How do input bias currents and source resistances contribute to output offset in op-amp circuits, and how can this error be minimized?

How do input bias currents and source resistances contribute to output offset in op-amp circuits, and how can this error be minimized?

• A. Bias currents do not affect offset in real op-amps.
• B. Offset voltages are only caused by input offset voltage, not bias currents.
• C. One can minimize by using large value source resistances.
• D. Bias currents through source resistances create offset voltages; minimize by matching source resistance seen by the noninverting input to Rin || Rf or by using bias compensation resistor; plus low bias current op-amps.

Answer: (D)

The key idea is that input bias currents flowing through resistances seen by the op-amp inputs create small voltages that get amplified, showing up as an output offset. If these drops differ between the two inputs, the closed-loop circuit no longer centers the output at zero when it should; the offset is basically the bias current times the impedance seen at each input, amplified by the gain. To minimize this, balance the impedance seen by both inputs so the voltage drops caused by the bias currents are as equal as possible. A practical way is to set the resistance seen by the noninverting input to Rin in parallel with Rf (the same impedance that the inverting input effectively sees). If perfect matching isn’t possible, add a bias compensation resistor at the noninverting input equal to Rin || Rf to force the bias-current-induced drops to cancel in the feedback loop. Using an op-amp with very low input bias currents also helps. Using large source resistances would actually worsen the offset because the bias currents produce larger drops across larger resistances. And bias currents do affect offset; offset voltage from the internal transistor mismatch is only part of the picture.

The key idea is that input bias currents flowing through resistances seen by the op-amp inputs create small voltages that get amplified, showing up as an output offset. If these drops differ between the two inputs, the closed-loop circuit no longer centers the output at zero when it should; the offset is basically the bias current times the impedance seen at each input, amplified by the gain.

To minimize this, balance the impedance seen by both inputs so the voltage drops caused by the bias currents are as equal as possible. A practical way is to set the resistance seen by the noninverting input to Rin in parallel with Rf (the same impedance that the inverting input effectively sees). If perfect matching isn’t possible, add a bias compensation resistor at the noninverting input equal to Rin || Rf to force the bias-current-induced drops to cancel in the feedback loop. Using an op-amp with very low input bias currents also helps.

Using large source resistances would actually worsen the offset because the bias currents produce larger drops across larger resistances. And bias currents do affect offset; offset voltage from the internal transistor mismatch is only part of the picture.