Sometimes, it’s a worthwhile thought experiment to speculate with a “what if?” question, as it broadens your thinking and perspective. Moore’s law—actually not a law in the physics sense, but rather a supposition—has defined the semiconductor industry since Gordon Moore proposed it in a short, readable, and remarkably prescient 1965 article in Electronics titled “Cramming more components onto integrated circuits.” It also contained several other impressively on-target predictions.
While the bulk of Moore’s article focused on digital integration, near the end, he did call out some of the differences between digital and linear circuits, a fact which is often overlooked by those who reference Moore’s predictions.
He wrote, “The lack of large-value capacitors and inductors is the greatest fundamental limitation to integrated electronics in the linear area. By their very nature, such elements require the storage of energy in a volume. For high Q it is necessary that the volume be large. The incompatibility of large volume and integrated electronics is obvious from the terms themselves. Certain resonance phenomena, such as those in piezoelectric crystals, can be expected to have some applications for tuning functions, but inductors and capacitors will be with us for some time… Other linear functions will be changed considerably. The matching and tracking of similar components in integrated structures will allow the design of differential amplifiers of greatly improved performance…. The integrated r-f amplifier of the future might well consist of integrated stages of gain, giving high performance at minimum cost, interspersed with relatively large tuning elements.”
Today’s reality is that we do have low-value IC-based inductors and capacitors as well as resistors. Many system-on-chip (SoC) designs use clever topologies that minimize both the number and value of these Ls and Cs, even if it requires more active but easily integrated devices. After all, analog IC designs aren’t simply the reduction of a discrete-device schematic into an IC.
At the same time, there has been tremendous progress in using new materials, fabrication techniques and automation to create passives which are unimaginably small. Many but not all of these use some sort of ceramic chip as the base for the physical construction.
Still, there’s room for ultra-tiny wirewound devices. For example, I just saw an announcement from Coilcraft about their 016008C Series, claimed to be the world’s smallest high-frequency wirewound chip inductor at just 0.47 mm × 0.28 mm × 0.35 mm (Figure 1).
Figure 1 This family of RF wirewound inductor measures a nearly invisible 0.47 mm × 0.28 mm × 0.35 mm form factor and is available in inductances from 0.45 nH to 24 nH. Source: Coilcraft
It’s available in 36 inductance values ranging from 0.45 nH to 24 nH. It features what they say is the highest Q factor on the market and very low DC resistance, lower than thin-film inductors—high Q is required to minimize the insertion loss in RF antenna impedance-matching circuits. The informative datasheet not only calls out the specifications, but also identifies the instrumentation used to measure each of those parameters.
While this compactness is nowhere near what an IC can achieve, it’s still a remarkable passive-component “shrink.” As a consequence, RF circuits which need this modest value of inductance will have additional flexibility in component placement and product packaging along with reduced parasitics.
There’s no question that inductors have come a long way since their days of being large wound coils of varying sizes, and capacitors as tangible metal or metallized surfaces. One strange but widely used example of a capacitor from “back in the day” was the so-called gimmick capacitor made of two wires twisted around each other as shown in Figure 2 (“What is a gimmick?).
Figure 2 The gimmick capacitor is true improvisation, usually made of two short wires twisted together. Source: Harry Lythall at SM0VPO/G4VVJ
Capacitance of the gimmick unit is highly dependent on wire size, insulation thickness, tightness of twist, and of course, length. Typical length is 1 to 2 centimeters and resultant capacitance is 2 to 5 pF/cm. Then there is inductive parasitics, of course, of a few microhenries. This crude but effective capacitor was used for interstage coupling in RF circuits, as shown in Figure 3, as well as for adding “just enough” needed capacitance to trim a circuit.
Figure 3 Gimmick capacitors have also been used in interstage coupling to increase the gain at the upper end of the tuning range. Here, note the schematic depiction for the gimmick placed over the grid end (that’s “vacuum-tube” talk) of the secondary-side winding. Source: www.radiomuseum.org
The good part is that you could start with a longer twisted wire and keep snipping until things work as desired; I suppose you could call it a user-programmable capacitor. The bad thing is that repeatability of this solution in a production setting is difficult.
Nonetheless, the gimmick capacitor has been used commercially because of its low cost, convenience, and feasibility of trimming each unit on the production line, as unit-to-unit consistent performance of RF circuits was a major problem in the early days. The gimmick is still used for functions such as fine-tuning antennas in the field to compensate for real-world variances that are dependent on location, installation specifics, wire gauge, parasitics, and end effects as well as “maybe a little extra capacitance here would help” scenarios.
Do you see a place where ultra-tiny inductors would make your RF-circuit design and layout easier? Or would the difficulties in handling visibility, prototyping, and probing circuits likely cause you unintended and unneeded difficulties?
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