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Ch3 Amplifier Basics. p. 1ECE 445: Biomedical Instrumentation
Amplifiers and Analog Signal Processing
Most bioelectric signals are small voltages in micro-volts range
currents in pA and nA range common
Small signals require amplification and filtering op-amp, resistors and capacitors
integrated circuit and surface-mount technology
Most modern signal processing tasks (filtering) are performedon a digital signal processor.
little change in amplification/filtering requirements over last 40 years but new interest in putting DSP algorithms into analog circuits
due to demand for low power portable/implantable instruments
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Ch3 Amplifier Basics. p. 2ECE 445: Biomedical Instrumentation
Ideal Op-Amp
Operational amplifier (op-amp) is a high-DC-gain differentialamplifier
Design circuits assuming op-amps are ideal then verify/modify using simulations/prototyping
Ideal op-amp modelopen loop gain: A = differential input resistance: Rd = output resistance: Ro = 0
input current = 0
output voltage: vo = 0 when v1-v2 = 0
0
0
o
d
o
RR
v
A
ideal op-amp small signal model
ideal op-amp
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Ch3 Amplifier Basics. p. 3ECE 445: Biomedical Instrumentation
Op-Amp Properties
Properties open-loop gain: ideally infinite: practical values 20k-200k
high open-loop gain
virtual short between + and - inputs
input impedance: ideally infinite: CMOS opamps are close to ideal
output impedance: ideally zero: practical values 20-100 zero output offset: ideally zero: practical value
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Ch3 Amplifier Basics. p. 4ECE 445: Biomedical Instrumentation
Basic Op-Amp Principles
Open loop gain: vo = A (v1-v2)
since A is very large, v1-v2 must be very small
When the op-amp output is in its linear range two input terminals are at (essentially) the same voltage
i.e., virtual ground between op-amp inputs rely on this for DC/bias calculations
Single vs. Dual Supply Voltage most modern ICs use single supply
ground in a dual supply becomes VDD/2 in single supply mid way between VDD and Ground
typical op-amp schematic symbol
vo, v1, v2 referenced to ground
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Ch3 Amplifier Basics. p. 5ECE 445: Biomedical Instrumentation
Basic Opamp Configuration
Voltage Comparator digitize input
assumes very high DC gain
Vcc = supply voltage
Negative Feedback output tied back into negative input
terminal generally avoid positive feedback
Voltage Follower
buffer prevents input signal from being
loaded down by a low-resistanceload
Rin =
Vref
Vout = Vcc (sign(Vin-Vref))
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6/19Ch3 Amplifier Basics. p. 6ECE 445: Biomedical Instrumentation
Inverting/Non-Inverting Configurations
Inverting Amplifier (uses negative feedback)
Non-Inverting Amplifier (also uses negative feedback)
i
f
i
o
R
R
v
vA
f
fi
i
f
i
o
R
RR
R
R
v
vA
1
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7/19Ch3 Amplifier Basics. p. 7ECE 445: Biomedical Instrumentation
More Opamp Configurations
Summing Amp weighted sum of
multiple inputs inverting or non??
Differential Amp match R1s and R2s inverting or non??
Single-Ended Amplifier Representation
Noise Amplification
even small interference at input gets amplified at output
inV outV
gnd gnd in
outv
VVA
signalnoise
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8/19Ch3 Amplifier Basics. p. 8ECE 445: Biomedical Instrumentation
Differential vs. Common Mode Signal
Define x+ = input at + terminal
x- = input at terminal
c = common mode signal on both inputs Differential inputs
Add common mode input crejected by differential amplifier (not amplified)
cmust be small enough to keep op-amp biased in linear operation
xxVout
)()( cxcxVout
x
x
2
xx
c
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9/19Ch3 Amplifier Basics. p. 9ECE 445: Biomedical Instrumentation
Noise in Differential Amplifiers
Global interference (e.g., supply voltage variations) assumed to be located far away from amp. input terminals
same interference on both the terminals
appear as common mode disturbance.
example: clock noise
Differential amplifiers amplify only the difference
reject the interference (common-mode)
+
-
-
+
inV
inV
outV
outV
common-mode
input noise
gone at
output
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10/19Ch3 Amplifier Basics. p. 10ECE 445: Biomedical Instrumentation
Desirable Properties of Amplifiers
High differential gain, Av
Low common mode gain, Acm= high common mode rejection
+
-
-
+
inV
inV outV
outV
inin
outoutv
VV
VVA
2
inin
outout
CMVV
VV
A2
inin VVCommon-mode signal
+
-
-
+
inV
inV
outV
outV
comm on mode rej ect ion ratio:
cm
v
CMRR A
A
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3-Op-Amp Instrumentation Amplifier
Differential amplifiers low common mode gain = Great!
lower than ideal input resistance Bad!
3-op-amp structure keeps low common mode gain
provides very high input resistance
why? call instrumentation amp
will discuss in detail later
1
122R
RRA
1comA
3
4
1
12d
2
R
R
R
RRG
t otal differential gain
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Comparator
Compare an input voltage vi to a reference voltage vref Output digital value (hi/low)
low if vi > vref why low and not hi?
high if vi < vref Output voltage = supply voltage
Op-amp comparator
Add hysteresis to improve noise immunity hysteresis = rising transition point different that falling transition point
R3 controls hysteresis
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Ch3 Amplifier Basics. p. 13ECE 445: Biomedical Instrumentation
Logarithmic Amplifiers
Uses non-linear current-voltage relationship of BJT in feedbackpath
Useful for computing logarithms and anti-logs for compressing and multiplying/dividing signals
S
CBE
I
IkV log
A=1
A=10
A=10
A=1
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Ch3 Amplifier Basics. p. 14ECE 445: Biomedical Instrumentation
Integrating/Differentiating Configurations
Integrating Amp
Differentiating Amp
t
o
dtiC
v1
f 2
dt
dvCi
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Ch3 Amplifier Basics. p. 15ECE 445: Biomedical Instrumentation
Converting Configuration
Current-to-Voltage
Voltage-to-Current
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Active Filters
Passive low pass filter
Active low pass filter
If Z1 is a resistor (R) and Z2 is a capacitor (1/sC) then
ffi
f
i
o
CRjR
R
jV
jV
1
1
)(
)(
ffCR1
0
0i
f
iff
f
i
ff
ff
i
f
i
o
11
)(1
])/1[()/(
)(
)(
sR
R
RCRj
R
R
RCjCjR
Z
Z
jV
jV
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Ch3 Amplifier Basics. p. 17ECE 445: Biomedical Instrumentation
Active Filters
Active high pass filter
ii
ii
i
f
i
o
CRj
CRj
R
R
jV
jV
1)(
)(
iiCR1
0
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Ch3 Amplifier Basics. p. 18ECE 445: Biomedical Instrumentation
Active Filters
)1)(1()(
)(
iiff
if
i
f
i
o
CRjCRj
CRj
R
R
jV
jV
Band Pass Filter
High Q (narrow frequency) Band Pass Filter
2-stage Band Pass Filter
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Ch3 Amplifier Basics. p. 19ECE 445: Biomedical Instrumentation
Non-ideal Characteristics
Offset voltage output not zero when the inputs to the amplifiers are equal
could be in order of millivolts
cancel offset voltage by adding an external nulling potentiometer
Temperature Drift offset voltage can drift by 0.1 microvolts over one degree variation
Finite (lower than infinite) input impedance can cause errors at input
High output impedance
limits load driving capabilities
Noise Thermal noise or high-frequency noise
Flicker noise: low-frequency noise
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