REV. A

AD8072/AD8073

–9–

APPLICATIONS

Overdrive Recovery

Overdrive of an amplifier occurs when the output and/or input

range are exceeded. The amplifier must recover from this overdrive

condition and resume normal operation. As shown in Figure 28,

the AD8072 and AD8073 recover within 75 ns from positive

overdrive and 30 ns from negative overdrive.

25ns

1V

Bandwidth vs. Feedback Resistor Value

The closed-loop frequency response of a current feedback am-

plifier is a function of the feedback resistor. A smaller feedback

resistor will produce a wider bandwidth response. However, if

the feedback resistance becomes too small, the gain flatness can

be affected. As a practical consideration, the minimum value of

feedback resistance for the AD8072/AD8073 was found to be

649

Ω. For resistances below this value, the gain flatness will be

affected and more significant lot to lot variations in device per-

formance will be noticed. Figure 29 shows a plot of the frequency

response of an AD8072/AD8073 at a gain of two with both feed-

back and gain resistors equal to 649

Ω.

On the other hand, the bandwidth of a current feedback ampli-

fier can be decreased by increasing the feedback resistance. This

can sometimes be useful where it is desired to reduce the noise

bandwidth of a system. As a practical matter, the maximum

value of feedback resistor was found to be 2 k

Ω. Figure 29

shows the frequency response of an AD8072/AD8073 at a gain

of two with both feedback and gain resistors equal to 2 k

Ω.

FREQUENCY – MHz

6.1

6.0

0.1

500

1

10

100

5.9

5.8

5.4

5.7

5.6

5.5

1 dB

DIV

0.1 dB

DIV

6

7

5

4

3

2

1

0

Capacitive Load Drive

When an op amp output drives a capacitive load, extra phase

shift due to the pole formed by the op amp’s output impedance

and the capacitor can cause peaking or even oscillation. The top

Ω, shows the output of one of the am-

plifiers of the AD8072/AD8073 when driving a 50 pF capacitor

as shown in the schematic of Figure 31.

The amount of peaking can be significantly reduced by adding

a resistor in series with the capacitor. The lower trace of Fig-

ure 30 shows the same capacitor being driven with a 25

Ω resis-

tor in series with it. In general, the resistor value will have to be

experimentally determined, but from 10

Ω to 50 Ω is a practical

range of values to experiment with for capacitive loads of up to a

few hundred pF.

20ns

50mV

Figure 30. Capacitive Low Drive

1k

1k

50

1k

50pF

Figure 31. Capacitive Load Drive Circuit