AD8031/AD8032

–13–

REV. B

Output Overdrive Recovery

Output overdrive of an amplifier occurs when the amplifier

attempts to drive the output voltage to a level outside its normal

range. After the overdrive condition is removed, the amplifier

must recover to normal operation in a reasonable amount of

time. As shown in Figure 40, the AD8031/AD8032 recover

within 100 ns from negative overdrive and within 80 ns from

positive overdrive.

2.5V

2.5V

TO GND

100ns

1V

50

Figure 40. Overdrive Recovery

Driving Capacitive Loads

Capacitive loads interact with an op amp’s output impedance to

create an extra delay in the feedback path. This reduces circuit

stability, and can cause unwanted ringing and oscillation. A

given value of capacitance causes much less ringing when the

amplifier is used with a higher noise gain.

The capacitive load drive of the AD8031/AD8032 can be in-

creased by adding a low valued resistor in series with the capaci-

tive load. Introducing a series resistor tends to isolate the

capacitive load from the feedback loop, thereby, diminishing its

influence. Figure 41 shows the effects of a series resistor on

capacitive drive for varying voltage gains. As the closed-loop

gain is increased, the larger phase margin allows for larger ca-

pacitive loads with less overshoot. Adding a series resistor at

lower closed-loop gains accomplishes the same effect. For large

capacitive loads, the frequency response of the amplifier will be

dominated by the roll-off of the series resistor and capacitive

load.

1000

10

100

01

4

CLOSED-LOOP GAIN – V/V

23

200mV STEP

WITH 30% OVERSHOOT

1

5

Figure 41. Capacitive Load Drive vs. Closed-Loop Gain

Output Stage, Open-Loop Gain and Distortion vs. Clearance

from Power Supply

The AD8031 features a rail-to-rail output stage. The output

transistors operate as common emitter amplifiers, providing the

output drive current as well as a large portion of the amplifier’s

open-loop gain.

Q37

R29

300

Q47

Q21

Q20

Q51

Q27

Q68

Q44

Q42

Q48

Q49

Q50

Q43

C5

1.5pF

I4

25 A

Q38

I1

25 A

DIFFERENTIAL

DRIVE

FROM

INPUT STAGE

I5

25 A

I2

25 A

C9

5pF

Figure 39. Output Stage Simplified Schematic

The output voltage limit depends on how much current the

output transistors are required to source or sink. For applica-

tions with very low drive requirements (a unity gain follower

driving another amplifier input, for instance), the AD8031 typi-

cally swings within 20 mV of either voltage supply. As the re-

quired current load increases, the saturation output voltage will

× R

the AD8031, the collector resistances for both output transistors

are typically 25

Ω. As the current load exceeds the rated output

current of 15 mA, the amount of base drive current required to

drive the output transistor into saturation will reach its limit,

and the amplifier’s output swing will rapidly decrease.

The open-loop gain of the AD8031 decreases approximately

linearly with load resistance and also depends on the output

voltage. Open-loop gain stays constant to within 250 mV of the

positive power supply, 150 mV of the negative power supply and

then decreases as the output transistors are driven further into

saturation.

The distortion performance of the AD8031/AD8032 amplifiers

differs from conventional amplifiers. Typically an amplifier’s

distortion performance degrades as the output voltage ampli-

tude increases.

Used as a unity gain follower, the AD8031/AD8032 output will

exhibit more distortion in the peak output voltage region around

the input stage architecture and is discussed in detail in the

section covering “Input Stage Operation.”