AD8031/AD8032
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.
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.
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.
I2
25µA
I1
25µA
Q51
Q42
Q47
DIFFERENTIAL
DRIVE
Q37
Q38
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,
FROM
C9
5pF
Q68
INPUT STAGE
R29
300Ω
Q20
Q27
Q21
V
OUT
C5
1.5pF
Q43
Q48
Q49
I4
25µA
OUTPUT OVERDRIVE RECOVERY
I5
25µA
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.
Q50
Q44
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
R
R
F
G
R
= R = 2kΩ
F
G
V
OUT
V
IN
R
L
I
LOAD × RC
where:
LOAD is the required load current.
50Ω
I
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.
V
V
R
= ±2.5V
S
= ±2.5V
IN
= 1kΩ TO GND
1V
L
100ns
Figure 45. Overdrive Recovery
Rev. C | Page 14 of 20
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