AD8005

–11–

REV. A

Increasing Feedback Resistors

Unlike conventional voltage feedback op amps, the choice of feed-

back resistor has a direct impact on the closed-loop bandwidth

and stability of a current feedback op amp circuit. Reducing the

resistance below the recommended value makes the amplifier

more unstable. Increasing the size of the feedback resistor

reduces the closed-loop bandwidth.

+5V

562

4.99k

2V (rms)

–5V

AD8005

0.2V (rms)

QUIESCENT CURRENT

475 A (MAX)

360 A (rms)

Figure 33. Saving Power by Increasing Feedback Resistor

Network

In power-critical applications where some bandwidth can be

sacrificed, increasing the size of the feedback resistor will yield

significant power savings. A good example of this is the gain of

+10 case. Operating from a bipolar supply (

±5 V), the quiescent

current is 475

µA (excluding the feedback network). The recom-

mended feedback and gain resistors are 499

Ω and 56.2 Ω

respectively. In order to drive an rms output voltage of 2 V, the

output must deliver a current of 3.6 mA to the feedback net-

work. Increasing the size of the resistor network by a factor of

10 as shown in Figure 33 will reduce this current to 360

µA.

The closed loop bandwidth will however decrease to 20 MHz.