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GROUND RETURNS FOR INPUT BIAS CURRENTS

Input bias currents are those dc currents that must flow to bias

the input transistors of an amplifier. These are usually transistor

base currents. When amplifying floating input sources such as

transformers or ac-coupled sources, there must be a direct dc

path into each input so that the bias current can flow. Figure 35

shows how a bias current path can be provided for the cases of

transformer coupling, capacitive ac-coupling, and a thermo-

couple application.

In dc-coupled resistive bridge applications, providing this path

is generally not necessary because the bias current simply flows

from the bridge supply through the bridge and into the amplifier.

However, if the impedances that the two inputs see are large and

differ by a large amount (>10 kΩ), the offset current of the input

stage causes dc errors proportional to the input offset voltage of

the amplifier.

THERMOCOUPLE

REF

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CAPACITIVELY COUPLED

REF

C

C

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TRANSFORMER

REF

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INCORRECT

CAPACITIVELY COUPLED

REF

C

R

R

C

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1

THERMOCOUPLE

REF

10MΩ

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TRANSFORMER

REF

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CORRECT