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Biomedical Electronics & Bioinstrumentation

Biomedical Electronics & Bioinstrumentation. Introduction to Amplifiers. Contents. Introduction to Biopotential Amplifier Principles of Differential Amplifier Fundamental of Instrumentation Amplifier. Biopotential Amplifiers.

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Biomedical Electronics & Bioinstrumentation

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  1. Biomedical Electronics & Bioinstrumentation Introduction to Amplifiers

  2. Contents • Introduction to Biopotential Amplifier • Principles of Differential Amplifier • Fundamental of Instrumentation Amplifier

  3. Biopotential Amplifiers • Biopotential amplifier is a term given to amplifiers used to process biopotential signals. • The designation applies to a large number of different types of amplifier. • Gain can be low, medium or high (x10, x100, x1000, x10000). • Some are ac-coupled, while some are dc-coupled.

  4. Biopotential Amplifiers • DC-coupling required where input signals are clearly DC or changes very slowly. • At frequencies as low as 0.05Hz, ac-coupling should be used instead of dc-coupling. • This is to overcome the electrode offset potential. • Also, the skin-electrode interface generates DC offsets.

  5. Biopotential Amplifiers • Low-Gain Amplifiers • Gain factors x1 and x10. • The unity-gain amplifier is mainly for isolation, buffering and possibly impedance transformation between signal source and readout device. • Used for measurement of action potentials and other relatively high-amplitude bioelectric events.

  6. Biopotential Amplifiers • Medium-Gain Amplifiers • Gain factors x100 and x1000. • Used for recording of ECG, EMG, etc. • High-Gain Amplifiers • Gain factors over x1000. • Used in very sensitive measurement such as EEG.

  7. Amplifier’s Desired Properties • Single-ended output, often differential input. • High CMRR. • Variable gain adequate for intended functions. • Frequency response suitable for applications. • Zero suppression.

  8. Differential Amplifiers • A differential amplifier produces an output voltage that is proportional to the difference between the voltage applied to the two input terminals.

  9. Differential Amplifiers R1 R2 V1 - VOut + R3 R4 V2

  10. Differential Amplifiers • The voltage gain for the differential signals is the same as for the inverting followers, provided the ratio equality of R2/R1 = R4/R3 is maintained. • Differential amplifiers are useful because it rejects common voltages while amplifying the differential signal of interest. • Example: Suppose equal 50Hz noise is present on each input, and one input is at 5Vdc while the other is at 2Vdc. The circuit removes the noise and amplifies the 3Vdc differential signal.

  11. Instrumentation Amplifiers • A solution to both high-gain and high-input impedance problem. • Uses 3 operational amplifiers. • Two input amplifier is connected in the non-inverting follower configuration. • 3rd amplifier is connected as a simple dc amplifier circuit.

  12. Instrumentation Amplifiers R2 R3 V1 + A1 - R1 - A3 VOut RG + R1 R2 R3 - A2 V2 +

  13. Instrumentation Amplifiers • VOut is given by: • The gain:

  14. Instrumentation Amplifiers • Advantages of using instrumentation amplifiers. • Ability to obtain high gain with low resistor values. • Extremely high input impedance. • Superior rejection of common-mode signals.

  15. Further Reading… • Carr, J.J. (2000). Introduction to Biomedical Equipment Technology. 4th Ed. Prentice Hall. • Chapter 7 • Floyd, T.L. (2008). Electronic Devices. 8th Ed., Prentice Hall. • Chapter 14 & 15

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