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參數(shù)資料
| 型號(hào)： | AN-242 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | LJT 18C 18#20 SKT RECP |
| 中文描述： | 應(yīng)用新的精密運(yùn)算放大器 |
| 文件頁(yè)數(shù)： | 3/16頁(yè) |
| 文件大?。?/td> | 267K |
| 代理商： | AN-242 |
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In production, altering resistors based on temperature test-
ing is to be avoided if at all possible. Therefore, the results
that can be obtained with simple nulling at room tempera-
ture and a fixed value for R2 are of interest. Figure 4 gives
this data for a range of parts with different initial bias cur-
rents. This was obtained from pre-production and initial-pro-
duction runs. The bias current variations were the result of
both h
FE
variations and changes in internal operating cur-
rents and represent the worst as well as best obtained.
They are therefore considered a realistic estimate of what
would be encountered among various production lots.
TL/H/7479–5
Figure 4. Compensated bias current for five representa-
tive units with a range of initial bias currents.
The circuit in Figure 3b was used with balanc-
ing at 25
§
C. High drift devices could be im-
proved further by altering R2.
Little comment need be made on these results, except that
the method is sufficiently predictable that another factor of
five reduction in worst case bias current could be made by
altering R2 based on the results of a single temperature run.
One disadvantage of the new circuit is that it is more sensi-
tive to supply variations than the old. This is no problem if
the supplies are regulated to 1%. But with worst regulation it
suffers because, with R2, the transistor no longer functions
as a regulator and because much tighter compensation is
obtained.
The circuit inFigure 5 uses pre-regulation to solve this prob-
lem. The added reference diode has a low breakdown so
that the minimum operating voltage of the op amp is unre-
stricted. Because of the low breakdown, the drop across R3
can no longer be considered constant. But it will vary linear-
ly with temperature, so this is of no consequence. The fact
that this reference can be used for other functions should
not be overlooked because a regulated voltage is frequently
required in designs using op amps.
InFigure 5, a divider is used so that the resistor feeding the
compensating current to the op amp can be reduced. There
will be an error current developed for any offset voltage
change across R6. This should not be a problem with the
LM11 because of its low offset voltage. But for tight com-
pensation, mismatch in the temperature characteristics of
R4 and R5 must be considered.
TL/H/7479–6
Figure 5. Bias current compensation for use with unregulated supplies.
Reference voltage is available for other circuitry.
3
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