George A. Scherer Western College for Women
Oxford, Ohio
lntegrated Circuits in the Instrumental Laboratory
Integrated circuits offer a new opportuuit,y to teachers who have limited electronic knowhow and small budgets, to design and construct instruments to be used iu their chemical iustrumeutation courses. Integrated circuits are basically tiny "active" packages (Fig. 1) of various combinations of transistors, diodes. resistors. and capacitors, requiri& ouly conuection to a few external element,s to produce com- I plete operating systems similar to discrctc comnonent transistorized systems found I in commercial electronic ill- ClrcUitr F!gu*e , 1. Three integrated on different poctoger. strumcnta with couvelitional circuitrv. Usiw~.inteerat,cd circuit,^ the inexperienced (in electronics) teacher can take advantage of the k~iowledgeoft,he best experts in the electronic field in circujt design, construction, and reliability.
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Thin Film Circuits
A variety of integrated circuits are now mass produced and therefore low in cost. Two basic processes are used-the thin film technique and the semiconductor technique. Thin film circuits are similar to conventional printed circuits except in size and thickness. The thicknesses of the coatings are measured in microns. These thin-film coatings are used mainly to fabricate resistors, capacitors, and inductors. Deposition of thin films is accomplished by transportation of matter (metallic or insulating) from a heated evaporation source to a condensation surface (substrate) in a vacuum. The shape and area of deposition of each layer is determined by masks which are originally made many times oversize and then reduced to their final minute form by high-resolution photo lithographic techniques. Semiconductor Circuits
A semiconductor integrated circuit incorporates several forms of semi-conductors as well as their associated resistors and capacitors on a common p or n type silicon substrate that is itself a semiconductor material. As bcforc, minute masks are used to cover the desired sections of the silicon wafer for various diffusion processes. One mask is used to create each conducting or insulating layer in the silicon substrate. Depending on the complexity of the circuit, up to 20 masks may be used to create a particular circuit. By proper arrangePresented hefore the Division of Chemical Education a t t h e National Meeting of the American Chemical Society, San Francisco, California, April 4, 1968.
ment of the various diffusion layers, it is possible to create transistors, diodes, resistors, and capacitors, each working because of the differences existing a t the interfaces between the various diffused layers. I n a conventional planar transistor, many transistors can be on the same wafer, but they will all share a common collector circuit. I n an integrated circuit transistor, the collectors are electrically separated from each other by an additional p-type diffusion which results in an additional diode, called the isolation diode, tied to the collector of each transistor. When these isolation diodes are reverse biased, the collectors of the individual transistors are isolated from other circuit elements. Hybrid Circuits
I n a semiconductor circuit, where a large number of compouents share- a common substrate, the substrate material must of ucccssity be a compromise. The substrate material m a y b e ideal for one component but far from ideal for another. This can compromise circuit operation. Or for some sub-circuits, a thin-film product may be bett,er. Because there are limits to what can be incorporated into a single chip, more complex circuits may require the assembly of several discrete chips. These hybrid or multi-chip circuits are still not physically large and for practical purposes are just as useful as a single integrated circuit. Sources
There are several manufacturers of integrated circuits. Among these are Radio Curporation of America, Electronic Components and Devices, Harrison, N. J. General Electric, Semiconductor Products Department, Syracuse, N. Y. Fairchild Semiconductor, Mountain Tlew, California A m ~ e r e xElectronic Comorabion. Slaterville, Rhode Island ~ a i o r o l a~. e m i e o n d u c t o ~ Box , 9&, phoenix; Arizona Texas Instroments, P.O. Box 6474, Dallas, Texas Sylvmia Electric Products, Inc., Wobunl, Mass. '
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They can he purchased from any electronics supply .. - house. Complete data sheets are available from the manufacturers. Applications
wave generotor.
Four specific applications of inexpensive, readily available, integrated circuits are given to illust r a t e the versatility of these devices in electronic circuits. Volume 46, Number 6, June 1969
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Square Wave Generator
Althouse' describes the construction of a wide-range signal generator (Fig. 2) built around a Fairchild pL914 integrated circuit which costs less than a dollar. This p1ast.i~-encapsulated integrated circuit has 4 silicon planar 2N708 transistors and G resistors in a TO-5 transistor-size case. The internal circuit of the pL914 is shown in Figure 3. It functions as a dual gate. The complete schematic diagram for the signal gemrator using the pL914 is shown in Figure 4. The generator puts out wellformed square waves from 5 Hz to 50 kHz and runs from a single 3-V battery. NEEDS HEEDS lirrm RlarstonZgives additional @ 0 0 0 applicatiolls of the ~ ~ WGUREJ i ~lmcarr-NUUBER OF%AIN UNITS &914 illtegrated cir- Figure 3. lnternol circuit of mrit,. One of these 'L914' ~ -~~~ ~ ~ ~ is a differential amplifier for iricreasing the sensitivity of a moving-coil current meter. Such an amplifier is drift free because the transistors are close together on a common slab of silicon and thus must inevitably be a t equal temperatures and have their characteristics closely matched.
Nios
Figure 7. Schematic diagram of half-watt oudio omplifler.
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Figure 10. CA3029.
Internal
circuit of
Figure I I. Malmrtadt rocket.
Modified
.
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watt audio amplifier using the CA3020. I t needs no heat sink and will dircctly drive a spcaker. I t has high gain and reasonable'distortio~lfigures. Decimal Counting Unit
Figure 4. Schematic diagram of square ware generator (from reference in footnote 1I.
Audio Amplifier
Lancaster3 describes a tiny general purpose audio amplifier (Fig. 5) which uses a Radio Corporation of America CA3020 integrated circuit. The CA3020 cont,ains a class B amplifier, a class A preamplifier, and a regulated power supply (Fig. G). I t requires a +9 V supply and is packaged in a 12-pin TO-5 metal can. Figure 7 is the complete schematic diagram for t,his half-
Lancaster4has designed a lowcost numerical readout device (Fig. 8) using four integrated circuits. One of these is a lIot,orola3IC7241'mhich is a quad two-imputgate, another is a 3lot,orola RIC715P which is a dual thrcc-imput,-gate and thc other t,mo are Motorola R'IC7901' which arc d u d JK flip-flop int,egrated circuits. Because of the complex miring involved in this counting unit it is almost mandatory that a priuted circuit hoard hc used. An etched and drilled board (as well as 21. completc kit of all parts) is available from Southwest Technical Products Corp., 219 W. Rhapsody, San Antonio, Tcxas 78216. This complete, resettable decimal couuter has n bright staggered-linc 0-9 incandescent readout. One can cascade as many of the decades toget,her as one needs for almost any digital display since the counting circuit has a "carry" output terminal. The intergated circuit,^ require 3.6 V and t,he incandeseerit readout bulbs 6 V. A line operated power supply is described by Lancast,er (Fig. 9). If a mechanical contact is used to nctunte thc counter it must he conditioned so t,hat it is noise-free. A monostable coutact coudit,ioning circuit nsiug a I'airchild pL914 integrated circuit is given by Lancaster. This produces a "bounccless" input signd. Operational Amplifier
Figure 5. Half watt oudio amplifier.
There are several integrated circuit operatioizal amplijLers available. The differential-input operational
and 39, 50 (January, IOFR). 3 L,INC.\STI:R, J ~ N P, o p ~ t l a ~Electlanies, . 27,
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(I)rcernlrer,
lOfi7l Figure 6. lnternol circuit of CA3020 (left) and functional black diagram of the CA320 (right).
400
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Journal of Chemical Education
4 L . \ ~ c . \ s ~ DON, ~ n , P o p u l a ~ Electronics, 28, 2 i (Febr~~nry, 1068).
amplifier is readily adapted to integrated circuit construction techniques because the stable dc amplifier configuration lends itself to the monolithic diffusion process used in the fabrication of silicon integrated circuits. One of these is Radio Corporation of America CA3029 (Fig. 10). I t consists basically of two differential amplifiers and a single-ended output circuit in cascade. It operates from a *6 V supply, which must be carefully balanced. This operational amplifier has been principally used to perform various mathematical functions, such as differentiation, integration, analog comparisons, and summation. A mount for integrated circuits has been designed so that experimental circuits could be easily connected and disconnected (Fig. 11). This is a modified Malmstadb
Enke tube socket with the integrated circuit leads permanently soldered to the pins of the socket. With this arrangement the resistors, capacitors, and other components of the illalmstadt-Enke Instrumentation Laboratory (manufactured by the Heath Company, Benton Harbor, Michigan) can be easily attached or detached from the integrated circuit. Conclusions
Networks built around integrated circuits can form operating systems for many of the functions of the electronic components found in commercial analytical instruments. The units are readily available, inexpensive and require a minimum of technical know-how for their use. This makes them valuable teaching devices in chemical instrumentation courses.
Volume 46, Number 6, June 1969
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