of interest.

    Regards, Thomas

    PA - Linear Applications Engineering

  • Hi Tom,

    Thanks for the support. When I increased the resistors in R1 and R2, the circuit behaves as I expected it to.

    Always appreciate benefiting from your expertise,
    Kareem