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.

CURRENT FEEDBACK COMPENSATION

Bipolar, nondegenerated, single-pole, and internally

compensated amplifiers have their bandwidths defined as

COMP

COMP

e

T

C

q

kT

I

C

r

f

π

=

π

=

2

2

1

where:

I is the collector current of the input transistor.

kT/q approximately equals 26 mV at 27°C.

the dynamic behavior of an amplifier is limited. Because

COMP

C

I

Rate

Slew

2

=

then

q

kT

f

Rate

Slew

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

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)

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

NOISE GAIN

1

10

1

10

100

10

100

1k

100

SLEW RATE