Biomedical Instrumentation Prof Dr Nizamettin AYDIN naydinyildiz edu
Biomedical Instrumentation Prof. Dr. Nizamettin AYDIN naydin@yildiz. edu. tr naydin@ieee. org http: //www. yildiz. edu. tr/~naydin 1
Amplifiers and Signal Processing
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 3
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. 4
Inside the Op-Amp (IC-chip) 20 transistors 11 resistors 1 capacitor 5
Ideal Characteristics • • • A = (gain is infinity) Vo = 0, when v 1 = v 2 (no offset voltage) Rd = (input impedance is infinity) Ro = 0 (output impedance is zero) Bandwidth = (no frequency response limitations) and no phase shift 6
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. 7
Inverting Amplifier o i Rf 10 V i i Ri -10 V - 10 V o i + Slope = -Rf / Ri (a) -10 V (b) (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. 8
Summing Amplifier Rf R 1 1 - R 2 2 o + 9
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. 10
Answer 3. 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. +10 Rf 100 k Ri 10 k i +15 V 5 k Rb 20 k vb o + Voltage, V i i + b /2 0 Time -15 V -10 (a) (b) o 11
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. - i + o 12
Noninverting Amplifier o i Ri i Rf 10 V Slope = (Rf + Ri )/ Ri -10 V i - i + o -10 V 13
Differential Amplifiers • Differential Gain Gd v 3 v 4 • 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 14
Instrumentation Amplifiers Differential Mode Gain Advantages: High input impedance, High CMRR, Variable gain 15
Comparator – No Hysteresis +15 v 1 > v 2, vo = -13 V v 1 < v 2, vo = +13 V v 2 -15 o i ref 10 V R 1 - -10 V o R 1 + ref R 2 -10 V If (vi+vref) > 0 then vo = -13 V else R 1 will prevent overdriving the op-amp vo = +13 V i
Comparator – With Hysteresis • Reduces multiple transitions due to m. V noise levels by moving the threshold value after each transition. o i ref R 1 With hysteresis 10 V - o R 1 + -10 V - ref R 2 R 3 i -10 V Width of the Hysteresis = 4 VR 3 17
Rectifier R x. R (1 -x)R D 1 D 2 o 10 V -10 V i + i R D 4 - 10 V -10 V D 3 (b) + (a) x. R • Full-wave precision rectifier: – For i > 0, D 2 and D 3 conduct, whereas D 1 and D 4 are reverse-biased. Noninverting amplifier at the top is active (1 -x)R - i D 2 vo + (a) 18
Rectifier R x. R (1 -x)R D 1 D 2 o 10 V -10 V i + i R D 4 - 10 V -10 V D 3 (b) + (a) • Full-wave precision rectifier: – For i < 0, D 1 and D 4 conduct, whereas D 2 and D 3 are reverse-biased. Inverting amplifier at the bottom is active x. Ri R i - D 4 vo + (b) 19
One-Op-Amp Full Wave Rectifier i Ri = 2 k Rf = 1 k v - o D RL = 3 k + (c) • 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. 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 Rf /9 Ic Rf i Ri - o + (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 -0. 66 V. 21
Logarithmic Amplifiers VBE Ic Ri vo 10 V VBE i Rf /9 9 VBE Rf -10 V - 1 i o + (a) (b) -10 V 10 (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. 22
Integrators for f < fc A large resistor Rf is used to prevent saturation 23
• A three-mode integrator With S 1 open and S 2 closed, the dc circuit behaves as an inverting amplifier. Thus o = ic and o can be set to any desired initial conduction. With S 1 closed and S 2 open, the circuit integrates. With both switches open, the circuit holds o constant, making possible a leisurely readout. 24
Differentiators • A differentiator – The dashed lines indicate that a small capacitor must usually be added across the feedback resistor to prevent oscillation. 25
Active Filters- Low-Pass Filter • A low-pass filter attenuates high frequencies i |G| Ri - Rf + o (a) Rf/Ri 0. 707 Rf/Ri fc = 1/2 Ri. Cf freq 26
Active Filters (High-Pass Filter) • A high-pass filter attenuates low frequencies and blocks dc. i Ci Ri - Rf + o (b) |G| Rf/Ri 0. 707 Rf/Ri fc = 1/2 Ri. Cf freq 27
Active Filters (Band-Pass Filter) • A bandpass filter attenuates both low and high frequencies. C f i Ci R i - Rf + |G| o (c) Rf/Ri 0. 707 Rf/Ri fc. L = 1/2 Ri. Ci fc. H = 1/2 Rf. Cf freq 28
Frequency Response of op-amp and Amplifier • • • Open-Loop Gain Compensation Closed-Loop Gain Bandwidth Product Slew Rate 29
Input and Output Resistance Rd ii i + d o Ro + - A d Typical value of Rd = 2 to 20 M io RL CL Typical value of Ro = 40 30
Phase Modulator for Linear variable differential transformer LVDT + + - 31
Phase Modulator for Linear variable differential transformer LVDT + + - 32
Phase-Sensitive Demodulator Used in many medical instruments for signal detection, averaging, and Noise rejection 33
The Ring Demodulator • If vc is positive then D 1 and D 2 are forward-biased and v. A = v. B. So vo = v. DB • If vc is negative then D 3 and D 4 are forward-biased and v. A = vc. So vo = v. DC 34
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