LB-9 판매점 - Universal Balancing Techniques

LB-9 반도체 회로 부품 판매점

Universal Balancing Techniques

National Semiconductor

Universal Balancing

Techniques

National Semiconductor

Linear Brief 9

August 1969

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.

00846002

R1 = 2000 R3 \ R4

R4 \ R3 ≤ 10 kΩ

00846001

FIGURE 1. Offset Voltage Adjustment for Inverting

Amplifiers Using 10 kΩ Source Resistance or Less

The circuit shown in Figure 1 is used to balance out the

offset voltage of inverting amplifiers having a source resis-

tance 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.

FIGURE 2. Offset Voltage Adjustment for Inverting

Amplifiers Using Any Type of Feedback Element

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%.

www.national.com

00846003

00846004

FIGURE 3. Offset Voltage Adjustment for

Non-Inverting Amplifiers

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.

FIGURE 4. Offset Voltage Adjustment for

Voltage Followers

Differential amplifiers are somewhat more difficult to bal-

ance. 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.

00846005

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-

terfering with the internal circuitry of the amplifier. The elec-

trical parameters of the amplifiers are tested and guaranteed

without balancing. Although it doesn’t usually happen, bal-

ancing could degrade performance.

www.national.com

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Universal Balancing Techniques

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