LM221H NSC [National Semiconductor], LM221H Datasheet - Page 7

• Manufacturer Part Number: LM221H
• Description: LM221/LM321 Precision Preamplifiers
• Manufacturer: NSC [National Semiconductor]

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% feedback such as a voltage
follower or integrator and the gain of the LM321 is high
When high closed loop gains are used for example A
1000 and 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 F bypass capacitor to ground Overall frequency com-
pensation then is achieved by an integrating capacitor
around the op amp
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
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
Typical Applications
Offset adjust
See table for frequency compensation
Bandwidth at unity-gain j
for 0 5 MHz bandwidth C
C
C
e
e
10
10
6
6
A
A
e
CL
CL
2 R
FIGURE 1 Low Drift Op Amp Using the LM321A as a Preamp
4
8
10
R
R
12
6
SET
SET
SET
R
4
SET
C
V
e
7
Table I shows typical values for the two compensating ca-
pacitors for various gains and operating currents
This table applies for the LM108 LM101A LM741 LM118
Capacitance is in pF
DESIGN EQUATIONS FOR THE LM321 SERIES
Null Pot Value should be 10% of R
Positive Common-Mode Limit
A
A
A
A
A
V
Closed
A
A
V
V
Loop
V
V
Gain
V
V
e
e
e
e
e
e
e
1000
100
500
10
50
1
5
Operating Current
120 k
68
15
10
1
–
–
–
Gain A
TABLE I
V
60 k
Current Set Resistor
130
27
15
3
1
–
–
1 2
V
R
a
c
SET
30 k
2
SET
270
b
56
27
10
c
5
3
1
–
R
6
SET
0 65V
0 6
12 k
b
680
130
68
15
TL H 7769 – 2
5
1
–
0 65V
R
SET
1300
c
6 k
270
130
27
10
3
–
50k