Section 14.5 Feedback
209
FIGURE 14.8 An amplifier without feedback.
The most significant point to note about control systems such as the one shown in Fig. 14.7 is that the output affects the input itself. Such systemsare called feedback systems (because information about the output is fed backto the input). The system is said to have negative feedback if it opposes achange in the input and positive feedback if it augments a change in the input.
The light control in Fig. 14.7 is a negative feedback because an increase inthe light intensity causes a decrease in the iris opening and a correspondingreduction of the light reaching the retina. Regulation of body temperatureby sweating or shivering is another example of negative feedback, whereassexual arousal is an example of positive feedback. In general, negative feedback keeps the system response at a relatively constant level. Therefore, mostbiological feedback systems are in fact negative.
We will illustrate the method of system analysis with an example from electrical engineering. We will analyze in these terms a voltage amplifier thathas part of its output fed back to the input. Let us first consider a simpleamplifier without feedback (see Fig. 14.8). The amplifier is an electric devicethat increases the input voltage (Vin) by a factor A; that is, the output voltageVout is Vout AVin
(14.1)
It is evident from this equation that the amplification A is simply determinedby the ratio of the output and input voltages; that is, A Vout
(14.2)
Vin
Now let us introduce feedback (Fig. 14.9). Part of the output (β × Vout) isadded back to the amplifier input so that the voltage at the input terminal ofthe amplifier (V V Vin + β × Vout
(14.3)
Here Vin is the externally applied voltage. The amplification of the total feedback system is
Afeedback Vout
(14.4)
Vin
210
Chapter 14 Electrical Technology
FIGURE 14.9 An amplifier with feedback.
Using the fact that V AVin , we can show that (see Exercise 14-2)
Afeedback
A
(14.5)
1 − Aβ β is a negative number, the amplification with feedback is smallerthan the amplification without feedback (i.e., Afeedback is smaller than A).
A negative β implies that the voltage is added out of phase with the externalinput voltage. This is negative feedback. With positive β, we have positivefeedback and increased amplification.
This type of analysis has the advantage that we can learn about the sys tem without a detailed knowledge of the individual system components. Wecan vary the frequency, the magnitude, and the duration of the input voltageand measure the corresponding output voltage. From these measurements,we can obtain some information about the amplifier and the feedback component without knowing anything about the transistors, resistors, capacitors, andother components that make up the device. We could, of course, obtain thisinformation and much more by a detailed analysis of the device in terms of itsbasic components, but this would involve much more work.
In the study of complex biological functions, the systems approach is often very useful because the details of the various component processes areunknown. For example, in the iris control system, very little is known aboutthe processing of the visual signals, the mechanism of comparing these signalsto the reference, or the nature of the reference itself. Yet by shining light atvarious intensities, wavelengths, and durations into the eye and by measuringthe corresponding changes in the iris opening, we can obtain significant information about the system as a whole and even about the various subunits. Herethe techniques developed by the engineers are useful in analyzing the system (see Exercises 14-3 and 14-4). However, many biological systems are socomplicated with many inputs, outputs, and feedbacks that even the simplifiedsystems approach cannot yield a tractable formulation.
211
14.6 Sensory Aids
Ear horns, in one form or another, have been used to aid hearing for thou sands of years. These devices improve hearing by collecting sound from anarea significantly larger than the pinna (see Chapter 12).
Electrical technology has led to the development of devices that greatly enhance hearing and in some cases even restore hearing. The restoration ofvision is far more challenging and while several avenues of research are beingpursued the final goal seems far in the future.
14.6.1 Hearing Aids
The first hearing aids became commercially available in the 1930s. They were relatively large cumbersome devices using battery-powered vacuum tubeamplifiers. The batteries had to be replaced daily.
The much smaller transistor amplifiers that became available in the 1950s made hearing aids truly practicable. Transistorized hearing aids were nowsmall enough to be placed in the ear. The application of digital computertechnology to hearing aids was another major improvement that allowed individual tailoring of the device to compensate for the specific hearing deficitsof the user. Using various feedback networks, modern hearing aids automatically adjust the volume of the sound so that quiet sounds can be heard andloud sounds not be painfully overwhelming.
14.6.2 Cochlear Implant
A cochlear implant functions differently from a hearing aid. A hearing aidsimply amplifies incoming sound compensating for the diminished functioning of the ear. A cochlear implant converts sound to electrical signals ofthe type produced by the inner ear in response to sound that enters the ear.
212
Chapter 14 Electrical Technology
3
2
1
5
4
FIGURE 14.10 Cochlear implant. 1. Sounds are picked up by the microphone.
2. The signal is then “coded” (turned into a special pattern of electrical pulses). 3. Thesepulses are sent to the coil and are then transmitted across the skin to the implant. 4. Theimplant sends a pattern of electrical pulses to the electrodes in the cochlea. 5. Theauditory nerve picks up these electrical pulses and sends them to the brain. The brainrecognizes these signals as sound.
The electrical signal is wirelessly transmitted to electrodes surgically implanted in the inner ear. The signals applied to the electrodes stimulate the auditorynerve to produce the sensation of sound. Thus the cochlear implant actuallymimics the functions of the ear and can restore partial hearing to the deaf.
A sketch of a cochlear implant system is shown in Fig. 14.10. The exter nal part of the system is small enough to be placed behind the ear. It consistsof a microphone, a signal processor, and a transmitter. The internal part consists of the receiver and an array of electrodes implanted and wound throughthe cochlea.
The microphone converts the sound to an electrical signal. Such electri cal signals as are produced by the microphone could themselves stimulate the
213
auditory nerve, but the neural signals produced by such stimulation wouldnot be interpreted by the brain as sound. In the normal ear the fluid filledcochlea processes the sound signal according to frequency such that the various frequency components of the incoming sound stimulate nerve endingsalong different parts of the basilar membrane (see Chapter 12). This type of afrequency-selective stimulation of the neural network provided by the cochleais essential if the signal is to be interpreted by the brain as sound.
One of the main challenges in the design of cochlear implants was the development of signal-processing techniques that duplicated the action of anormal cochlea. Much of the work in this area was done in the 1950s and 60s.
First experiments with human implants began in the mid-1960s and continued through the 1970s. In 1984, FDA approved implantation into adults andshortly after, into children.
Usually a person receiving an implant is not immediately able to hear sounds properly. A period of training and speech therapy are needed beforethe full benefits of the device are realized.
EXERCISES
14-1. From the data in the text, compute the capacitance of the capacitor in the defibrillator and calculate the magnitude of the average current flowingduring the pulse.
14-2. Verify Eq. 14.5.
14-3. Draw a block diagram for the following control systems. (a) Control of the body temperature in a person. (b) Control of the hand in drawing aline. (c) Control of the reflex action when the hand draws away froma painful stimulus. Include here the type of control that the brain mayexercise on this movement. (d) Control of bone growth in response topressure.
14-4. For each of the control systems in Exercise 14-3, discuss how the sys tem could be studied experimentally.
14-5. Discuss the controversy surrounding cochlear implants.
Optics Light is the electromagnetic radiation in the wavelength region between about400 and 700 nm (1 nm 10−9 m). Although light is only a tiny part of theelectromagnetic spectrum, it has been the subject of much research in bothphysics and biology. The importance of light is due to its fundamental rolein living systems. Most of the electromagnetic radiation from the sun thatreaches the Earth’s surface is in this region of the spectrum, and life hasevolved to utilize it. In photosynthesis, plants use light to convert carbondioxide and water into organic materials, which are the building blocks ofliving organisms. Animals have evolved light-sensitive organs which are theirmain source of information about the surroundings. Some bacteria and insectscan even produce light through chemical reactions.
Optics, which is the study of light, is one of the oldest branches of physics.
It includes topics such as microscopes, telescopes, vision, color, pigments,illumination, spectroscopy, and lasers, all of which have applications in thelife sciences. In this chapter, we will discuss four of these topics: vision,telescopes, microscopes, and optical fibers. Background information neededto understand this chapter is reviewed in Appendix C.
15.1
Vision
It has been estimated that about 70% of a person’s sensory input is obtainedthrough the eye. The three components of vision are the stimulus, which
214
215
is light; the optical components of the eye, which image the light; and thenervous system, which processes and interprets the visual images.
15.2 Nature of Light photons. For a given frequency f of the radiation, each photon has a fixed amountof energy E which is
E hf
(15.1)
where h is Planck’s constant, equal to 6.63 × 10−27 erg-sec.
In our discussion of vision, we must be aware of both of these properties of light. The wave properties explain all phenomena associated with the propagation of light through bulk matter, and the quantum nature of light must beinvoked to understand the effect of light on the photoreceptors in the retina.
15.3 Structure of the Eye
The focusing of the light into an image at the retina is produced by the cur ved surface of the cornea and by the crystalline lens inside the eye. The focusing power of the cornea is fixed. The focus of the crystalline lens, however, isalterable, allowing the eye to view objects over a wide range of distances.
In front of the lens is the iris, which controls the size of the pupil, or entrance aperture into the eye (see Chapter 14). Depending on the intensityof the light, the diameter of the aperture ranges from 2 to 8 mm. The cavity ofthe eye is filled with two types of fluid, both of which have a refractive indexabout the same as water. The front of the eye, between the lens and the cornea,
216
Chapter 15 Optics FIGURE 15.1 The human eye.
is filled with a watery fluid called the aqueous humor. The space between thelens and the retina is filled with the gelatinous vitreous humor.
15.4
Accommodation accommodation. When the ciliary muscle is relaxed, the crystalline lens isfairly flat, and the focusing power of the eye is at its minimum. Under theseconditions, a parallel beam of light is focused at the retina. Because lightfrom distant objects is nearly parallel, the relaxed eye is focused to viewdistant objects. In this connection, “distant” is about 6 m and beyond (seeExercise 15-1).
The viewing of closer objects requires greater focusing power. The light from nearby objects is divergent as it enters the eye; therefore, it must befocused more strongly to form an image at the retina. There is, however, alimit to the focusing power of the crystalline lens. With the maximum contraction of the ciliary muscle, a normal eye of a young adult can focus onobjects about 15 cm from the eye. Closer objects appear blurred. The minimum distance of sharp focus is called the near point of the eye.
The focusing range of the crystalline lens decreases with age. The near point for a 10-year-old child is about 7 cm, but by the age of 40 the near point
