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% feedbacksuch as a voltage

follower or integratorand the gain of the LM321 is high

1000and 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

001 mF 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

for 05 MHz bandwidth C

4

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

4

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

8

Table I shows typical values for the two compensating ca-

pacitors for various gains and operating currents

TABLE I

Closed

Current Set Resistor

Loop

Gain

120 kX

60 kX

30 kX

12 kX

6kX

1

68

130

270

680

1300

5

15

27

56

130

270

10

10

15

27

68

130

50

1

3

5

15

27

100

–

1

3

5

10

500

–

–

1

1

3

1000

–

–

–

–

–

This table applies for the LM108 LM101A LM741 LM118

Capacitance is in pF

DESIGN EQUATIONS FOR THE LM321 SERIES

Operating Current

Positive Common-Mode Limit

(

Typical Applications

Offset adjust

See table for frequency compensation

TLH7769 2

FIGURE 1 Low Drift Op Amp Using the LM321A as a Preamp

7