Chapter 2 Aspects of Technology

Chapter 2 Aspects of Technology Now that we have covered some elements of physics in Chapter 1 we can continue with our survey of basic concepts by touching on a number of topics from analog electronics. We concentrate here on describing the large-scale technology of circuit elements, on how they are constructed. We review what is meant by an analog waveform, an analog filter, the transistor amplifier and the operational amplifier. We shall see how a transistor or an operational amplifier can be used as a gate, in preparation for our discussion of digital electronics in Chapter 3. Energy Sources The Chemical Cell intended. Cells are connected in series and in parallel The most common small-scale source of electrical to form batteries of 9 volts, 12 volts etc., capable of energy is the chemical cell. Chemical cells are con- delivering various currents (Figure 2-2). structed from various materials, usually of two chem- ically dissimilar substances, called a cathode and an anode, separated by a liquid or a paste medium called electrolyte. The anode serves as a source of electrons which are driven by chemical action through the electrolyte to the cathode. Thus the anode takes on a positive potential, the cathode a negative potential. An example is the carbon-zinc type whose internal structure is drawn in Figure 2-1. Figure 2-2. At the top are shown common consumer type chemical cells of 1.5V. The batteries (bottom) of 6 and 9V consist of two or more cells connected in series or in parallel and encapsulated in a single convenient container. Cells and batteries are designed to have a charge capacity expressed in ampere-hours (Ah) or milliampere- hours (mAh). Capacities for typical cell types are listed in Table 2-1. The larger the current drawn from Figure 2-1. Internal structure of the carbon-zinc cell. a battery the shorter is its lifetime. Charge capacity is roughly related to the amount of chemical mass in the cell and therefore indirectly to the cell’s volume. A cell A chemical cell is designed to produce an electro- for a digital watch or a hearing aid might be tiny motive force (emf) of 1.5 volts and to have a size and a whereas a battery in a nuclear submarine might be as shape appropriate to the device for which it is large as an average refrigerator. Research is being 2-1 Aspects of Technology carried out in major corporations like Union Carbide, solar cell has a lifetime that, in principle, is infinite. Sony and others to produce cells of ever-increasing The physics of the silicon solar cell is basically the capacity and lifetime.1 physics of the PN junction diode that we have dis- cussed in Chapter 1. We shall concentrate here on the Table 2-1. Charge Capacities of Some Cell Types.2 practical uses of the cell as an alternative source of power and the practical details of its power output for Type Description Elements Capacity various light and load conditions. D Gen Purpose Carbon-Zn 1500 Each silicon solar cell (Figure 2-3) can convert solar C Gen Purpose Carbon-Zn 700 energy directly into electrical energy by a process AA General Carbon-Zn 300 called photovoltaic conversion. Essentially a large-area AAA Heavy Duty Zn Chloride 120 PN junction diode, the cell is made from two pieces of silicon fused together. One piece is doped so as to yield an excess of free electrons (N type) while the other is doped so as to yield a deficiency of free electrons or Example Problem 2-1 an excess of holes (P type). One layer of the cell is Cell Lifetime made thin enough to enable photons of light to pene- trate to the junction and there to interact with free Ordinary flashlights use D cells. A fresh D cell has a electrons. A free electron, in absorbing a photon, typical charge capacity of 1500 mAh. If 25 mA are acquires enough energy to take part in electrical con- drawn from the cell continuously, how long in hours duction. This means that the number of minority should the cell last? charge carriers in each semiconductor type increases —holes in the P-material and free electrons in the N- Solution: material. These carriers, if they reach the junction The number of milliampere-hours can be written I x before recombining, cross the junction in response to t, where I is in mA and t is in hours. Thus the cross-junction electric field. Once across the junc- t = 1500 mA-hours/25 mA = 60 hours. tion they are free to move through an external circuit The D cell should be expected to last 60 hours. and deliver power to a load. The Power Supply Next to the chemical cell the most common source of electrical energy in a laboratory is a power supply. Basically, a power supply converts the input from the mains at 110 V AC to some DC voltage at a (possibly metal variable) DC current. One such instrument you will annular ring use in this course is the Agilent Model E3640A pro- grammable power supply. This supply can be made metal P-TYPE SILICON electrodes to function as a voltage source or as a current source. base plate N-TYPE SILICON More details on this instrument can be found in Appendix A. Figure 2-3. The disk-shaped silicon solar cell. The Solar Cell The solar cell is a less common source of electrical A single cell is typically able to deliver about 0.5 volt energy than is the chemical cell, though its importance to an open circuit (called the open circuit voltage VOC) increases daily. Many hand calculators used by stud- and a certain maximum current to a shorted load ents today are powered by solar cells. In contrast to (called the short circuit current ISC). To form a practical the chemical cell, the solar cell, by its name, derives power source, a number of cells must be connected in energy not from the dissociation of chemicals, but series to form arrays with voltage outputs of 6, 9, 12 from sunlight or the ambient light in buildings. The volts, and so forth, and in parallel to give a desired 2-2 Aspects of Technology output current. Most arrays have a flat geometry, con- This curve is obtained by connecting the array to a sistent with the need to capture maximum sunlight. load resistor and then graphing I as a function of V as Some are fabricated on a glass substrate and are the resistance is changed. You can see that as the load fragile while others are made on a metallic backing resistance increases the output current decreases. and are flexible, enabling them to be bent into conven- Superimposed on the figure is the output power P ient shapes and to be used in demanding applications (the product of I and V). P goes through a maximum as in pleasure boats and spacecraft. Connecting a solar for a certain V and therefore also for a certain resis- array to a circuit is simple—you connect the array to tance R. R is equal to the array’s internal resistance. the circuit with two wires. Thus maximum power is obtained from an array Though we have described a silicon solar array as a when it is connected to a load whose resistance is power source, the power it can deliver is relatively equal to the internal resistance of the array. low; it is therefore not often used in a stand-alone way. Most often it functions as a trickle charger for a The Selenium Photocell higher-power primary source like a lead-acid battery The selenium photocell functions in practice much or a gell-cell. Under normal conditions the battery like a solar array in that it converts solar energy into supplies power to the main load (house wiring, etc) electrical energy. The advantage of selenium photo- and is independent of the array. When convenient voltaic cells over other cells is that their response is (during periods of non-usage), the battery is very close to that of the human eye. Their efficiency as recharged by being disconnected from the main load energy converters of the total spectrum is not as high and connected to the array. as other photocells, and so they are not used as Silicon solar arrays are commonly described by sources of energy as are solar cells. three parameters: the maximum power, PMAX, they can Figure 2-5 shows the cross-section of an idealized deliver to a load of a common type (like a lead-acid barrier-layer selenium photocell. The steel support battery), the open circuit voltage, VOC, and the short plate “A” provides the rear (positive) contact, and circuit current, ISC. These parameters are quoted for the carries a layer of metallic selenium “B”, which is a few array for one full sun, which is the illumination hundreds of a millimeter in thickness. “C” is a thin received in an outdoor position on the equator at high transparent electrically-conductive layer applied by noon on a summer day. cathodic sputtering; it is reinforced along its edge by a sprayed on negative contact ring “D” and protected IV Characteristic of a Solar Cell from damage by lacquering. The rear support of the Many of the properties of a silicon solar cell or array photocell is protected from corrosion by a metallic are described by its IV characteristic curve (Figure 2- spray coating “E”; this also improves electrical 4).

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