### NSC LB-9

```IC op amps are widely accepted as a universal analog
component. Although the circuit designs may vary, most
devices are functionally interchangeable. However, offset
voltage balancing remains a personality trait of the particular
amplifier design. The techniques shown here allow offset
voltage balancing without regard to the internal circuitry of
the amplifier.
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Universal Balancing Techniques
National Semiconductor
Linear Brief 9
August 1969
Universal Balancing
Techniques
FIGURE 2. Offset Voltage Adjustment for Inverting
Amplifiers Using Any Type of Feedback Element
R1 = 2000 R3 \ R4
R4 \ R3 ≤ 10 kΩ
FIGURE 1. Offset Voltage Adjustment for Inverting
Amplifiers Using 10 kΩ Source Resistance or Less
This technique of supplying a small voltage effectively in
series with the input is also used for adjusting non-inverting
amplifiers. As is shown in Figure 3, divider R1, R2 reduces
the voltage at the arm of the pot to ± 7.5 mW for offset
adjustment. Since R2 appears in series with R4, R2 should
be considered when calculating the gain. If R4 is greater than
10 kΩ the error due to R2 is less than 1%.
The circuit shown in Figure 1 is used to balance out the
offset voltage of inverting amplifiers having a source resistance of 10 kΩ or less. A small current is injected into the
summing node of the amplifier through R1. Since R1 is 2000
times as large as the source resistance the voltage at the
arm of the pot is attenuated by a factor of 2000 at the
summing node. With the values given and ± 15V supplies the
output may be zeroed for offset voltages up to ± 7.5 mW.
If the value of the source resistance is much larger than 10
kΩ, the resistance needed for R1 becomes too large. In this
case it is much easier to balance out the offset by supplying
a small voltage at the non-inverting input of the amplifier.
Figure 2 shows such a scheme. Resistors R1 and R2 divide
the voltage at the arm of the pot to supply a ± 7.5 mW
adjustment range with ± 15V supplies.
This adjustment method is also useful when the feedback
element is a capacitor or non-linear device.
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FIGURE 3. Offset Voltage Adjustment for
Non-Inverting Amplifiers
FIGURE 4. Offset Voltage Adjustment for
Voltage Followers
A voltage follower may be balanced by the technique shown
in Figure 4. R1 injects a current which produces a voltage
drop across R3 to cancel the offset voltage. The addition of
the adjustment resistors causes a gain error, increasing the
gain by 0.05%. This small error usually causes no problem.
The adjustment circuit essentially causes the offset voltage
to appear at full output, rather than at low output levels,
where it is a large percentage error.
Differential amplifiers are somewhat more difficult to balance. The offset adjustment used for a differential amplifier
can degrade the common mode rejection ratio. Figure 5
shows an adjustment circuit which has minimal effect on the
common mode rejection. The voltage at the arm of the pot is
divided by R4 and R5 to supply an offset correction of ± 7.5
mV. R4 and R5 are chosen such that the common mode
rejection ratio is limited by the amplifer for values of R3
greater than 1 kΩ. If R3 is less than 1k the shunting of R4 by
R5 must be considered when choosing the value of R3.
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FIGURE 5. Offset Voltage Adjustment for
Differential Amplifiers
The techniques described for balancing offset voltage at the
input of the amplifier offer two main advantages: First, they
are universally applicable to all operational amplifiers and
allow device interchangeability with no modifications to the
balance circuitry. Second, they permit balancing without in-
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terfering with the internal circuitry of the amplifier. The electrical parameters of the amplifiers are tested and guaranteed
without balancing. Although it doesn’t usually happen, balancing could degrade performance.
2
Universal Balancing Techniques
Notes
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whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
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