Input Bias Current Errors In An Audio Amp

[eev_carl]

Hi,

I built an inverting amp and was experimenting with a resistor from the positive input versus a wire to GND.  For audio applications, is this resistor important?  The op amp I'm using, LME49720NA, has an IB of 10nA.

I was reading this article which uses an extra resistor in Figure 3: http://www.analog.com/media/en/training-seminars/tutorials/MT-038.pdf .

Thanks,
Carl

[Wimberleytech]

Hi,

I built an inverting amp and was experimenting with a resistor from the positive input versus a wire to GND.  For audio applications, is this resistor important?  The op amp I'm using, LME49720NA, has an IB of 10nA.

I was reading this article which uses an extra resistor in Figure 3: http://www.analog.com/media/en/training-seminars/tutorials/MT-038.pdf .

Thanks,
Carl

IB can be as high as 72nA according to the spec.  Without extra resistor in the + leg, this could result in 0.72mV offset (for a 10k resistor).
Datasheet also indicates input offset current to be as high as 65nA, so for 10k resistor this could add 0.65mV offset voltage. 

Without the extra resistor, you might see 0.72mV input-referred offset.
With the extra resistor, you might see 0.65mV input-referred offset.

Appears to be a wash.  I don't think you need it.

[David Hess]

Ideally the resistor is not needed for audio.  It adds noise and the amplifier can be configured for AC coupling so DC offsets are removed or minimized.

[floobydust]

Ib causes big problems in audio use, as the output coupling capacitor's polarity has to be decided. Does the electrolytic cap (+) or (-) go to the op-amp's output?
So sometimes a little intentional DC offset is desirable, from omitting the extra resistor.

I've seen in Neve consoles the NE5534's age and output offset drift and change polarity, which aggravated output coupling capacitor failures.