Why does the datasheet only has one input bias current value?

Question

There should be two input bias currents, one flowing into or out of the inverting input and the other, the non-inverting input. Their difference is the input offset current. A typical datasheet only states one value for the input bias current. Is that bias current going into the inverting or the non-inverting input? Or is that an average of the two input bias currents? Or perhaps something else no specified on the datasheet?

If I assume \$ I_{B+} = 600 \text{nA} \$ and \$ I_{B-} = 400 \text{nA} \$ then I would get an average value of \$ 500 \text{nA} \$ and a difference value of \$ 200 \text{nA} \$. The calculation matches the values on the specification sheet where input bias current = 500nA and input offset current = 200nA. The leads me to assume that the single input bias current value provided on the datasheet is actually an average.

A similar question was asked here Input bias current in an opamp and its value in a datasheet but I can't find a relevant answer there or elsewhere on the web.

Answer 1

In a voltage feedback opamp¹, the two input circuits are identical, at least up until the stage where their difference is taken. This helps with things like low tempco of offset voltage. The specification for bias current is therefore for each input individually.

On the data sheet you've shown, you would expect each bias current to never exceed 1.5uA over the temperature range, and the two be within 500nA of each other over the range. This is not a specification for the average of the two bias currents.

The important thing to bear in mind is that bias and offset current specify two totally different things about the opamp.

Bias current. Smaller is generally good. But it has to be compromised for noise, speed, common mode range etc with any given technology. The clever designer will use high gain transistors, reduce capacitances, use various circuit tricks (including compensating the bias current with an internal bias source) to minimise the external bias current for a given performance specification.

Offset current. Smaller is generally good. This is a measure of how well one input tracks the other, and stays tracking the other with temperature changes, and with power changes. This is important as if the user can match the two input resistances, changes in bias current will cancel out if the offset stays small. The two input circuits cannot occupy the same physical space, so variations of doping density and temperature across the die will unbalance the inputs. The more expensive opamp will actually interleave multiple transistors from the two input circuits, so even with a temperature and doping gradient across the chip, on average they will still track.

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¹ - a current feedback opamp has very different inverting and non-inverting inputs.

Answer 2

Because it's the 741; it's the oldest opamp still commercially available, and because it was a challenge to specify these values reliably in the 1960s, when this was invented. Because nobody caring about these specs would subsequentially be able to rely on this, TI / its predessors and competitors simply never cared enough to spec these.

Honestly, if you care about opamp specs at all, the 741 is the wrong choice. Pretty much end of story; it's a dinosaur, and not an impressive one at that – its use case is really limited to

• least-cost designs where bias currents largely don't matter
• 1968 to say, rough guess, 1975 legacy designs that were tuned to work with a µA741 and re-designing it would be a hassle
• people on the internet copying copies of copies of stolen designs from magazines that copied them elsewhere, all the way back to the late 60's/early 70's