AD8029/AD8030/AD8040 Data Sheet
Rev. B | Page 16 of 24
THEORY OF OPERATION
03679-0-051
IN–
IN+
R
5
R
6
R
7
R
8
R
1
R
2
R
3
R
4
M
TOP
I
TAIL
M
BOT
OUTPUT
BUFFER
–V
S
R
TH
I
TH
+V
S
–1.2V
+V
S
V
OUT
Q
10
Q
11
C
MT
C
MB
–V
S
AD8029 ONLY
TO DISABLE
CIRCUITRY
DISABLE
Q
3
Q
4
Q
2
Q
1
Q
9
Q
8
Q
7
Q
6
Q
5
OUT IN
COM
SPD
Figure 50. Simplified Schematic
The AD8029 (single), AD8030 (dual), and AD8040 (quad) are
rail-to-rail input and output amplifiers fabricated using Analog
Devices’ XFCB process. The XFCB process enables the
AD8029/ AD8030/AD8040 to operate on 2.7 V to 12 V supplies
with a 120 MHz bandwidth and a 60 V/µs slew rate. A
simplified sche-matic of the AD8029/AD8030/AD8040 is
shown in Figure 50.
INPUT STAGE
For input common-mode voltages less than a set threshold
(1.2 V below V
CC
), the resistor degenerated PNP differential
pair (comprising Q
1
toQ
4
) carries the entire I
TAIL
current,
allowing the input voltage to go 200 mV below –V
S
. Conversely,
input common-mode voltages exceeding the same threshold
cause I
TAIL
to be routed away from the PNP differential pair and
into the NPN differential pair through transistor Q
9
. Under this
condition, the input common-mode voltage is allowed to rise
200 mV above +V
S
while still maintaining linear amplifier
behavior. The transition between these two modes of operation
leads to a sudden, temporary shift in input stage transconduc-
tance, g
m
, and dc parameters (such as the input offset voltage
V
OS
), which in turn adversely affect the distortion performance.
The SPD block shortens the duration of this transition, thus
improving the distortion performance. As shown in Figure 50,
the input differential pair is protected by a pair of two series
diodes, connected in anti-parallel, which clamp the differential
input voltage to approximately ±1.5 V.
OUTPUT STAGE
The currents derived from the PNP and NPN input differential
pairs are injected into the current mirrors M
BOT
and M
TOP
, thus
establishing a common-mode signal voltage at the input of the
output buffer.
The output buffer performs three functions:
1. It buffers and applies the desired signal voltage to the
output devices, Q
10
and Q
11
.
2. It senses the common-mode current level in the output
devices.
3. It regulates the output common-mode current by
establishing a common-mode feedback loop.
The output devices Q
10
and Q
11
work in a common-emitter
configuration, and are Miller-compensated by internal
capacitors, C
MT
and C
MB
.
The output voltage compliance is set by the output devices’
collector resistance R
C
(about 25 Ω), and by the required load
current I
L
. For instance, a light equivalent load (5 kΩ) allows the
output voltage to swing to within 40 mV of either rail, while
heavier loads cause this figure to deteriorate as R
C
× I
L
.
