1. BME 311: BIOMEDICAL INSTRUMENTATION I Lecturer: Ali Işın
FACULTY OF ENGINEERING
DEPARTMENT OF BIOMEDICAL ENGINEERING
BME 311: BIOMEDICAL INSTRUMENTATION I Lecturer: Ali Işın
Lecture Note 2: Amplifiers and Signal Processing for Biomedical Instrumentation
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
Copyright 2014 A. IŞIN. All rights reserved.

2. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
Applications of Operational Amplifier In Biological Signals and Systems
The three major operations done on biological signals using Op-Amp:
Amplifications and Attenuations
DC offsetting:
add or subtract a DC
Filtering:
Shape signal’s frequency content
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

3. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
Ideal Op-Amp
Most bioelectric signals are small and require amplifications
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.
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

4. Inside the Op-Amp (IC-chip)
20 transistors 11 resistors 1 capacitor
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

5. Ideal Characteristics
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
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

6. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
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.
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

7. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
Inverting Amplifier
10 V
(b) i o
Slope = -Rf / Ri
-10 V Ri i o i Rf + -
(a)
An inverting amplified. Current flowing through the input resistor Ri also flows through the feedback resistor Rf .
The input-output plot shows a slope of -Rf / Ri in the central portion, but the output saturates at about ±13 V.
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

8. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
Summing Amplifier 1 o - + R2 R1 Rf 2
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

9. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
Example 2.1
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.
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

10. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
Answer 2.1
We assume that vb, the balancing voltage at vi=5 V. For vo=0, the current through Rf is zero. Therefore the sum of the currents through Ri and Rb, is zero. i v b o - +
+15V
+10
Time
i + b /2
-10
(a)
(b)
5 kW
-15 V Rb
20 kW Ri
10 kW Rf
100 kW
Voltage, V
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

11. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
Follower ( buffer)
Used as a buffer, to prevent a high source resistance from being loaded down by a low-resistance load. In another word it prevents drawing current from the source. o i + -
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

12. Noninverting Amplifier
Slope = (Rf + Ri )/ Ri
-10 V Rf o i i + - Ri
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

13. Differential Amplifiers
Differential Gain Gd
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
Typical values range from 100 to 10,000
Disadvantage of one-op-amp differential amplifier is its low input resistance v3 v4
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

14. Instrumentation Amplifiers
Differential Mode Gain
Advantages: High input impedance, High CMRR, Variable gain
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

15. Comparator – No Hysteresisv2
+15
-15
v1 > v2, vo = -13 V
v1 < v2, vo = +13 V uo ui
uref
10 V
-10 V ui uo - + R1 R2
uref
If (vi+vref) > 0 then vo = -13 V else vo = +13 V
R1 will prevent overdriving the op-amp
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

16. Comparator – With Hysteresis
Reduces multiple transitions due to mV noise levels by moving the threshold value after each transition.
Width of the Hysteresis = 4VR3 uo ui
- uref
10 V
-10 V
With hysteresis ui uo - + R1 R2 R3
uref
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

17. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
Rectifier - +
(a) D3 R i D2 D1 D4 xR
(1-x)R
Full-wave precision rectifier:
For i > 0, D2 and D3 conduct, whereas D1 and D4 are reverse-biased. Noninverting amplifier at the top is active
10 V
(b)
-10 V o i
(a) D2 v o i - + xR
(1-x)R
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

18. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
Rectifier - +
(a) D3 R i D2 D1 D4 xR
(1-x)R
Full-wave precision rectifier:
For i < 0, D1 and D4 conduct, whereas D2 and D3 are reverse-biased. Inverting amplifier at the bottom is active
10 V
(b)
-10 V o i
(b) D4 v o i - +
xRi R
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

19. One-Op-Amp Full Wave Rectifier
For i < 0, the circuit behaves like the inverting amplifier rectifier with a gain of For i > 0, the op amp disconnects and the passive resistor chain yields a gain of +0.5.
(c) D v o i - +
Ri = 2 kW
Rf = 1 kW
RL = 3 kW
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

20. Logarithmic Amplifiers
Uses of Log Amplifier
Multiply and divide variables
Raise variable to a power
Compress large dynamic range into small ones
Linearize the output of devices
(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 10-7 to 10-2 and the range of vo is -.36 to V.
(a) Rf Ic
Rf /9 o Ri i - +
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

21. Logarithmic AmplifiersRf Ic
Rf /9 o Ri i - +
VBE
9VBE
(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.
(b)
10 V
-10 V v o i 1
10
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

22. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
Integrators
for f < fc
A large resistor Rf is used to prevent saturation
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

23. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
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.
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

24. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
Differentiators
A differentiator
The dashed lines indicate that a small capacitor must usually be added across the feedback resistor to prevent oscillation.
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

25. Active Filters- Low-Pass Filter
A low-pass filter attenuates high frequencies + - Ri Rf
(a) ui uo
|G|
freq
fc = 1/2RiCf
Rf/Ri
0.707 Rf/Ri
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

26. Active Filters (High-Pass Filter)
A high-pass filter attenuates low frequencies and blocks dc. Ci + - Ri ui uo
(b) Rf
|G|
freq
fc = 1/2RiCf
Rf/Ri
0.707 Rf/Ri
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

27. Active Filters (Band-Pass Filter)
A bandpass filter attenuates both low and high frequencies. + - ui uo
(c) Rf Ci Ri Cf
|G|
freq
fcL = 1/2RiCi
Rf/Ri
0.707 Rf/Ri
fcH = 1/2RfCf
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

28. Phase Modulator for Linear variable differential transformer LVDT+ -
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

29. Phase Modulator for Linear variable differential transformer LVDT+ -
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

30. Phase-Sensitive Demodulator
Used in many medical instruments for signal detection, averaging, and Noise rejection
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014

31. BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014
The Ring Demodulator
If vc is positive then D1 and D2 are forward-biased and vA = vB. So vo = vDB
If vc is negative then D3 and D4 are forward-biased and vA = vc. So vo = vDC
BME 311 LECTURE NOTE 2 - ALİ IŞIN, 2014