AD8031B Analog Devices, AD8031B Datasheet - Page 13

• Manufacturer Part Number: AD8031B
• Description: 2.7 V/ 800 uA/ 80 MHz Rail-to-Rail I/O Amplifiers
• Manufacturer: Analog Devices

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 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
increase linearly as I
current and R
the AD8031, the collector resistances for both output transistors
are typically 25 . 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
V
the input stage architecture and is discussed in detail in the
section covering “Input Stage Operation.”
REV. B
DIFFERENTIAL
INPUT STAGE
CC
–0.7 V. This unusual distortion characteristic is caused by
Figure 39. Output Stage Simplified Schematic
DRIVE
FROM
C
is the output transistor collector resistance. For
Q20
25 A
LOAD
I1
25 A
I4
Q21
Q42
Q50
R
Q43
Q37
C
, where I
300
R29
Q48
Q38
Q44
Q51
25 A
LOAD
I5
Q27
is the required load
Q68
I2
25 A
1.5pF
C5
5pF
C9
Q47
Q49
V
OUT
–13–
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.
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.
Figure 41. Capacitive Load Drive vs. Closed-Loop Gain
1000
100
10
1
0
V
200mV STEP
WITH 30% OVERSHOOT
Figure 40. Overdrive Recovery
S
1V
= +5V
R
R
1
F
S
R
V
V
= R
= 20
L
S
IN
R
= +1k
S
=
=
CLOSED-LOOP GAIN – V/V
G
= 0 , 5
= 2k
2.5V
2.5V
2
TO GND
V
IN
R
AD8031/AD8032
G
R
G
50
3
R
R
S
F
= 20
R
F
100ns
R
L
V
OUT
R
R
4
S
S
= 5
R
V
C
OUT
S
L
= 0
5