Pick any CMOS or JFET amplifier and you’ll get the lowest possible input bias (Ib) current, right?
Not so fast. If you mean low Ib compared to that of bipolar, then yes. But if you mean sub 10pA, there are other factors to consider. Did you say temperature? Yes, that’s one of the factors, but I was really talking about room temp specs. And one factor you must be aware of is the common mode.
Some of us look at the spec table on the second or third page of the datasheet to determine the value of Ib. Do you look at the top of the page (spec table) where it usually says Vs=5, 10 or 36V and Vcm=Vs/2?
This phenomenon isn’t a design flaw and is due to the leakage of the ESD diodes. As you start shifting from the midpoint, the leakage currents from the diodes fail to cancel each other out. As a result the input bias current increases. How do you get around that? The obvious cure is to operate at the common mode “sweet spot” so that your Ib is at its lowest. What if your application circuit doesn’t allow you to choose the optimum common mode voltage?
No worries, there are a few exceptions. The OPA320 is one amplifier where Ib is pretty flat for about 3V across the Vcm. The LMP7721, another very low current device, uses “special” low leakage diodes with a bootstrap to prevent Ib from rising across the common mode.
The same concept holds true for low power devices. Not Ib but Iq, the quiescent current also varies with the common mode.
So, the next time you’re shopping for a low current op amp, scroll all the way down and check out the Vcm before making a decision.
Thanks for reading! Be sure to leave any questions about what I covered in this post in the comments section below.
