AD8011 데이터 시트보기 (PDF) - Analog Devices
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AD8011 Datasheet PDF : 16 Pages
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AD8011
(error current times the open-loop inverting input resistance) that
results (see Figure 7), a more exact low frequency closed-loop
transfer function can be described as
AV
=
G
1
+
G × RI
TO
+
RF
TO
=
G
1+
G
AO
+
RF
TO
for noninverting (G is positive).
AV
=
G
1
+
1–G
AO
+
RF
TO
for inverting (G is negative).
where G is the ideal gain as previously described. With RI = TO /AO
(open-loop inverting input resistance), the second expression
(positive G) clearly relates to the classical voltage feedback op amp
equation with TO omitted due to its relatively much higher value
and thus insignificant effect. AO and TO are the open-loop dc
voltage and transresistance gains of the amplifier, respectively.
These key transfer variables can be described as
AO
=
R1 ×
(1 –
gmf
gmc
×|A2|
× R1)
R1 × |A2|
and
TO =
2
Therefore
RI
=
1 – gmc ×
2 × gmf
R1
where gmc is the positive feedback transconductance (not shown)
and 1/gmf is the thermal emitter resistance of devices D1/D2 and
Q3/Q4. The gmc × R1 product has a design value that results in a
negative dc open-loop gain of typically –2500 V/V (see Figure 8).
+VS
RS
LN
LS
VP
IE
TO (s)
AO (s)
LI
ZI
LS
VO
RL
CL
CP
RN
RF
–VS
Z I = OPEN LOOP INPUT IMPEDANCE = CI || RL
Figure 7. ZI = Open-Loop Input Impedance
Though atypical of conventional CF or VF amps, this negative
open-loop voltage gain results in an input referred error term
(VP–VO/G = G/AO + RF/TO) that will typically be negative for G,
greater than +3/–4. As an example, for G = 10, AO = –2500,
and TO = 1.2 MΩ, results in an error of –3 mV using the AV
derivation above.
This analysis assumes perfect current sources and infinite transistor
VAs. (Q3, Q4 output conductances are assumed zero.) These
assumptions result in actual versus 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 (VP) modulating the output conductances of Q3/Q4,
resulting in RI less negative than derived here. For inverting
operation, the actual versus model dc error terms are relatively
much less.
80
70
60
50
40
30
20
10
0
–10
–20
–30
1E+03
1E+04
PHASE
GAIN
AO(s)
1E+05 1E+06 1E+07
FREQUENCY (Hz)
1E+08
–90
–100
–110
–120
–130
–140
–150
–160
–170
–180
–190
–200
1E+09
Figure 8. Open-Loop Voltage Gain and Phase
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 approxima-
tion and an intuitive understanding of its primary ac small signal
characteristics.
For inverting operation and high noninverting gains, these
transfer equations provide a good approximation to the actual
ac performance of the device.
To accurately quantify the VO versus VP relationship, AO(s)
and TO(s) need to be derived. This can be seen by the following
nonexpanded noninverting gain relationship
VO (s) /VP (s) =
G
G
AO [s]
+
RF
TO [s]
+
1
with
AO (s)
=
R1× gmf ×| A2 |
1 – gmc × R1
Sτ1
1 – gmc × R1
where R1 is the input resistance to A2/A2B, and τ1 (equal to
CD ϫ R1 ϫ A2) is the open-loop dominate time constant,
and
TO
(s)
=
|
A2 | ×R1
2
sτ1+ 1
–10–
REV. C
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