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Inleiding Meten
en Modellen – 8C120
Prof.dr.ir. Bart ter Haar Romeny
Dr. Andrea Fuster
Faculteit Biomedische Technologie
Biomedische Beeld Analyse
www.bmia.bmt.tue.nl
8C120 - 2010
Chapter 3 - Webster
Operational Amplifiers
and Signal Processing
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Applications of Operational Amplifier
In Biological Signals and Systems
The three major operations done on biological signals using
Op-Amps:
1) Amplifications and Attenuations
2) DC offsetting: add or subtract a DC
3) Filtering: Shape signal’s frequency content
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Ideal Op-Amp
Most bioelectric signals are small and require amplifications
Figure 3.1 Op-amp equivalent circuit.
The two inputs are 1 and 2. A differential voltage between them causes current
flow through the differential resistance Rd. The differential voltage is multiplied by
A, the gain of the op amp, to generate the output-voltage source. Any current
flowing to the output terminal vo must pass through the output resistance Ro.
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Inside the Op-Amp (IC-chip)
20 transistors
11 resistors
1 capacitor
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Ideal Characteristics
12345-
A = (gain is infinity)
Vo = 0, when v1 = v2 (no offset voltage)
Rd = (input impedance is infinity)
Ro = 0 (output impedance is zero)
Bandwidth = (no frequency response limitations) and no phase shift
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Two Basic Rules
Rule 1
When the op-amp output is in its linear range, the two input terminals
are at the same voltage.
Rule 2
No current flows into or out of either input terminal of the op amp.
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Inverting Amplifier
o
10 V
i
Rf
-10 V
i
i
Ri
10 V
i
-
o
Slope = -Rf / Ri
+
-10 V
(a)
(b)
vo
Rf
Ri
vi
Rf
vo
G
vi
Ri
Figure 3.3 (a) An inverting amplified. Current flowing through the input resistor Ri
also flows through the feedback resistor Rf . (b) The input-output plot shows a
slope of -Rf / Ri in the central portion, but the output saturates at about ±13 V.
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Summing Amplifier
Rf
R1
1
-
R2
2
o
+
v1 v2
vo R f
R1 R2
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Example 3.1
The output of a biopotential preamplifier that measures the electro-oculogram is
an undesired dc voltage of ±5 V due to electrode half-cell potentials, with a
desired signal of ±1 V superimposed. Design a circuit that will balance the dc
voltage to zero and provide a gain of -10 for the desired signal without saturating
the op amp.
Ri
Rf
10 kW
100 kW
+10
i
+15V
5 kW
Rb
20 kW
vb
o
+
Voltage, V
i
i + b /2
0
Time
-15 V
-10
(a)
o
(b)
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Follower (buffer)
Used as a buffer, to prevent a high source resistance from being
loaded down by a low-resistance load. In other words:
it prevents drawing current from the source.
-
i
vo vi
o
+
G 1
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Noninverting Amplifier
o
i
Ri
i
Rf
10 V
Slope = (Rf + Ri )/ Ri
-10 V
10 V
i
-
i
vo
o
+
R f Ri
Ri
vi
-10 V
G
R f Ri
Ri
Rf
1
Ri
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Differential Amplifiers
v3
Differential Gain Gd
v4
vo
R4
Gd
v4 v3 R3
R3
vo
R4
Common Mode Gain Gc
For ideal op amp if the inputs are equal then the output
= 0, and the Gc =0. No differential amplifier perfectly
rejects the common-mode voltage.
Common-mode rejection ratio CMMR
Gd
CMRR
Gc
R4
R3
R4
vo
(v4 v3 )
R3
Typical values range from 100 to 10,000
Disadvantage of one-op-amp differential amplifier is its low input resistance
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Instrumentation
Amplifiers
Differential Mode Gain
v3 v4 i( R2 R1 R2 )
v1 v2 iR1
v3 v4 2 R2 R1
Gd
v1 v2
R1
2 R2 R1 R4
v2 v1
vo
R1
R3
Advantages: High input impedance, High CMRR, Variable gain
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Comparator –
No Hysteresis
v1 > v2, vo = -13 V
v1 < v2, vo = +13 V
+15
v2
-15
ui
uref
10 V
R1
-
R1
-10 V
uo
+
uo
uref
R2
ui
-10 V
If (vi+vref) > 0 then vo = -13 V
else
R1 will prevent overdriving the op-amp
vo = +13 V
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Comparator –
With Hysteresis
Reduces multiple transitions due to mV noise levels by moving the
threshold value after each transition.
uo
ui
R1
With hysteresis
10 V
-
uref
uo
R1
+
-10 V
10 V
- uref
R2
R3
ui
-10 V
Width of the Hysteresis = 4VR3
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Rectifier
o
10 V
-10 V
R
xR
(1-x)R
D1
i
D2
-10 V
-
i
+
(b)
R
D4
-
10 V
D3
o=
i
xR
x
(1-x)R
-
+
i
(a)
D2
vo
+
(a)
Full-wave precision rectifier:
a) For i > 0,
D2 and D3 conduct, whereas D1 and D4 are reverse-biased.
Noninverting amplifier at the top is active
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Rectifier
xRi
i
R
xR (1-x)R
D1
-
D2
+
D4
(a)
o
10 V
R
D4
-
vo
+
-
i
R
D3
o=
i
-10 V
10 V
i
x
-10 V
+
(a)
(b)
Full-wave precision rectifier:
b) For i < 0,
D1 and D4 conduct, whereas D2 and D3 are reverse-biased.
Inverting amplifier at the bottom is active
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Rectifier
i
Ri = 2 kW
Rf = 1 kW
v
-
o
D
RL = 3 kW
+
(c)
One-op-amp full-wave rectifier. For i < 0, the circuit behaves like the
inverting amplifier rectifier with a gain of +0.5. For i > 0, the op amp
disconnects and the passive resistor chain yields a gain of +0.5.
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Logarithmic Amplifiers
Rf /9
Ic
VBE
i
Ri
VBE
Rf
-
o
+
IC
0.06 log
IS
vi
vo 0.06 log
13
Ri 10
(a)
Figure 3.8 (a) A logarithmic amplifier makes use of the fact that a
transistor's VBE is related to the logarithm of its collector current.
For range of Ic equal to 10-7 to 10-2 the range of vo is -.36 to -0.66 V.
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Logarithmic Amplifiers
VBE
Ic
Ri
vo
10 V
VBE
i
Rf /9
9VBE
Rf
-10 V
10 V
-
1
i
o
+
(a)
(b)
-10 V
10
Figure 3.8 (a) With the switch thrown in the alternate position, the circuit
gain is increased by 10. (b) Input-output characteristics show that the
logarithmic relation is obtained for only one polarity; 1 and 10 gains are
indicated.
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Logarithmic Amplifiers
Uses of Log Amplifier:
1. Multiply variable
2. Divide variable
3. Raise variable to a power
4. Compress large dynamic range into small ones
5. Linearize the output of devices
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1
vo
Ri C f
t1
v dt v
i
Integrators
ic
0
Zf
Vo ( j )
Vi ( j )
Zi
Rf
Vo j
Vi j Ri jR f Ri C
A large resistor Rf is used to prevent saturation
vo R f
vi
Ri
for f < fc
1
fc
2R f C f
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Integrators
Figure 3.9 A three-mode integrator With S1 open and S2 closed, the dc circuit
behaves as an inverting amplifier. Thus o = ic and o can be set to any desired
initial conduction. With S1 closed and S2 open, the circuit integrates. With both
switches open, the circuit holds o constant, making possible a leisurely readout.
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The output of the piezoelectric sensor may be fed
directly into the negative input of the integrator as
shown below. Analyze the circuit of this charge
amplifier and discuss its advantages.
Example 3.2
is
R
C
isC = isR = 0
vo = -vc
-
dqs/ dt = is = K dx/dt
uo
isC
isR
+
FET
Piezo-electric
sensor
1
vo
C
t1
0
Kdx
Kx
dt
dt
C
Long cables may be used without changing sensor sensitivity or time constant.
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Differentiators
dvi
vo RC
dt
Zf
Vo ( j )
jRC
Vi ( j )
Zi
Figure 3.11 A differentiator The dashed lines indicate that a small capacitor
must usually be added across the feedback resistor to prevent oscillation.
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Active Filters- Low-Pass Filter
Vo j
1
Gain = G =
Vi j
Ri 1 jR f C f
Cf
Rf
|G|
ui
Ri
-
Rf
+
uo
(a)
Rf/Ri
0.707 Rf/Ri
MMA
fc = 1/2RiCf
freq
Active filters
(a) A low-pass filter attenuates high frequencies
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Active Filters (High-Pass Filter)
Gain = G =
Vo j R f jRi Ci
Vi j
Ri 1 jRi Ci
ui
Ci
Ri
-
Rf
uo
+
(b)
|G|
Rf/Ri
0.707 Rf/Ri
MMA
fc = 1/2RiCf
freq
Active filters
(b) A high-pass filter attenuates low frequencies and blocks dc.
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Cf
Active Filters (Band-Pass Filter)
jR f Ci
Vo j
Vi j 1 jR f C f 1 jRi Ci
|G|
Ci
ui
Ri
-
Rf
+
uo
(c)
Rf/Ri
0.707 Rf/Ri
MMA
fcL = 1/2RiCi
fcH = 1/2RfCf
Active filters
(c) A bandpass filter attenuates both low and high frequencies.
freq
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Frequency Response of op-amp and Amplifier
Open-Loop Gain
Compensation
Closed-Loop Gain
Loop Gain
Gain Bandwidth Product
Slew Rate
Offset voltage
Bias current
Noise
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Input and Output Resistance
-
Rd ud
ii
ui
+
Ro
+
-
vi
Rai
( A 1) Rd
ii
Typical value of Rd = 2 to 20 M
Aud
uo
io
RL
CL
vo
Ro
Rao
io
A 1
Typical value of Ro = 40
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Chapter 3 - Webster
Operational Amplifiers
and Signal Processing
Wikipedia:
Operational
Amplifier
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