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AD8011 Scheda tecnica(PDF) 10 Page - Analog Devices

Il numero della parte AD8011
Spiegazioni elettronici  300 MHz Current Feedback Amplifier
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Produttore elettronici  AD [Analog Devices]
Homepage  http://www.analog.com
Logo AD - Analog Devices

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REV. C
–10–
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
A
G
GR
T
R
T
G
G
A
R
T
V
I
O
F
OO
F
O
=
+
×
+
=
++
11
for noninverting (G is positive).
A
V
O
F
O
G
G
A
R
T
=
++
1
1 –
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
A
Rgmf
A
g
mc
R
O
=
××
×
12
1
1
|
|
( –
)
and
T
RA
O
=
×
12
2
|
|
Therefore
R
g
mc
R
g
mf
I
=
×
×
1
1
2
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).
RS
LN
TO (s)
AO (s)
VP
ZI
IE
LI
RN
CP
RF
+VS
–VS
LS
RL
CL
VO
LS
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 M
Ω, results in an error of –3 mV using the A
V
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.
1E+03
1E+04
1E+05
1E+06
1E+07
1E+08
1E+09
80
70
60
50
40
20
10
FREQUENCY (Hz)
30
–90
–100
–110
–120
–160
PHASE
GAIN
0
–10
–20
–30
–170
–180
–190
–200
–130
–140
–150
AO(s)
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
Vs
V s
G
G
As
R
Ts
OP
O
F
O
() /
( )
[]
[]
=
++ 1
with
As
Rg
mf
A
g
mc
R
S
g
mc
R
O ()
||
=
××
×
×
12
11
1
11
τ
where R1 is the input resistance to A2/A2B, and
τ1 (equal to
CD
R1
A2) is the open-loop dominate time constant,
and
T
s
AR
s
O ()
||
=
×
+
21
2
11
τ


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