The purpose of the balancing resistor is to reduce the static input voltage formed by the op amp input bias current on the resistor, resulting in a detailed reading. (reducing the offset voltage)
When the input bias current of the op amp is small, or the signal is large, and its influence can be neglected, the balancing resistor can be omitted.
R2=R1//Rf-------------------- Calculation formula of balance resistance, parallel connection of resistance on the inverting arm = resistance of the same phase arm
R2=R1//Rf
If the op amp is an ideal op amp, if the input is 0, then:
However, the actual op amp has offset voltage (VIO), offset current (IIO), and input bias current (IIB). According to the offset current, the definition of the input bias current is:
Solve the equations and get:
Use KCL on the inverting side to have:
Considering the offset voltage, the actual inverting terminal voltage is:
and:
Substituting all the parameters, the solution is:
When R2=R1//Rf,
At this time, the offset voltage generated by the input bias current IIB is zero.
The resistors connected to the op amp input are balanced so that the op amp's bias current does not create an additional offset voltage. However, some circuits do not require high offset voltages. For example, AC audio amplifiers, some op amp bias current is very small, even if the input resistance is not balanced, it will not affect the offset voltage. These circuits can not balance the input resistance. Especially for CMOS-based op amps, the input bias current is a few pA, and the effect of adding or not balancing resistors is negligible.
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