What is input bias current in an op-amp, and how does it influence offset in high-impedance sources?

• A. Input bias current is the small DC current into the input terminals; in high-impedance circuits it creates voltage drops across source resistances, causing output offset.
• B. AC current through feedback network.
• C. Current drawn from power rails to bias transistors.
• D. Current that flows to ground from output.

Answer: (A)

Input bias current is the small DC current that flows into or out of the op-amp’s input terminals. This current comes from the transistor biasing inside the input stage and exists even with no signal. When the connected source has high impedance, this bias current flowing through the source resistance creates a voltage drop (V = I_bias × R_source). That drop appears as an extra input signal and is then amplified, showing up as an output offset. The higher the source impedance, the larger the offset for the same bias current, which is why high-impedance sources are particularly sensitive to input bias errors. To reduce this effect, use a low-bias-current op-amp or add matching/compensation resistors to balance the bias currents, or lower the source impedance. The other options describe currents that aren’t related to the input bias effect on offset at high-impedance sources.