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參數資料
| 型號： | AD8011* |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Micropower Op Amp, Comparator and Reference; Package: MSOP; No of Pins: 8; Temperature Range: 0°C to +70°C |
| 中文描述： | 300兆赫。 1毫安電流反饋放大器 |
| 文件頁數： | 10/16頁 |
| 文件大小： | 219K |
REV. B
–10–
AD8011
This analysis assumes perfect current sources and infinite tran-
sistor V
A
s (Q3, Q4 output conductances are assumed zero).
These assumptions result in actual vs. model open loop voltage
gain and associated input referred error terms being less accurate
for low gain (G) noninverting operation at the frequencies below
the open loop pole of the AD8011. This is primarily a result of
the input signal (V
P
) modulating the output conductances of
Q3/Q4 resulting in R
I
less negative than derived here. For invert-
ing operation, the actual vs. model dc error terms are relatively
much less.
AC TRANSFER CHARACTERISTICS
The ac small signal transfer derivations below are based on a
simplified single-pole model. Though inaccurate at frequencies
approaching the closed-loop BW (CLBW) of the AD8011 at low
noninverting external gains, they still provide a fair approximation
and a intuitive understanding of its primary ac small signal
characteristics.
For inverting operation and high noninverting gains these trans-
fer equations provide a good approximation to the actual ac
performance of the device.
To accurately quantify the V
O
vs. V
P
relationship, both A
O
(s)
and T
O
(s) need to be derived. This can be seen by the following
nonexpanded noninverting gain relationship:
V
O
(
s
)/
V
P
(
s
)
=
G
G
A
O
[
s
]
+
R
F
T
O
[
s
]
+
1
with
A
O
(
s
)
=
R
1
×
g
mf
×
|
A
2|
1
–
g
mc
×
R
1
S
τ
1
1
–
g
mc
×
R
1
1E+03 1E+04 1E+05 1E+06 1E+07 1E+08 1E+09
FREQUENCY
–
Hz
80
70
60
50
40
20
10
30
G
–
–
90
P
–
–
100
–
110
–
120
–
160
PHASE
GAIN
0
–
10
–
20
–
30
–
170
–
180
–
190
–
200
–
130
–
140
–
150
A
O
(s)
Figure 30. Open-Loop Voltage Gain and Phase
(error current times the open loop inverting input resistance)
that results (see Figure 29), a more exact low frequency closed
loop transfer functions can be described as:
A
V
=
G
1
+
G
×
R
I
T
O
+
R
F
T
O
=
G
1
+
G
A
O
+
R
F
T
O
for noninverting (G is positive)
A
V
=
G
1
+
1
–
G
A
O
+
R
F
T
O
for inverting (G is negative)
R
S
L
N
T
O
(s)
A
O
(s)
V
P
Z
I
IE
L
I
R
N
C
P
R
F
+V
S
–
V
S
L
S
R
L
C
L
V
O
L
S
Z
I
= OPEN LOOP INPUT IMPEDANCE = C
I
|| R
L
Figure 29. Z
I
= Open-Loop Input Impedance
where G is the ideal gain as previously described. With
R
I
= T
O
/A
O
(open-loop inverting input resistance), the second
expression (positive G) clearly relates to the classical voltage
feedback
“
op amp
”
equation with T
O
omitted do to its relatively
much higher value and thus insignificant effect. A
O
and T
O
are
the open-loop dc voltage and transresistance gains of the ampli-
fier respectively. These key transfer variables can be described as:
A
R
gmf
A
g
R
R
A
R
gmc
×
2
R
gmf
O
O
I
mc
T
=
×
×
×
=
×
=
×
1
2
1
1
1
2
2
1
1
|
|
(
–
)
|
|
;
and
therefore
Where g
mc
is the positive feedback transconductance (not
shown) and 1/g
mf
is the thermal emitter resistance of devices
D1/D2 and Q3/Q4. The g
mc
×
R1 product has a design value that
results in a negative dc open loop gain of typically
–
2500 V/V (see
Figure 30).
Though atypical of conventional CF or VF amps, this negative
open-loop voltage gain results in an input referred error term
(V
P
–
V
O
/G = G/A
O
+ R
F
/T
O
) that will typically be negative for G
greater than +3/
–
4. As an example, for G = 10, A
O
=
–
2500 and
T
O
= 1.2 M
, results in a error of
–
3 mV using the A
V
deriva-
tion above.
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