Characteristics of an Ideal Op-Amprhabash/ELG4135L2.pdf · 2007. 9. 13. · Ad vo AdvId Acm vIcm...
Transcript of Characteristics of an Ideal Op-Amprhabash/ELG4135L2.pdf · 2007. 9. 13. · Ad vo AdvId Acm vIcm...
![Page 1: Characteristics of an Ideal Op-Amprhabash/ELG4135L2.pdf · 2007. 9. 13. · Ad vo AdvId Acm vIcm CMRR =20 log = + 3 ... • For the ideal op-amp, the common-mode output signal is](https://reader036.fdocuments.in/reader036/viewer/2022071510/612eef581ecc515869431ff9/html5/thumbnails/1.jpg)
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Characteristics of an Ideal Op-Amp
• Infinite input impedance
• Zero output impedance
• Zero common-mode gain, or, infinite common-mode rejection
• Infinite open-loop gain A
• Infinite bandwidth.
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Difference Amplifier• A difference amplifier is one that responds to the difference between the two
signals applied at its input and ideally rejects signals that are common to the two inputs.
21Idv
IcmvIv −=
+-
-+
+-
2Idv
22Idv
IcmvIv +=
2Idv
Icmv
12 IvIvIdv −=
)21(2
1IIIcm vvv +=
cmAdA
IcmvcmAIdvdAov
log20CMRR =
+=
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More Characteristics of Op-Amp
• Since the ideal op-amp responds only to the difference between the two input signals, the ideal op-amp maintains a zero output signal when the two input signals are equal.
• When the two input signals are unequal, there is what is called a common-mode input signal.
• For the ideal op-amp, the common-mode output signal is zero. This characteristic is referred to as common-mode rejection.
• Another characteristic, because op-amp is biased by both positive and negative power supplies, most op-amps are direct coupled devices (no coupling capacitors are required on the input). Accordingly, the two input voltages can be DC.
• Because the OP is composed of transistors biased in the active region by the DC power supply, the output voltage is limited.
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Difference Amplifier
R2
R1
R3
R4
vI1
vI2
vo
-
+
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Figure 8.10
A Single Difference or Differential Amplifier
( )121
2 vvR
Rvout −=
1
2
1
2
R
RdA
IdvR
Routv
=
=
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Figure 8.14, 8.15
Instrumentation Amplifier Input (a) and output (b) stages of Instrumentation amplifier
+=
−=
1
2
21
21
R
R
R
R
vv
vA Fout
V
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Design Example: Determine the range required for resistor R1 in the instrumentation amplifier to realize a differential gain adjustable from 5 to
500. Assume RF = 2R, so that the difference amplifier gain is 2.
• Assume R1 is a combination of a fixed resistor R1f and a variable resistor R1v. Assume R1v = 100 kΩ
Ω=Ω=
+=
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=
++=
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fR
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fR
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vvoutv
VA R1f
R1v
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Figure 8.35
Op-amp Differentiator
dt
tdvCRtv S
SFout
)()( −=
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Large Signal Operation of Op Amp
• Like other amplifiers, op amps operate linearly over a limited range of output voltages.
• Another limitation of the operation of op amps is that their output current is limited to a specified maximum. For example, the op amp 741 is specified to have a maximum output current of ±20 mA.
• Read Example 2.5.
• Another phenomenon that can cause nonlinear distortion when large output signals are present is that of slew-rate limiting. This means there is a specific maximum rate of change possible at the output of a real op amp. This maximum is known as the slew rate (SR) of the op amp and isdefined as
maxSR
dtodv
=
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Design Example. Design a difference amplifier with a specified gain and minimum differential input resistance.
Design the circuit such that the differential gain is 30 and theminimum differential input resistance is Ri = 50 kΩ
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Design Example: Calculate the common-mode rejection ratio of a differential amplifier. Consider the difference amplifier shown in page 2.
Let R2/R1 = 10 and R4/R3 = 11. Determine CMRR (dB)
dB 6.410.0833
10.0421020logCMRR(dB)
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)2
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cmAdAcmvcmAdvdAovcmvdvov
dvcmvdv
cmvvo
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Figure 8.20
Op-amp Circuits Employing Complex Impedances
S
F
S
out
S
F
S
out
Z
Zj
V
V
Z
Zj
V
V
+=
−=
1)(
)(
ω
ω
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Figure 8.21, 8.23
Active Low-Pass Filter
Normalized Response of Active Low-pass Filter
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Figure 8.24, 8.25
Active High-Pass Filter
Normalized Response of Active High-pass Filter
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Figure 8.26, 8.27
Active Band-Pass Filter
Normalized Amplitude Response of Active Band-pass Filter
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Figure 8.30
Op-amp Integrator
∫∞−
−=t
SFS
out dttvCR
tv )(1
)(
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Figure 8.35
Op-amp Differentiator
dt
tdvCRtv S
SFout
)()( −=