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Original question:

OPA209: High Speed Amplifier Application Notes

LMP7721: High Speed Amplifier Application Notes

Sreehari kamsala
Prodigy 90 points
Part Number: LMP7721

Hi all,

I designed a Two stage op amp ( made of CMOS ) which has a bias current of 100pA.  Is there any method or trick to reduce it further. Is there any input bias current cancellation amplifier circuitry needed.

Kindly please Suggest

Thanks in advance.

Sreehari

  • 0
    TI__Guru 64680 points
    LMP7721 max IB at 125C is +/-5pA so if you used this part in your design how do you get 100pA? Please show your circuit schematic?
    The input bias in CMOS op amps is caused by the reverse-bias leakage current of ESD protection diode, which may have opposite polarity; for that reason, it is not possible to cancel it.
  • 0 in reply to Marek Lis
    Prodigy 90 points

    Hello sir,

    Thanks for the response.

    I didn't use ESD protection diodes. Still I'm geting bias current.

    The schematics of my Op Amp and the way i used for measuring Bias currents and its results are attached below.

    Suggest me a way to reduce the bias current further.

    Thanks in advance.

    Sreehari

  • 0 in reply to Sreehari kamsala
    TI__Guru 64680 points

    Sreehari,

    If you have designed a simple op amp with discrete transistors, the input bias current, IB, will be significantly higher than an integrated solution simply because of the several-orders of magnitude larger geometry of the input transistors used (much larger area of the gate oxide).  Also, in order to minimize such leakage current, you must properly clean the circuit using ultrasonic immersion process to remove any residue deposits or flux from your board - see link below:

    https://www.kyzen.com/processes/ultrasonicbatch-immersion-cleaning-processes/

  • 0 in reply to Marek Lis
    Prodigy 90 points

    Hello Sir,

    Sorry for the delay and Thanks for your response.

    Is there any method to add some external circuitry to reduce the input bias current. Also I have to maintain high input impedance.

    Thanks in Advance

    Sreehari

  • 0 in reply to Sreehari kamsala
    Guru 140200 points
    Hi Sreehari,

    I don't want to recommend this, but many many years ago when we had to decrease the input bias current of an OPAmp we have connected a high ohmic resistor from the input to the upper or lower supply voltage sometimes. But this will only work (if at all...), if the input voltage is nearly constant and if the input bias current does not show any relevant drift.

    But why not using the LMP7721, as Marek already recommended?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 90 points
    Hello Sir,

    Thanks for the another way which is not Recommended. On chip resistance may be difficult to manufacture. Why I'm not using LMP7721? The reason is, i just want to know how will be the internal circuitry, that's why i tried with simulation.

    Thanks for your Precious time sir.
    Sreehari