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Original question:

Electret model used in TIDU765 design

OPA172: impedance matching in TIDU765 design

Luciano Martinez99
Prodigy 30 points
Part Number: OPA172

Greetings everyone,

I am going through the TIDU765 design (John Caldwell) and I need to know if it would not be better to add an adaptation network between the microphone output and C3, since the input impedance of the TIA is very low, while the output impedance of the microphone is typically 2.2kohm.

Thanks!

Luciano

  • 0
    TI__Expert 6625 points

    Hi Luciano,

    What kind of adaptation network are you trying to use? The reason that I ask is to make sure that it does not affect the overall functioning of the TIA circuit. Also can you please elaborate on what you mean by low input impedance for a TIA circuit?

    Best Regards,

    Bala Ravi

  • 0 in reply to Bala Ravi
    Prodigy 30 points

    Hi Bala Ravi,

    I'm thinking in a PI network (capacitor in serie and inductors to ground). I think that this network doesn't affect the DC biasing circuit.

    If I Ignoring R4, C5 and R6, the impedance seen from the output of the microphone towards C3 is frequency dependent, but in the frequency range of interest (10Hz-10Khz), the impedance is small in comparison with R1.

    It would not be better, for example, if R1 = 13.7kohm that the impedance seen towards C3 (at to say f =1khz) will be Z=13.7kohm?

    Best regards and thank you,

    Luciano

  • 0 in reply to Luciano Martinez99
    TI__Expert 6625 points

    Hi Luciano,

    Can you please share a picture of your schematic? 

    Yes, I agree that the impedance seen towards C3 will be frequency dependent, for the frequencies of interest (10Hz - 10kHz), the impedance will range from 7.234 kohms to 7.234 ohms, which is significantly lower in comparison with R1 (14 kohm). 

    When f = 1kHz, the impedance of C3 is 72.34 ohms, is this what you're referring to? As you can read from Page 4 of the reference design: "If the impedance of the C3 is much less than R1 at audio frequencies, then Iac will flow through C3 and not R1". 

    Best Regards,

    Bala Ravi