AD8031 Analog Devices, AD8031 Datasheet - Page 14

• Manufacturer Part Number: AD8031
• Manufacturer: Analog Devices

Specifications of AD8031

• -3db Bandwidth: 80MHz
• Slew Rate: 35V/µs
• Vos: 1mV
• Ib: 450nA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 15nV/rtHz
• Vcc-vee: 2.7V to 12V
• Isy Per Amplifier: 1.6mA
• Packages: DIP,SOIC,SOT

AD8031/AD8032
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
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
where:
I
R
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.
INPUT STAGE
LOAD
C
is the output transistor collector resistance.
I
is the required load current.
LOAD
DRIVE
FROM
× R
Figure 44. Output Stage Simplified Schematic
C
Q20
25µA
I1
25µA
I4
Q21
Q42
Q50
Q43
Q37
300Ω
R29
Q48
Q38
Q44
Q51
25µA
I5
Q27
Q68
I2
25µA
1.5pF
C5
5pF
C9
Q47
Q49
V
OUT
Rev. D | Page 14 of 20
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 V
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.
1V
R
V
V
R
F
CC
S
IN
L
= R
= ±2.5V
= 1kΩ TO GND
= ±2.5V
− 0.7 V. This unusual distortion characteristic is
G
Figure 45. Overdrive Recovery
= 2kΩ
V
IN
R
G
50Ω
R
F
R
L
100ns
V
OUT