Input Bias Offset Current

All op-amps have the input bias current, which is the current drawn by the input terminals. It is a leakage current that sources or sinks at both input terminals. Depending on the type of input transistor, the bias current can flow in or out of the input terminals.

The op-amp datasheet usually specifies the input bias current as the average value of the input bias current IB+ at the non-inverting terminal and the input bias current IB- at the inverting terminal.

The input offset current (IOS) is equal to the difference between the input bias current at the non-inverting terminal (IB+) minus the input bias current at the inverting (IB- ) terminal of the amplifier. Offset current is typically an order of magnitude less than bias current.

InputOffsetCurrent.png
Figure 1

The magnitudes can range from nA down to pA. Generally speaking, MOSFET op-amps have smaller bias currents than BJT types.
Bipolar transistor: ~nA
MOSFET/JFET: ~pA

Figure 2 shows the input bias current specification of MCP604.

mcp604_ib_ios.png
Figure 2

Amp-titudes Episode 2 – Input Bias and Offset Current:

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