AD744
EXTERNAL FREQUENCY COMPENSATION
Even though the AD744 is useable without compensation in
most applications, it may be externally compensated for even
more flexibility. This is accomplished by connecting a capacitor
between Pins 5 and 8. Figure 28, a simplified schematic of the
AD744, shows where this capacitor is connected. This feature is
useful because it allows the AD744 to be used as a unity gain
voltage follower. It also enables the amplifier to drive capacitive
loads up to 2000 pF and greater.
+V
S
400 A
300
–IN
NULL /
COMPENSATION
NULL /
DECOMPENSATION
1k
1k
8k
–V
S
300
+IN
5pF
2mA
The following section provides tables to show what C
COMP
values
will provide the necessary compensation for given circuit configurations
and capacitive loads. In each case, the recommended C
COMP
is a
minimum value. A larger C
COMP
can always be used, but slew rate
and bandwidth performance will be degraded.
Figure 30 shows the AD744 configured as a unity gain voltage
follower. In this case, a minimum compensation capacitor of
5 pF is necessary for stable operation. Larger compensation ca-
pacitors can be used for driving larger capacitive loads. Table I
outlines recommended minimum values for C
COMP
based on
the desired capacitive load. It also gives the slew rate and band-
width that will be achieved for each case.
+V
S
OUTPUT
1 F
0.1 F
AD744
COMPENSATION
V
IN
C
COMP
5pF
V
OUT
1 F
0.1 F
Figure 28. AD744 Simplified Schematic
–V
S
The slew rate and gain bandwidth product of the AD744 are in-
versely proportional to the value of the compensation capacitor,
C
COMP
. Therefore, when trying to maximize the speed of the
amplifier, the value of C
COMP
should be minimized. C
COMP
can
also be used to slow the amplifier to a point where the slew rate
is perfectly symmetrical and well controlled. Figure 29 sum-
marizes the effect of external compensation on slew rate and
bandwidth.
20
100
Figure 30. AD744 Connected as a Unity Gain
Voltage Follower
Table I. Recommended Values of C
COMP
vs.
Various Capacitive Loads
Gain
1
1
1
Max
C
LOAD
(pF)
50
150
2000
C
COMP
(pF)
5
10
25
Slew Rate
(V/ s)
37
25
12.5
–3 dB
Bandwidth
(MHz)
6.5
4.3
2.0
GAIN BANDWIDTH – MHz
2
10
SLEW RATE – V/ s
0.2
1.0
Figures 31 and 32 show the AD744 as a voltage follower
with gain and as an inverting amplifier. In these cases, external
compensation is not necessary for stable operation. How-
ever, compensation may be applied to drive capacitive loads
above 50 pF. Table II gives recommended C
COMP
values, along
with expected slew rates and bandwidths for a variety of load
conditions and gains for the circuits in Figures 31 and 32.
C
LEAD
*
R1*
+V
S
1 F
0.1 F
R2*
0.02
0
10
C
COMP
– pF
100
0.1
1000
Figure 29. Gain Bandwidth and Slew Rate vs. C
COMP
AD744
V
IN
V
OUT
OPTIONAL
C
COMP
1 F
0.1 F
*SEE
TABLE II
–V
S
Figure 31. AD744 Connected as a Voltage Follower
Operating at Gains of 2 or Greater
–8–
REV. C
AD [ ANALOG DEVICES ]
