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TIPD181: Help modifying this circuit so that roll off starts around 20hz

Noah Klugman
Prodigy 30 points
Part Number: TIPD181

Hello,

I am looking to compare the mag of frequencies between 35 and 250hz captured from an electret condenser microphone. I would like to use this reference design to get started but noticed a roll off in the transfer function around the frequencies I am interested in. Can you help me think about how to modify this circuit to support my application? Is it just a modification of eq. 16 from the doc? Or, can you point me towards another amplifier design I should be looking at? 

Thanks!

  • 0
    TI__Genius 15410 points

    Hi Noah,

    The capacitor C2 would need to be recalculated by using equation 7 and 8 to modify the pole location of the low pass filter.

    Also, the capacitor C3 would need to be recalculated using equation 11 to modify the corner frequency of the high-pass filter. And also C5 would need to be recalculated using equation 16 and 17 like you have mentioned.

    The recommended changes to the design assumes that all other aspects of the design are the same as in the document (R2 value, microphone output sensitivity, etc.). After making the changes to the design I recommend performing all of the design verification done in section 4 (simulation) of the document to ensure the design meets all of your requirements and the circuit is stable.

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    Prodigy 30 points
    Hi Tim,

    Thanks for the quick response. Can you help me understand why C2 would be required to move? It would be convient to use a new value of C2 as a low pass filter around 250hz, but with it currently at 133725Hz, it shouldn't be required to be recalulated to change the left side of the transfer function right?

    Best,
    Noah
  • 0 in reply to Noah Klugman
    TI__Genius 15410 points

    Hi Noah,

    C2 doesn't necessarily have to change. I think I may have misunderstood your question and thought that you wanted to change the design to only pass 35 to 250Hz. There will be no issues meeting your design requirements with C2 the way it is in the document. However, decreasing the usable bandwidth from 20kHz to 250Hz will slightly decrease the noise of the system because you are decreasing the bandwidth of the circuit.

    Thank you,

    Tim Claycomb