AD8031/AD8032

Data Sheet

Rev. G | Page 14 of 20

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.

DIFFERENTIAL

DRIVE

FROM

INPUT STAGE

Q37

Q47

Q21

Q20

Q51

Q27

Q68

Q44

Q42

Q48

Q49

Q50

Q43

VOUT

Q38

I1

25µA

I2

25µA

C9

5pF

C5

1.5pF

I5

25µA

I4

25µA

R29

300

Ω

Figure 44. Output Stage Simplified Schematic

The output voltage limit depends on how much current the

output transistors are required to source or sink. For applications

with low drive requirements (for instance, a unity gain follower

driving another amplifier input), the AD8031 typically swings

within 20 mV of either voltage supply. As the required current

load increases, the saturation output voltage increases linearly as

ILOAD × RC

where:

ILOAD is the required load current.

RC is the output transistor collector resistance.

For 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 reaches its

limit, and the amplifier’s output swing rapidly decreases.

The open-loop gain of the AD8031 decreases approximately

linearly with load resistance and 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, the distortion

performance of the amplifier degrades as the output voltage

amplitude increases.

Used as a unity gain follower, the output of the AD8031/

AD8032 exhibits more distortion in the peak output voltage

region around VCC − 0.7 V. This unusual distortion characteristic is

caused by the input stage architecture and is discussed in detail

in the Input Stage Operation section,

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 45, the AD8031/AD8032 recover

within 100 ns from negative overdrive and within 80 ns from

positive overdrive.

RL

50

Ω

VIN

VOUT

100ns

1V

VS = ±2.5V

VIN = ±2.5V

RL = 1kΩ TO GND

RF = RG = 2kΩ

RG

RF

Figure 45. Overdrive Recovery