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TLV6741: The quiescent operating point of TLV6741 is not correct

Torry Huang
Prodigy 20 points
Part Number: TLV6741

Tool/software:

Hi,

We built a mic pre-amp for electret microphone as following:

The power supply of 4.6V is from LDO, which is from TI, too. We measured the power ripple, is quite clean.

When the circuit is powered up, Vin+=2.3V, Vin-=0V, Vout=4.6V. It seems the quiescent operating point is not set correctly. When we reduce Rf from 10K to 4.5K or <4.5K, the circuit work correctly. Please help to take a look what caused the issue when Rf=10K.

Thank you and regards

Torry

  • 0
    TI__Guru 59926 points

    Hello Torry,

    The op-amp is working, but it is acting like a comparator. VIN+ > VIN- = output high (4.6V).

    Possibilities:

     1. Short somewhere on IN- node (0V? Should also be 2.3V)

     2. Bad/open Rf

     3. Leaky (reversed) 100uF cap

     4. Damaged TLV6741 input

    With the power off, what do you measure in ohms mode between VIN- pin and GND? If it is in the <100 ohms range, then there is a short.

    What happens if you remove CG? DC-wise, the output should then match VIN+ (but you will have no audio gain).

    Is the 100uF cap an electrolytic or tantalum? If polarized, is the negative lead to GND? When reversed, these capacitors will draw large currents (up to several mA). Lower values of RF will provide more current to overcome the leakage.