![single supply op amp offset single supply op amp offset](https://i.stack.imgur.com/DfscP.png)
Since Kirchhoff’s law tells us that current must flow in circles, this current also must return to its origin through a DC path. If an op amp uses bipolar transistors in its input stage, a base current must be supplied from somewhere to bias them into their active operating region. Nihal Kularatna, in Modern Component Families and Circuit Block Design, 2000 2.3.2.2 Input Bias CurrentĪnother DC parameter of op amps is input bias current ( I B). The input impedance of the noninverting configuration is determined by the change in input voltage divided by the change in bias current due to it. It is irrelevant for the closed-loop inverting configuration, since the actual impedance seen at the op-amp input terminals is reduced to near zero by feedback.
![single supply op amp offset single supply op amp offset](https://www.petervis.com/Electronics_Kits/Op_Amp_Headphone_Amplifier/Single_Rail_Op_Amp_Power_Supply/half-vcc-offset.gif)
Input impedance is rarely quoted as a parameter on op-amp data sheets since bias currents are a better measure of actual effects. The significance of input bias and offset currents is twofold: they determine the steady-state input impedance of the amplifier, and they result in added voltage offsets. Note that even the 25☌ figure for JFETs can be misleading, because it is quoted at 25☌ junction temperature: many JFET op-amps take a fairly high supply current and warm up significantly in operation, so that the junction temperature is actually several degrees or tens of degrees higher than ambient.
![single supply op amp offset single supply op amp offset](https://sound-au.com/p07_fig2.gif)
Industry-standard JFET op-amps are therefore no better than bipolar ones at high temperatures, though precision JFET and CMOS still show nanoamp levels at the 125☌ extreme. Junction field-effect transistor (JFET) and CMOS devices routinely achieve input currents of a few picoamps or tens of picoamps at 25☌, but because this is almost entirely reverse-bias junction leakage it increases exponentially with temperature (see Section 4.1.3). Precision bipolar op-amps achieve less than 20 nA while some devices using current nulling techniques can boast picoamp levels. There is a well-established trade-off between bias current and speed high speeds require higher first-stage collector currents to charge the internal node capacitance faster, which in turn requires higher bias currents. Input bias currents of bipolar devices range from a few microamps down to a few nanoamps, with most industry-standard devices offering better than 0.5 μA. Peter Wilson, in The Circuit Designer's Companion (Fourth Edition), 2017 Bias Current Levels