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5/12/2017 841005155 1/4 Example: The Input Bias Current Q: How do input bias currents I B 1 and IB 2 affect amplifier operation? A: Consider both inverting and non-inverting configurations. Inverting Configuration R2 i2 vi R1 i1 v1 IB 1 vo v2 IB 2 + In this case, we apply KCL and we find: i1 However, we still find v1 v2 0 (neglecting the input offset voltage) by virtue of the virtual short. 5/12/2017 841005155 Therefore: i1 vi R1 and 2/4 i2 vo R2 and combining results: vi v o IB 1 R1 R2 The output voltage is thus: R vo 2 vi R1 IB 1 R1 Note again that if IB 1 0 , the result reduces to the expected inverting amplifier equation: R vo 2 vi R1 The second term in the above expression ( IB 1 R1 ) therefore represents another output offset voltage! It appears that we should keep the value of R1 small to minimize the output offset voltage. However, what will this do to the amplifier input resistance? 5/12/2017 841005155 3/4 Non-Inverting Configuration R1 R2 i1 i2 IB 1 vi v1 IB 2 vo v2 + Neglecting the input offset voltage, we can use the virtual short to determine that: v1 vi and KCL provides the same result as that of the inverting amplifier: ii i2 IB 1 where: i1 0 v1 R1 i2 v1 vo R2 and: 5/12/2017 841005155 4/4 Combining, we find: vi vi vo IB 1 R1 R2 or rearranging: R vo 1 2 vi IB 1R2 R1 Again, we find that this result is simply the ideal non-inverting expression: R2 1 vo vi R 1 with an added output offset voltage term: IB 1 R2 In this case, we find that this offset voltage is minimized by making feedback resistor R2 small. In general, we find that the effects of the input bias currents can be minimized by using small resistor values. However, we will find that there is an additional strategy for minimizing the effects of input bias currents!