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參數(shù)資料
| 型號(hào)： | LM221 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | LM221/LM321 Precision Preamplifiers |
| 中文描述： | LM221/LM321精密前置放大器 |
| 文件頁數(shù)： | 7/11頁 |
| 文件大?。?/td> | 207K |
| 代理商： | LM221 |
Frequency Compensation
UNIVERSAL COMPENSATION
The additional gain of the LM321 preamplifier when used
with an operational amplifier usually necessitates additional
frequency compensation. When the closed loop gain of the
op amp with the LM321 is less than the gain of the LM321
alone, more compensation is needed. The worst case situa-
tion is when there is 100% feedbackDsuch as a voltage
follower or integratorDand the gain of the LM321 is high.
When high closed loop gains are usedDfor example A
V
e
1000Dand only an addition gain of 200 is inserted by the
LM321, the frequency compensation of the op amp will usu-
ally suffice.
The frequency compensation shown here is designed to op-
erate with any unity-gain stable op amp.Figure 1 shows the
basic configuration of frequency stabilizing network. In oper-
ation the output of the LM321 is rendered single ended by a
0.01
m
F bypass capacitor to ground. Overall frequency com-
pensation then is achieved by an integrating capacitor
around the op amp.
Bandwidth at unity-gain
j
12
2
q
R
SET
C
for 0.5 MHz bandwidth C
e
4
10
6
R
SET
For use with higher frequency op amps such as the LM118
the bandwidth may be increased to about 2 MHz.
If the closed loop gain is greater than unity, ‘‘C’’ may be
decreased to:
C
e
4
10
6
A
CL
R
SET
ALTERNATE COMPENSATION
The two compensation capacitors can be made equal for
improved power supply rejection. In this case the formula for
the compensation capscitor is:
C
e
8
10
6
A
CL
R
SET
Table I shows typical values for the two compensating ca-
pacitors for various gains and operating currents.
TABLE I
Closed
Loop
Gain
Current Set Resistor
120 k
X
60 k
X
30 k
X
12 k
X
6 k
X
A
V
e
1
A
V
e
5
A
V
e
10
A
V
e
50
A
V
e
100
A
V
e
500
A
V
e
1000
68
15
10
1
–
–
–
130
27
15
3
1
–
–
270
56
27
5
3
1
–
680
130
68
15
5
1
–
1300
270
130
27
10
3
–
This table applies for the LM108, LM101A, LM741, LM118.
Capacitance is in pF.
DESIGN EQUATIONS FOR THE LM321 SERIES
Gain A
V
&
1.2
c
10
6
R
SET
Null Pot Value should be 10% of R
SET
Operating Current
&
2
c
0.65V
R
SET
Positive Common-Mode Limit
&
V
a
b
D
0.6
b
0.65V
c
50k
R
SET
(
Typical Applications
*
Offset adjust.
2
See table for frequency compensation.
TL/H/7769–2
FIGURE 1. Low Drift Op Amp Using the LM321A as a Preamp
7
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