Datasheet

Data Sheet AD829
Rev. I | Page 13 of 20
supply voltages should be used. Figure 38 is a graphical extension
of Table 4, which shows the slew rate/gain trade-off for lower
closed-loop gains, when using the shunt compensation scheme.
Figure 38. Value of C
COMP
and Slew Rate vs. Noise Gain
CURRENT FEEDBACK COMPENSATION
Bipolar, nondegenerated, single-pole, and internally
compensated amplifiers have their bandwidths defined as
COMP
COMP
e
T
C
q
kT
I
Cr
f
π
=
π
=
2
2
1
where:
f
T
is the unity-gain bandwidth of the amplifier.
I is the collector current of the input transistor.
C
COMP
is the compensation capacitance.
r
e
is the inverse of the transconductance of the input transistors.
kT/q approximately equals 26 mV at 27°C.
Because both f
T
and slew rate are functions of the same variables,
the dynamic behavior of an amplifier is limited. Because
COMP
C
I
RateSlew
2
=
then
q
kT
f
RateSlew
T
π= 4
This shows that the slew rate is only 0.314 V/µs for every mega-
hertz of bandwidth. The only way to increase the slew rate is to
increase the f
T
, and that is difficult because of process limitations.
Unfortunately, an amplifier with a bandwidth of 10 MHz can
only slew at 3.1 V/µs, which is barely enough to provide a full
power bandwidth of 50 kHz.
The AD829 is especially suited to a form of current feedback
compensation that allows for the enhancement of both the full
power bandwidth and the slew rate of the amplifier. The voltage
gain from the inverting input pin to the compensation pin is
large; therefore, if a capacitance is inserted between these pins,
the bandwidth of the amplifier becomes a function of its feed-
back resistor and the capacitance. The slew rate of the amplifier
is now a function of its internal bias (2I) and the compensation
capacitance.
Table 4. Component Selection for Shunt Compensation
Follower Gain Inverter Gain R1 (Ω) R2 (Ω) C
LEAD
(pF) C
COMP
(pF) Slew Rate (V/µs) −3 dB Small Signal Bandwidth (MHz)
1 Open 100 0 68 16 66
2 −1 1 k 1 k 5 25 38 71
5 −4 511 2.0 k 1 7 90 76
10 −9 226 2.05 k 0 3 130 65
20 −19 105 2 k 0 0 230 55
25
−24
105
2.49
0
0
230
39
100 −99 20 2 k 0 0 230 7.5
00880-038
NOISE GAIN
C
COMP
(pF)
SLEW RATE (V/µs)
1 10
1
10
100
10
100
1k
100
SLEW RATE
C
COMP
V
S
= ±15V