A WIZARD'S ELECTRONICS COMPANION
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A WIZARD'S ELECTRONICS COMPANION |
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Once you know the nuts and bolts of the basics (which I've already given you), what you need is to begin familiarizing yourself with an open-ended list of circuit configurations, along with occassional tricks about how to analyze them. I will try to analyze each configuration in detail. Remember, this is not a rigorous analytical text, but one aimed at helping you create practical circuits using the simplest intuitive approaches.
Beyond mathematical modeling, there are some concepts that will help you understand circuit design. These are the most important ones.
A circuit is a collection of electrical components engineered to work together and forming a source of power and some sort of load upon which work is done. A subcircuit is some portion of a circuit.
A circuit stage is a group of electronics components which work closely together, typically in some fairly common configuration. For example, a Class A bipolar transistor amplifying stage can be found here. Traditionally, it usually refers to certain common groupings of parts quite dependent on other parts nearby. One or more stages can be put together to create an even more generic application and are called functional blocks A DC current source, for example, might be both a circuit stage and a functional block. When we don't care (or don't know) the details of a circuit's inner workings, we can simply designate it with a labelled rectangle and refer to it as a black box. These three are fairly loosely-used terms, so they needn't be stressed over.
Just as in my definition of a circuit given above, every circuit can be broken down conceptually to any degree you like. One or more components can be thought of as a source for a load represented by one or more components.
Perhaps the most important, and single most misunderstood concept. The term ground was coined before electronics proper, and the Earth itself was used as an important part of a circuit. This is still true where an Earth ground is used for safety reasons and when it is used as a ground plane for an antenna. Even though resistance can be quite high in dry ground, once the current manages to spread about ten feet or so in all directions, the effective resistance will have dropped quite low. Obviously, the salty ocean makes a darn good ground. If I recall correctly, Canada(?) uses a three phase power distribution system which (when load-balanced) allows any one phase to pull virtually all its current from the other two phases, thereby negating the need for a specific return power line. The Earth itself is used as the system return because, on the whole, the current needed to flow in the ground is fairly negligable. In modern and portable systems, except for power-line safety grounding, a connection to the physical ground is just not needed. In practice, unless your circuit is battery-powered or is a simple plug-in device like a lamp, you will likely encounter safety-grounds.
A horror story. When I worked as a senior technologist at Curtis PMC (a maker of motor controllers for battery-powered vehicles), I was a responsible for the maintenance of some large battery packs and technician support for various projects. To support a second test area requiring heavy currents, I risked installing a set of contactors to provide the upper half of the battery to the other group, to alleviate some of the uneven discharge and load problems we had. As the groups were on separate workbenches and the battery was isolated from Earth ground, I didn't anticipate problems. One day I closed my emergency kill switch to make some measurements and watched in consternation as a mushroom cloud rose to the ceiling over the other group's workbench. What had happened? My bench had inadvertantly grounded the B- of the lower-half of the battery pack through the oscilloscope probe when the "cheater" (a three-prong adapter which allows you to float a ground or use it in a two-prong outlet) was removed from my scope by someone who desperately needed one (to their credit, cheaters usually aren't required most of the time.) At the same time, the same Earth-ground was used on the other group's power-strip which connected to their computer power-supply, which connected to their emulator board, which connected to their microprocessor emulator pod, which was attached to the microprocessor mounted on our product, whose five-volt ground was connected to the product's B-, which connected to the group's B- battery ground, which was, relative to mine at +48V. A ground loop of destructive proportions which shorted 48V across a bunch of wires, pins, and connections.
Most everyday ground-loop problems are noise problems which arise because people forget that the ground is the other half of the circuit and just as important. Any conductor has resistance and inductance and will seem to generate noise by having a voltage induced across a length of it from (1) electrical, magnetic, and RF noise impinging on the conductor, and (2) currents flowing from various parts of the circuit. Older cars were routinely wired to use the chassis of the car as the power return, or ground, but then noise wasn't a big problem.
To reduce outside noise, two wires may be twisted together so that any electrical noise will strike both conductors at the same time with the same amount of induced noise. If the voltage between the two wires is measured (a differential measurement) most of the noise will cancel out, like both ends of a surfboard rising over a gentle ocean swell. Also, two conductors may be concentric (as in a coaxial cable) which will have the advantage of a twisted pair, but also have a shielding effect as the outer conductor will absorb some of the noise and (hopefully) effectively short-out the noise field. This effect is similar to the field effect in a Van de Graf electrostatic generator. In spite of the tremendous voltage outside the globe, the force on charges inside the globe is very small because the same force is pushing from all sides. Another variant is to place a twisted-pair of wires inside an outer shield. Yet another is to have three concentric conductors, where the two inner ones are the signal and return, and the outermost one is grounded.
To reduce inside noise, remember that your "ground" is half your circuit. Power or signals might daisy-chain from one place to another via a pair of conductors, as long as you remember there are losses along both conductors which shift your voltages. Don't, for example, tap a ground from a circuit which pulls some fair current into a low-current ground line that is, say, carrying an analog sound signal. If you have a circuit with both analog and digital signals, separate their power sources, as the high-frequency noise generated by digital switching will just destroy a sensitive analog signal. Naturally, keep their power returns separate, too, for that same reason. Watch the ground paths when you are hooking up a lot of equipment together. The sneak paths for ground (as I described above) can be truly devious. You can't just "ground everything" and expect it to solve all your grounding problems.
One of the more "advanced" tricks, once you've done the above, is the concept of a shield. If you place a conducting sheet between a noise source and your circuit, some of the noise will be "shorted-out" in the conductor, some will be reflected, and some will pass through. You may or may not decide to try "grounding" the sheet (now I think you can see how this might make things worse!) I was confused the first time I saw traces leading nowhere on a circuit board. This is called a guard trace. The idea is to assume the source of a signal is about as strong as its going to get and nowhere else will there be as good a reference for it. If you attach two conductors to the signal itself, but allow only the least exposed one to connect to the signal load point at the end of its travel, any noise "trapped" on the guard trace has to travel back to the source point before it gets to your signal. This works especially well with coaxial cable where both the inner and outer conductors are attached to the same signal point at the source, but only the inner conductor attaches to where the signal is going. There is a certain capacitance between the inner and outer conductors. If the same signal is essentially on both, then the differential voltage is essentially DC and no signal will flow between the inner and outer conductors. If you had grounded the outer shield poorly, the noise differences would introduce a lot of noise between them. It can be a bit of an art form, but if you use your head and remember the basics, it will vastly improve the grounding in your systems.
One in which all the current flows through one branch. Different voltages will likely occur across each component in the series circuit, but the current is the same in each component (figure 5a.) If you have an confusion visualizing the action of voltage and current in series and parallel circuits, use the water flow analogy again.
Allows the current to divide into branching streams, summing together again at the bottom as in figure 5b. Different currents may be flowing in each branch, but the voltage across each branch is the same. If a limited current is provided to both branches together, as in figure 5c, the currents will naturally divide according to their relative total resistances. Once again, use the water flow analogy to help you visualize.
Is a trickle of electrons, or a voltage, you give a circuit as distinguished from the signal. It can refer to the main power for the circuit, but it usually refers to the "pull" on a signal you achieve by connecting a signal node to a power source through a component in order to add that voltage or current to the signal so that it can best be applied to the next stage. A detector circuit might have a threshold of 3V, so you might bias it to 2.9V to greatly increase its sensitivity so only 0.1V of signal is needed to trigger it.
Is a form of bias which uses the signal itself to help stabilize / destabilize the circuit, by taking some of an amplified signal's output and feeding it back and mixing it with the input. If the amplifier inverts the signal, then they can mix destructively, which is known as negative feedback. This has a number of useful properties, including that of limiting amplification to a known, fixed value. Unlimited, an amplifier will have a mountainous peak in its response to various frequencies of signal. If you slice that "mountain" flat and fairly low, it will not only have a known amplification, but also have a wider frequency response at that precise amplification. Digital circuits often use feed-forward to force the circuit into the saturated on / off regions at each end of a transistor's biasing range -- it effectively becomes a logic switch.
Can be created by placing a capacitor in-line with the signal and between two stages, or (more expensively and with other arcane considerations, a transformer.) Many circuits have different DC bias requirements between the output of the previous stage and the input of the next. Likewise, an AC BLOCK is sometimes needed in high-frequency radio circuits to prevent the high-frequency signal from getting onto the power supply rails. A DC bias is provided through a resistor, and an inductor added in series with it provides no appreciable increase in DC resistance, but may increase the AC reactance considerably.
An inductor often used in radio frequency (RF) circuits in which there are two separate frequency ranges being accounted for. Typically, an inductor will be connected from a signal node to one of the power supply rails to feed DC power to the circuit node. At DC, the inductor presents little or no resistance to the current, but the operating frequency range of the circuit is so high that the inductor's reactance will block any significant RF from getting from the circuit node to the power supply.
This is usually a failure in a circuit. Quite simply, the electricity is not going where it's needed, but is being rerouted around the area via a shortcut which allows it to do no work where we want it to. Shorts are also used as a conceptual tool in some basic analysis schemes: "Imagine a short circuit here. What would be the result on the circuit's operation?" This is a useful conceptual tool for trying to determine the robustness of some circuits: "What if the user draws unlimited amounts of current from my power supply? How do I design it so it won't blow up?" In a few circumstances, a short is designed into a circuit, usually called a crowbar, which is used when you do not want the voltage or current to exceed a certain level in some part of your circuit under any circumstances, and you might even accept the destruction of part of your circuit when and if the crowbar is triggered. Nowadays, with improvements in technology, more and more power losses due to undesireable resistances are being eliminated. In principle, most information processing energy that's not otherwise going to be dissipated in some transducer could simply be shuttled around with electronic switches, capacitors, and inductors in some advanced superconducting way.
A switch will close or open a circuit. It can also be a failure mode as when a component blows up and leaves a physical gap in the circuit path. Like shorts, opens are also used as conceptual tools.
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