limitacions ao
TRANSCRIPT
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No Idealitats dels AOs
Limitacions dels Amplificadors
operacionals reals
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Some Opams of Analog Devices
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OA Schematic
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Non-ideal effects
Finite
open-loop
gain
Constant GxBW
V i
V o R2
+
-
R1
+
-
-
+
V id V o
+
-
+
-
-
+
G
BW
G BW
)()(
1
11
s A s A oV
)(
)( BW s
BW G s AV
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Non-ideal effects
Finite
input impedance.
Non-zero output impedance.
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OA Schematic
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Non-ideal effects
Input current
Due to biasing requirements or leakage.
Typically
~10 nanoamperes
for bipolar op-amps,
tens of picoamperes
for JFET input stages, and
only a few pA for MOSFET input stages.
Input offset voltage
Voltage required across the op-amp's inputterminals to drive
the
output voltage
to 0
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How to take into account V OS
, IB
and I OS
?
V id V o
+
-
+
-
-
+ real
V id
V o
+
-
+
-
-
+ ideal
+ V OS
V idV o
+
-
+
- -
+ ideal
N
P
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Non-ideal effects: CMRR
Differential-mode gain
A perfect operational amplifieramplifies only the voltage
difference between its twoinputs, completely rejecting allvoltages that are common toboth.
Common-mode gain
However, the differential input stage ofan operational amplifier is never
perfect, leading to the amplification ofthe common input voltage. Thestandard measure of this defect iscalled the common-mode rejection ratio
(denoted CMRR).
V idV o
+
-
+
--
+
V pV o
+
-
+
-
-
+
V n+
-
nn p po V AV AV
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CMRR
V pV o
+
-
+
-
-
+
V n+
-
n pd V V V
2
n pCM
V V V
nn p po V AV AV
22
d
CM n
d
CM po
V V A
V V AV
CM n pd n po V A AV A A
V 2
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CMRR
V pV o
+
-
+
-
-
+
V n+
-
n pd V V V 2n p
CM V V V
0V V V
ACM
d
od
0V V V
Ad
CM
oCM
CM CM d d o V AV AV
CM
d
A ACMRR log20
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CMRR
V pV o
+
-
+
-
-
+
V n+
-
CM
d
A ACMRR log20
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CMRR
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How to take into account CMRR effect?
V id V o
+
-
+
--
+ ideal +
V CMCMRR
V id
V o
+
-
+
-
-
+ real
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Non-ideal effects
Limited
output current
Most op-amps
are
designed
to limit
the
output current
so as not
toexceed
a specified
level
around
25
mA
for a type
741 IC op-amp
thus
protecting
the
op-amp
and associated
circuitry
from
damage.
Noise
Amplifiers
generate
random
voltage
at the
output even
when
there
isno signal
applied.
Drift
Real op-amp
parameters
are
subject
to slow
change
over time andwith
changes
in temperature, input conditions, etc.
Temperature
effects
All parameters change with temperature.
Temperature
drift of the
input offset voltage
is especially
important.
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Non-ideal effects
Power-supply
rejection
ratio
Reflects how well the op-amp can reject changes in its supply
voltage.
Use
of bypass capacitors
can
improve
the
PSRR.
CC
OS
V
V PSRR log20
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Non-linear limitations
Saturation
Maximum
output voltage
value.
It
is close
to the
power
supply.
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Rise Time
Rise
Time
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Slew Rate
Slew
Rate
Represents the maximum rate of change of asignal.
It
is usually
specified
in volts per microsecond.
Max.
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OA Schematic
Current
source
with
IOMAX
Capacitor
Integrator
dt I C
k V sourceo1
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Slew Rate
dt I C
k V sourceo1
C I
k dt
dV sourceo
SRC
I k
C I
k dt
dV sourceo maxmaxmax
t AV o sin t Adt
dV o cos At Adt
dV omax
max
cos
SRdt
dV omax SR f A
SR A
2 f SR A
2 ASR f
2
dt d
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Some Opams of Analog Devices