Linear Integrated Circuits
|
|
Here is a sample of what passed as big news in the electronics world in 1965 as reported in none other than Electronics World magazine. Linear integrated circuits were beginning to be designed into commercial products and a lot of effort and money was invested in promoting the newfangled technology to the public. Prices were rapidly falling as acceptance increased. The truth is the vast majority of the general public had no idea what the difference was between vacuum tube and semiconductor equipped radios, televisions, phonographs, tape recorders, etc., from a performance standpoint. What they did notice was the smaller size, lack of warm-up time, and lower power consumption (i.e., less heat). Prices were about the same at the beginning of the technology transition. Some anti-semiconductor naysayers tried to argue that at least with tube equipment you had a chance of fixing a malfunctioning unit simply by replacing a $1 tube, but failed to note that the equivalent semiconductor product almost never experienced a failure. Of course there were some crappy transistorized products, but that was the exception rather than the rule. Linear Integrated Circuits
Table 1 - Characteristics of a small sampling of linear IC's. Commercial linear integrated circuits can be adapted for use in consumer electronics equipment. However, two major roadblocks lie in the way: one is the device cost, and the other is the total lack of inductive elements. A linear integrated circuit is one whose output varies linearly with its input, just like a conventional amplifier using vacuum tubes or transistors. It will do everything that its big brothers will do, with some important differences. It is extremely tiny (an entire multi-transistor circuit can be contained within a TO-5 transistor case), and it requires far less operating power (in many cases measured in micro watts rather than the milliwatts of conventional circuitry). However, it delivers a somewhat smaller output power. In one type of linear integrated circuit, all circuit components and interconnections are created by carefully controlled molecular diffusion; they have the desired electrical characteristics and are molecularly bonded together to form a monolithic structure. Circuit reproducibility and reliability are thus greatly increased over a similar circuit fabricated from conventional broader tolerance components soldered or welded to a circuit board. Most linear integrateds presently being fabricated are used in analog computers. Since most consumer electronic circuitry is analog in operation, the question naturally arises, "Why can't linear integrated circuits be used in radio or TV sets?" The answer is - they can, if the obstacles of high unit price and lack of frequency-selective elements can be overcome. Several companies have fabricated integrated circuit receivers and transceivers for the military, and one manufacturer has a hearing aid which uses a linear integrated circuit amplifier. However, although there are some prototype consumer applications, the present cost of these items is prohibitive in the price-conscious consumer area. A glance at Table 1 shows the electrical characteristics of a small sampling of available linear IC's. A directory of some integrated circuit manufacturers is included in the article on page 49 of this issue. All presently available linear IC's are forms of untuned amplifiers having various bandwidths. No units in themselves are capable of being tuned to a particular r.f, or i.f. frequency. One company is considering the use of a broadband linear IC in an oscilloscope probe so as to create a variable gain de-vice. At present, most scope and v.t.v.m. probes either have no gain or are loss types, as vacuum-tube or transistor circuits are too bulky to be mounted within a hand-held probe. In another area, development is under way to fabricate a broadband booster that can be used within a CCTV cable. This will eliminate the bulky line amplifiers and their mounts presently being used. It is even possible to mount a broadband amplifier within a dipole receiving antenna so as to create a high-gain receiving antenna without using valuable chassis space for vacuum-tube or transistor signal boosters. Linear LC's for use as r.f. or i.f. amplifiers (with external tuning components) are presently available for use to 300 mc., with gains up to 30 db, while oscillator-mixer combinations that can operate from the broadcast band to the u.h.f. region are obtainable. This has led several companies to consider using linear IC's along with outboard miniature tuned circuits together on a small printed board. Another company recently announced an integrated audio amplifier having a one-watt power output. Industry engineers claim that when the cost of such composite circuits comes down to present circuit costs, then there is a very good probability that these new devices will find their way into consumer products. The major reduction in consumer electronics equipment dimensions came with the introduction of the transistor. At this point, size reduction was considerable, as the components used in conjunction with transistors had also undergone a great size reduction. However, some components do not readily lend themselves to miniaturization. The CRT in scopes and TV sets, meters, switches, loudspeakers, batteries, and operating controls, such as potentiometers and knobs, are already near their lowest practical physical limits for efficient operation. Hence, the use of linear IC's in a circuit will not necessarily mean an automatic over-all size reduction of most devices. All they could contribute is an increase in circuit re-liability because of the reduced number of circuit interconnections. There is another, often overlooked, limit to size reduction. Some semiconductor integrated circuit packages, each occupying typically one thousandth of a cubic inch, can dissipate 100 mw. In a practical system, a group of these tiny circuits could realize a density of 200 per cubic inch. This produces a power density of 20 watts per cubic inch, meaning that air cooling is not sufficient and either liquid or thermoelectric cooling must be used. This will tend to offset the size advantage of the small circuit dimensions. Tuning Problem The lack of frequency-selective elements within linear IC's has been mentioned several times. The basic problem is that when an inductor is scaled down in physical size (so that it can be included within the TO-5 can), the "Q" decreases as the square of the scaling factor. Even with relatively large thin-film substrates, it is not possible, at this time, to obtain useful values of inductance without expensive special techniques. Simple 74-inch special thin-film inductors of 1 to 2 μhy. have been fabricated, but they are not usable below v.h.f. where "Q's" up to 2.5 are realized. Many other techniques for creating frequency-selective circuits without the use of inductors have been tested. These include passive feedback using piezoelectric resonators, active feedback with parallel-T or distributed RC null networks, and the principle of the reactance tube. None of these has proven feasible, however, and the search for a relatively high-"Q," relatively low operating frequency inductor, or inductance simulator, is still high on the list of integrated circuit research goals. At the present time, engineers researching consumer applications of linear IC's have to make do with "outboard" microminiature tuned circuits coupled to existing active-element IC's, There is no doubt in the minds of consumer equipment engineers that linear IC's will come to the consumer field. Within the next year or so, some radio, TV, or audio systems will use some integrated circuits, once the barrier of unit cost has been removed or reduced to the point where the price differential is not as great as at present. The lack of suitable tuning elements will not deter this introduction. As previously mentioned, equipment size is limited by other components within the system. In the area of research, development is now under way on a "device on a slice." In this approach, instead of a large number of identical circuits being formed on a slice of silicon, one over-all interconnected system, such as a complete radio except for non-electronic items, or a TV set except for the high-power circuits, can be laid down on a single one-inch diameter silicon slice. The development of entirely new circuit elements may change the .design of future consumer electronics equipment. One laboratory is presently experimenting with an acoustic-resonant transistor that is theoretically capable of producing power at its resonant frequency without the need for frequency-selective components. This particular device lends itself admirably to local oscillator use in receivers and to use in low-power r.f. stages in transmitters.
Posted December 8, 2022 |
|
