REV. C
AD744
–8–
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.
5pF
+IN
300⍀
–IN
NULL /
COMPENSATION
NULL /
DECOMPENSATION
300⍀
COMPENSATION
2mA400A
+V
S
OUTPUT
–V
S
1k⍀1k⍀ 8k⍀
Figure 28. AD744 Simplified Schematic
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.
C
COMP
– pF
2
0
GAIN BANDWIDTH – MHz
20
0.02
0.2
10 100 1000
10
SLEW RATE – V/s
100
0.1
1.0
Figure 29. Gain Bandwidth and Slew Rate vs. C
COMP
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.
AD744
1F 0.1F
–V
S
C
COMP
5pF
1F 0.1F
+V
S
V
OUT
V
IN
Figure 30. AD744 Connected as a Unity Gain
Voltage Follower
Table I. Recommended Values of C
COMP
vs.
Various Capacitive Loads
Max –3 dB
C
LOAD
C
COMP
Slew Rate Bandwidth
Gain (pF) (pF) (V/s) (MHz)
1 50 5 37 6.5
1 150 10 25 4.3
1 2000 25 12.5 2.0
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.
AD744
1F
0.1F
–V
S
C
COMP
1F
0.1F
+V
S
V
OUT
V
IN
OPTIONAL
R2*
C
LEAD
*
R1*
*SEE TABLE II
Figure 31. AD744 Connected as a Voltage Follower
Operating at Gains of 2 or Greater