chapter 26 bioelectronics and...

Physics Including Human Applications

Chapter 26 Bioelectronics and Instrumentation

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Chapter 26 BIOELECTRONICS AND INSTRUMENTATION

GOALS When you have mastered the content of this chapter, you will be able to achieve the following goals:

Definitions

Define each of the following terms and use each term in an operational definition:

feedback interference noise

linearity signal-to-noise ratio

amplification stability

frequency response

Electronic Devices

Explain the basis of operation and potential use for each of the following:

diode rectifier differential amplifier

transistor amplifier oscilloscope

operational amplifier

Oscilloscopes

Evaluate the specifications provided for commercially produced oscilloscopes

PREREQUISITES Before beginning this chapter you should have achieved the goals of Chapter 21, Electrical Properties of Matter, Chapter 22, Basic Electrical Measurements, Chapter 23, Magnetism, and Chapter 25, Alternating Currents.



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Chapter 26 BIOELECTRONICS AND INSTRUMENTATION

26.1 Introduction Many of the advances that have been made in the life sciences in recent years are the result of the use of electronics in modern bioinstrumentation. Electronic devices called transducers can convert forms of energy that are not detectable by human senses into easily detected and recorded information. Electronic amplifiers make possible the study of heart potentials, muscle potentials, nerve action potentials, and brain waves in physiology laboratories. It is not necessary to be able to design your own instruments, but it is important to understand the basis of operation of modern electronic instrumentation so that you can use it intelligently. It is helpful to know the limitations of your instruments and thereby make sure that you use them correctly. As you improve your understanding of the basic operation of electronics, you will be able to increase your use of electronic instrumentation and you will discover many different approaches to the study of life science problems.

The advent of solid state electronics has greatly improved instrumentation. Technology has developed the compact integrated circuits (IC) which incorporate many components into complete, single-unit, complex circuits. The IC is the building block of modern electronic instrumentation. The discrete elements such asdiodes and transistors may still be used, but if the circuit has wide enough applicability, it is likely to be produced as an IC.

26.2 Instrumentation Characteristics Assume we are interested in studying the response of a human eye to light flashes (see Figure 26.1). The stimulus in this case is a light, and the transducers might be a television camera monitoring the subject's eyes and a microphone monitoring the subject's heart rate. The feedback control can provide random light flashes with possible variations in light intensity and color.



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In more sophisticated systems the feedback control and recording parts of the system can be handled by a minicomputer (or any other versatile electronic control system). The signal processing would include matching the impedance of the transducer to the impedance of the amplifier as well as shaping of the signal to assure linearity of output signal with input signal. By linearity we mean that the output signal is a linear function of the input signal:

output signal = (constant1)(input signal) + constant2 (26.1)

The linearity criteria is very important if quantitative relationships are sought between input and output signals.

In many cases the signal we obtain from our experimental system is small. In fact, it may be too small to measure with our recording instrument. Our signal must be made larger, or amplified, before we record it. We can use an electronic instrument called an amplifier for this purpose. The ratio of the output signal to input signal is called the amplification factor. We can choose to amplify the current, voltage, or power of our experimental signal.

Considerable effort is spent in making linear amplifiers for wide ranges of frequency responses and amplitudes of input signals. Again the matching of the transducer with the amplifier is important in order to maximize the measuring capability of the system. The frequency response of either the transducer or the amplifier refers to the range of frequencies to which the system responds without distortion.

In all present-day bioscience research laboratories there are a large number and variety of electronic devices. The air is literally full of weak electromagnetic waves generated as a by- product of electronic instrumentation. These electromagnetic waves can cause random signals which interfere with our detection systems. This random signal interference is called interference noise. The interference noise is due to coupling of environmental energy sources into the experimental system.

One of the important sources of interference noise is the 60-Hz noise picked up by electromagnetic induction from all of the AC equipment in the laboratory. Shielded coaxial cable should be used, and in some cases it might be necessary to isolate the subject in a shielded screen room in order to minimize the noise picked up by the electrodes attached to the subject. Interference noise must be reduced before amplification because the amplifier will amplify the noise along with the signal under study.

One way to characterize the quality of our experimental system is to compute the ratio of the average amplitude of the signal we are measuring to the average amplitude of the interference noise. We call this the signal-to-noise ratio for the system, and it should be greater than 10. In life science applications the signals are usually so small that interference noise must be kept at a minimum in order to maintain a signal-to-noise ratio that allows adequate measurements.

Finally, it is desirable to have the system as stable as possible. The stability of the electronics is a measure of the system's ability to return to equilibrium after an input disturbance. The stability of a system can often be improved by taking a small portion



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of the output signal, changing its polarity and feeding it back into the input side of the system. This is called negative feedback (Section 2.6) or feedback control.

26.3 Diodes Modern solid state electronics is based on the physical properties of semiconductor materials such as germanium and silicon. Devices such as thermistors, diodes, transistors, solar cells, and integrated circuits are examples of semiconductor devices. Solid state electronics is based on the ability to vary the conductivity of the semiconductor materials by varying the number of current carriers per unit volume. Besides electrons (in n-type semiconductors), it is possible to make semiconductors with holes, or positive charge carriers (in p-type semiconductors).

Semiconductor diodes are made by joining a n-type and a p-type semiconductor. When the p-type region has a higher potential than the n-type region, the current is maintained as electrons fall into holes at the junction with an equal number of holes being created as electrons are pulled from the p-type region. This configuration is a forward biased diode that has low impedance to current. When the polarity is reversed, the electrons are pulled out of the n-type region and the holes are pulled out of the p-type region, leaving the junction void of current carriers. This situation corresponds to very high impedance to current and this is the reversed biased diode that blocks current flow. (See Figure 26.2 for the current characteristics of a diode.) Thus the diode can function as an on-off current switch which is controlled by the polarity of the voltage applied to the diode.

When a current source of alternating polarity is connected to a diode as shown in Figure 26.3, the result is an output voltage of a single polarity corresponding to the forward biased polarity of the diode. The diode is said to have rectified the current. Such a circuit is called a rectifier circuit.



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Notice that the arrowhead part of the symbol for a diode points in the direction for positive current. The primary use of diodes is as rectifiers. Another use of semiconductor diodes in life science research is that of temperature transducer. The voltage drop across a forward biased diode is quite sensitive to temperature. This sensitivity is linear, being about -2.5 mV/°C for silicon diodes. Two circuits for diode temperature transducers are illustrated in Figure 26.4. The best results are obtained with very small diodes which have a small thermal inertia and with a diode current of approximately 100 mA

26.4 Transistors The transistor is a three terminal semiconductor (silicon or germanium) device. The three terminals are called emitter, base, and collector, and transistors are designated as either pnp or npn types. This notation refers to the nature of the majority carriers in the emitter, base, and collector respectively. The p represents a majority of positive carriers (holes) and the n represents a majority of negative carriers (electrons). In the schematic representation of a transistor an arrow is shown at the emitter to indicate the direction of positive current Figure 26.5.

In a circuit with two input wires and two output wires, a three- terminal device such

as a transistor can be installed in three different ways.



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The three different transistor amplifier configurations with their characteristics are shown in Figure 26.6. In each case, a small current injected to the base region results in the output current or voltage amplification. The common emitter configuration is a good compromise between power gain and output impedance and thus is the most widely used transistor circuit.

Let us consider a specific use for each of the transistor configurations shown in Figure 26.6. A thermocouple is a temperature transducer that produces a small voltage output (~ millivolt) at low output impedance. For the measurement of thermocouple output with a typical voltmeter, the common-base configuration provides an ideal amplifier to use between the thermocouple and the meter.

A photomultiplier is a light transducer that provides a small current signal at a high output impedance. It is frequently desirable to transmit this signal over a coaxial cable of low impedance (50 W) to an output device or recorder. The common-collector configuration is the ideal choice as a preamplifier at the photomultiplier tube to match the impedance of the tube with the coaxial cable for optimum signal transfer.

The photo-relay system is a typical application for the common-emitter configuration. In this case a solar cell is the light detector that is used to control a relay that in turn controls lights, counters, or some other power requiring device. The common-emitter circuit provides maximum power gain to activate the relay which controls the output device.

These are only three simple applications of the transistor circuits shown in Figure 26.6.



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26.5 Operational Amplifiers Let us consider a particular integrated circuit that is called an operational amplifier (op-amp). The op-amp is a high-gain amplifier that exhibits a frequency response from DC to at least 30 MHz. These devices are relatively low in cost and very versatile in applications. The ideal op-amp has very high input impedance, very low output impedance, and very high amplification. (The DC voltage gain is between 104 and 109 for maximum amplification.) The very high gain means that the output of the op-amp is determined by the negative feedback connection in the system Figure 26.7.

The amplification of this configuration shown in Figure 26.7 is given by the voltage ratio,

amplification = Vout /Vin = - R2 /R1 (26.2)

where Vout is the output voltage and Vin is the input voltage, and R1 and R2 are the resistances shown in the figure.

The op-amp can be used as a differential amplifier. A differential amplifier is designed to amplify the difference between two input signals. This form of amplifier is used in life science applications where it can greatly reduce the interference noise that is common to both input signals. For example, to measure an ECG you may connect an electrode to one wrist and one ankle. The two input signals are the inputs to a differential amplifier as shown in Figure 26.8, the interference noise that is common to these signals will not be amplified.



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The operational amplifier is a very versatile circuit. With proper modifications in the external circuit elements, an op-amp can be used to add, integrate, or differentiate input voltages as shown in Figure 26.9. See Footnote 1.



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26.6 The Oscilloscope The oscilloscope is designed to display voltages as a function of time. Because of its variable voltage sensitivity and wide range of response speeds, the oscilloscope is a very useful instrument in the laboratory (Figure 26.10). The central element of the instrument is the cathode-ray tube upon which the waveform of the signal is displayed. This waveform is traced by a beam of electrons writing on the phosphorescent coating of the tube face. The signal under study can be monitored visually by the experimenter or photographed for a permanent record. The position of the electron beam is controlled by electric fields applied across two pairs of deflecting plates, called the horizontal and vertical deflection plates as shown in Figure 26.10.

For most applications in the life sciences the internal time base of the oscilloscope is used. The horizontal deflection is provided by using an internal sweep generator connected internally to the horizontal amplifier. This sweep generator is a sawtooth wave that has a slope linear in time. A control switch on the scope allows the operator to select the appropriate sweep rate, denoted by time per centimeter deflection on the switch. For example, 1 msec/cm is a setting for which each centimeter of horizontal deflection corresponds to 1 millisecond. The signal to be studied as a function of time is the input to the vertical amplifier; a switch allows selection of the proper vertical sensitivity designated in volts per centimeter. A vertical sensitivity of 10 mV/cm means that each centimeter of vertical deflection corresponds to a signal amplitude of 10 millivolts. Let us consider two specific applications to illustrate the use of the oscilloscope. Suppose we wish to measure the conduction velocity of a nerve impulse along the sciatic nerve of a frog. The nerve is extracted and placed in a wet nerve cell where it rests on silver electrodes equally spaced along the chamber. A typical set up for this experiment is illustrated in Figure 26.11.



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The nerve is stimulated by a voltage pulse from a stimulator (an electronic pulse generator with variable pulse amplitude, frequency, and duration). This stimulation (if above threshold) will produce a nerve impulse that travels along the nerve. As this impulse passes the electrodes, it produces a small voltage (a few millivolts) across adjacent electrodes. This is the input signal for the vertical input (set on 10 mV/cm). In order to use the time base of the oscilloscope as an interval timer, it is necessary to start the sweep of the beam with the stimulator pulse. The time base should be set for 0.5 msec/cm, and the stimulator should be at a frequency of 50 pulses/sec. Each pulse is of 0.1 msec in duration. The amplitude of the stimulator pulse is increased until a signal is noted, that is, until a vertical deflection is noted on the oscilloscope tube when the impulse passes the electrodes connected to vertical input. The horizontal distance from the start of the sweep to the vertical impulse can be converted to the time it took the impulse to travel from the stimulator electrode to the vertical pickup electrode, that is, distance (cm) x 0.5 msec/cm. The distance between the stimulator electrode and vertical pickup electrode divided by this time gives the speed of the nerve impulse between these two points. In this example it was necessary to start the sweep with an external signal. This flexibility of the trigger source for the sweep of the oscilloscope adds much to its versatility.

As another example of oscilloscope use, consider the study of the heart potentials (ECG). In this case we are studying a waveform (ECG potential) that is periodic, and we wish to measure the period and observe the shape of the waveform. In this application it is necessary to trigger the scope on one of the voltage pulses that serves as the vertical input. The set-up that might be used is shown in Figure 26.12.

The synchronization of the sweep with the periodic input signal is achieved by switching the trigger selection switch to internal, which means trigger voltage will be provided by internal vertical amplifier of the oscilloscope shown in Figure 26.10. The trigger level can be adjusted to start the horizontal beam sweep at the initiation of a vertical signal. The time between heart beats can be determined by reading the distance between waveforms along the horizontal axis and multiplying this distance in centimeters by the time base setting. The vertical sensitivity should be 500 mV/cm.

Finally, consider the evaluation of a set of specifications provided by a manufacturer of oscilloscopes:

"Vertical bandwidth of DC to 15 MHz." This means that the vertical amplifier will faithfully amplify signals whose frequency may range from dc to 15 MHz.

"Vertical sensitivity of 10 mV/cm to 50 V/cm in twelve calibrated positions." This gives the range of input signals that can be studied on the cathode-ray tube screen. This screen is typically 6 cm vertically by 10 cm horizontally.

"Vertical input impedance of 1 meg ohm." This means the effective impedance across the input terminals is 1,000,000 W, and thus the oscilloscope is a good voltage measuring device.

"May have as many as 22 time bases from 2 sec/cm to 0.2 msec/cm." This gives the range of horizontal time bases available.



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"Trigger sensitivity of internal, 1 cm display, external, 0.5 V peak to peak." This information tells you that on internal trigger a vertical signal of 1 cm on any vertical sensitivity will trigger the time base sweep and that on the external trigger a voltage pulse of 0.5 V is needed to trigger the sweep.

These are the most important specifications to consider when selecting an oscilloscope, and they should be matched with the demands of your work.

Questions

1. For a heart rate of 72 beats/min, what would be a good time base setting?

SUMMARY Use these questions to evaluate how well you have achieved the goals of this chapter. The answers to these questions are given at the end of this summary with the number of the section where you can find related content material.

Definitions

1. Suppose you are trying to measure a small-amplitude, short-time pulse in a biological system. Fill in the following blanks. The sample will be placed in a copper cage, grounded to a water pipe, to reduce the a. _________and thereby increase the b.___________ before sending the signal into an electronic device for c.___________ . The wide d.___________ of the detector is desirable so that the pulse shape will not be changed. to make quantitative measurements of the amplitude of the pulse the e.____________ of the system must be calibrated. To make consistent repeated measurements, the f.____________ of the system will be improved by using a negative g._________ control feature.

Electronic Devices

2. List two uses of a diode.

3. What kind of circuit element is a forward-biased diode?

4. What kind of circuit element is a reversed-biased diode? 5. Match the following characteristics to these transistor configurations: common base,

common collector, common emitter a. low input impedance b. high input impedance c. low output impedance d. high output impedance e. high power gain f. low power gain

g. high voltage gain h. no voltage gain i. high current gain j. no current gain k. moderate output impedance l. moderate input impedance

6. List the ideal characteristics of an operational amplifier.

7. List four ways an op-amp can be used to process signals.



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8. What does a differential amplifier amplify?

9. What is displayed on an oscilloscope tube?

Oscilloscopes

10. List at least four characteristics that are specified for an oscilloscope.

Answers 1. a. interference noise b. signal- to-noise ratio c. amplification d. frequency response e. linearity f. stability g. feedback (Section 26.2) 2. rectifier, temperature transducer (Section

26.3) 3. low impedance, high current flow (Section

26.3) 4. high impedance almost no current flow

(Section 26.3) 5. common base: f, g, j, a, d; common collector:

f, h, i, b, c; common emitter: e, g, i, k, l (Section 26.4)

6. infinite input impedance, zero output impedance, infinite amplifications (Section 26.5)

7. to amplify signals, to amplify the difference between two signals, to add signals, to integrate signals, to differentiate signals (Section 26.5)

8. the difference between two signals (Section 26.5)

9. the voltage versus time curve for a signal (Section 26.6)

10. bandwidth, voltage sensitivity, time base range, input impedance, trigger sensitivity (Section 26.6)

ALGORITHMIC PROBLEMS Listed below are the important equations from this chapter. The problems following the equations will help you learn to translate words into equations and to solve single-concept problems. Equations output signal = (constant1)(input signal) + constant2 (26.1) amplification = Vout /Vin (26.2) Problems 1. A linear electronic device gave output voltages of 25 V and 75 V for input voltages of

5 V and 10 V respectively. What are the values of the linearity constants for this device?

2. A common-base transistor amplifier is reported to have an amplification factor of 50. What input voltage is needed to obtain a 1-V output signal?

Answers 1. constant1 = 10, constant2 = -25 V 2. 0.02 V



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EXERCISES These exercises are designed to help you apply the ideas of a section to physical situations. Where appropriate, the quantitative answer is given in brackets at the end of the exercise.

Section 26.3

1. What can you conclude about the ohmic nature of a diode? (Compare a I vs V plot of an ohmic resistor with Figure 26.2.

2. A thermocouple is a linear temperature transducer that generates an emf of about 1 mV when the reference junction is at 0°C, and the probing junction is at room temperature (27°C). Compare the sensitivity of a diode thermometer with a thermocouple. [a diode is about 60 times more sensitive]

Section 26.4

3. Describe which of the transistor configurations you would use for each of the following situations (show a sketch of each):

a. crystal microphone (high-impedance output) to audio-amplifier through a coaxial cable

b. ECG potentials for display on scope with 10 mV/cm sensitivity

c. photocell with small current output (microamps) to be monitored with milliamp meter.

Section 26.5

4. Given a voltmeter (1 V full scale), an op-amp, resistors of 10, 100, 1,000, 10,000, and 100,000 W, and a thermocouple, you wish to monitor the thermocouple with the meter. The thermocouple output is about 1 mV, sketch the system you could use showing the resistors used in the op-amp set up [Op amp: Rf = 105Ω; Ri = 10 2 Ω]

Section 26.6

5. Given four unknowns (one each of a capacitor, inductor, resistor, and diode) and a 100-V AC and 100-Volt DC source, explain how you could determine which unknown is which by using an oscilloscope with both AC and DC amplifiers.



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PROBLEMS The following problems may involve more than one physical concept.

6. Design an experiment to measure human reaction time to either a light or sound stimulus. Show a sketch of your set up. (Be sure to show clearly trigger mode selection). Answer

7. Design a biofeedback system that might be used to condition a subject to reduce the temperature of a fingertip. Answer

8. Using diodes and op-amps, design a differential thermometer system that measures small differences between two temperatures. Answer

FOOTNOTE

1. An excellent reference for anyone interested in learning more about using operational amplifiers and other solid state devices is S. A. Hoenig and F. L. Payne, How to Build and Use Electronic Devices Without Frustration, Panic, and Mountains of Money, or an Engineering Degree, Boston: Little, Brown and Company, 1973

chapter 26 bioelectronics and...

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