AN-763

Application Note

Rev. C | Page 4 of 8

R1

1k

Ω

V–

V+

V–

V+

VIN

100k

Ω

R2

100

Ω

C3

1k

Ω

R4

R3

C2

VCC

VEE

VCC

VEE

1/2

AD8657

1/2

AD8657

Figure 6. Low Power Composite Amplifier

A composite amplifier can be used to optimize the dc and ac

characteristics. Figure 6 shows an example using the AD8657,

which offers many circuit advantages. The bandwidth is

increased substantially and the input offset voltage and noise

of the AD8657 becomes insignificant because they are divided

by the high gain of the amplifier. The circuit offers a high

bandwidth, a high output current, and a very low power

consumption of less than 100 μA.

EXTERNAL COMPENSATION TECHNIQUES

Series Resistor Compensation

The use of external compensation networks may be required

to optimize certain applications. Figure 7 shows a typical

representation of a series resistor compensation to stabilize an

op amp driving capacitive loads. The stabilizing effect of the

series resistor can be thought of as a means to isolate the op

amp output and the feedback network from the capacitive load.

The required amount of series resistance depends on the part

used, but values of 5 Ω to 50 Ω are usually sufficient to prevent

local resonance. The disadvantage of this technique is a

reduction in gain accuracy and extra distortion when driving

nonlinear loads.

VIN

VOUT

R2

CL

RL

Figure 7. Series Resistor Compensation

TIME (10µs/DIV)

GND

RL = 10kΩ

CL = 2nF

Figure 8. Capacitor Load Drive Without Resistor

TIME (10µs/DIV)

GND

RL = 10kΩ

RS = 200Ω

CL = 2nF

CS = 0.47µF

Figure 9. Capacitor Load Drive with Resistor