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
V
e
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
mF
bypass capacitor to ground Overall frequency com-
pensation then is achieved by an integrating capacitor
around the op amp
12
2
q
R
SET
C
4
for 0 5 MHz bandwidth C
e
6
R
SET
10
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 鈥樷€楥鈥欌€?may be
decreased to
Bandwidth at unity-gain
j
C
e
4
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
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
Current Set Resistor
120 kX
68
15
10
1
鈥?/div>
鈥?/div>
鈥?/div>
60 kX
130
27
15
3
1
鈥?/div>
鈥?/div>
30 kX
270
56
27
5
3
1
鈥?/div>
12 kX
680
130
68
15
5
1
鈥?/div>
6 kX
1300
270
130
27
10
3
鈥?/div>
This table applies for the LM108 LM101A LM741 LM118
Capacitance is in pF
DESIGN EQUATIONS FOR THE LM321 SERIES
1 2
c
10
6
R
SET
Null Pot Value should be 10% of R
SET
2
c
0 65V
Operating Current
R
SET
Gain A
V
Positive Common-Mode Limit
V
a
b
0 6
b
0 65V
c
50k
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
Typical Applications
Offset adjust
See table for frequency compensation
TL H 7769 鈥?2
FIGURE 1 Low Drift Op Amp Using the LM321A as a Preamp
7
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