3

Components and Devices

On a road map, symbols indicate geographical features such as cities, highways, airports, railroad tracks, and other landmarks. The same rule applies to schematics in electricity and electronics. Specialized symbols portray conductors, switches, resistors, capacitors, inductors, transistors, and other circuit elements. Whenever engineers invent a component or device, they create a new schematic symbol for it.

Resistors

Resistors rank among the simplest electronic components. As the term implies, they resist or impair the flow of electric current. Engineers express resistance (the extent of current impairment) in units called ohms. Most real-world resistors have values ranging from approximately 1 ohm up to millions of ohms. Once in a while, you’ll encounter resistors with values less than 1 ohm, or values in the thousand-millions (billions) or million-millions (trillions) of ohms.

Regardless of their ohmic value, nearly all fixed resistors have schematic symbols that look like Fig. 3-1A or B. The two horizontal lines at the left and right (A) or the top and bottom (B) depict wires called leads that protrude from the ends of the physical component. Some resistors have rigid metal terminals such as pins or lugs that don’t necessarily come out of the ends.

Figure 3-2 shows a “transparent” pictorial of a carbon-composition fixed resistor with wire leads on both ends. Figure 3-3 shows pictorials of two other types of resistors: wirewound (A) and film (B). You can denote any resistor of the sort shown in Fig. 3-2 or Fig. 3-3 with either of the symbols in Fig. 3-1.

FIG. 3-1   Symbol for a fixed-value resistor. In a schematic, it can appear horizontal (A) or vertical (B).

FIG. 3-2   “Transparent” pictorial of a carbon-composition resistor.

FIG. 3-3   Pictorials showing the anatomy of a wirewound resistor (A) and a film type resistor (B).

A variable resistor has an ohmic value that you can adjust by moving a slide or tap along the resistive element. You set the resistance to a specific value, where it remains until you deliberately change it. The circuit “sees” the component as a fixed resistor at any given time.

When a circuit contains a variable resistor, the schematic reveals that fact. Figure 3-4 shows a common symbol for a variable resistor with two terminals. Some variable resistors have three terminals. Figure 3-5 shows two examples of schematic symbols for a three-terminal variable resistor known as a potentiometer or rheostat, depending on the method of construction.

FIG. 3-4   Symbol for a two-terminal variable resistor.

FIG. 3-5   Symbols for three-terminal variable resistors, also known as potentiometers or rheostats (depending on the method of manufacture). At A, the center terminal connects to one end terminal to obtain, in effect, a two-terminal component. The resistor at B has three independent terminals.

Figure 3-6 shows a variable resistor of the wirewound type, manufactured to expose an uninsulated coil of resistance wire. You can adjust a sliding metallic collar, which goes around the body of the resistor, to intercept different points along the coil. A flexible conductor connects the collar to one of the two end leads. The collar shorts out more or less of the coil turns, depending on where it rests along the length of the coil. As you move the collar to the right along the wire coil, the ohmic value between the two end leads decreases.

FIG. 3-6   Pictorial of a wirewound variable resistor with the movable middle sleeve connected to one of the fixed end leads.

Figure 3-7A is a functional drawing of a rotary potentiometer. Figure 3-7B shows its schematic symbol. The symbol has three distinct contact points. When you rotate the control shaft, the resistance varies between the center contact and the end contacts. Figure 3-8 is a pictorial of a typical real-world potentiometer.

FIG. 3-7   Functional drawing of a rotary potentiometer (A) and its schematic symbol with corresponding connections (B).

FIG. 3-8   Pictorial of a potentiometer suitable for mounting on the front panel of an electronic system such as a radio receiver.

The schematic symbol for a resistor, all by itself, says nothing about its ohmic value, or anything else about the component such as its power rating or physical construction. You’ll often see specifications for the component written alongside the resistor symbol, but these details might instead appear in a separate “components list” and referenced to an alphabetic/numeric designation printed next to the schematic symbol (such as R1, R2, R3, and so on).

Capacitors

Capacitors are electronic components that can block direct current (DC) while passing alternating current (AC). They can also store energy in the form of an electric field. The basic unit of capacitance is the farad (symbolized F). One farad represents a huge electrical quantity, so most real-world capacitors are rated in tiny fractions of a farad: microfarads or picofarads. One microfarad (symbolized µF) equals a millionth of a farad (0.000001 F). One picofarad (symbolized pF) equals a millionth of a microfarad (0.000001 µF) or a trillionth of a farad (0.000000000001 F).

Figure 3-9 shows the most common symbol for a fixed capacitor. The curved side should go to electrical ground, or to the circuit point more nearly connected to electrical ground. On occasion, you’ll see alternative symbols such as those in Fig. 3-10A or B.

FIG. 3-9   Standard symbol for a fixed capacitor. The curved line represents the plate (or set of plates) that’s electrically closer to ground.

FIG. 3-10   Alternate symbols for fixed capacitors. At A, air dielectric; at B, solid dielectric.

Many types of capacitors exist. Some are nonpolarized devices, meaning that you can connect them in either direction and they’ll work equally well. Others are polarized, having a positive and a negative lead or terminal. You must connect such a capacitor so that any DC voltage across it has the correct polarity.

Unless the symbol includes a polarity sign, it indicates a nonpolarized capacitor, which can have metal plates surrounding ceramic, mica, glass, paper, or other solid nonconductive material (and sometimes air or a vacuum). The nonconductive material, known as a dielectric, separates the metal parts of the component. A typical fixed-value capacitor comprises two tiny sheets (or sets of sheets) of conductive material that lie physically close to each other but are kept electrically apart by the dielectric layer.

Figure 3-11 shows the symbol for a polarized capacitor. It looks like the symbol for a nonpolarized capacitor, but a plus (+) sign appears on one side. The plus sign tells you that the positive terminal of the component should go to the more positive part of the external circuit. Occasionally, a minus (−) sign will appear on the opposite side instead of, or in addition to, the plus sign. The minus sign indicates that the negative terminal of the capacitor should go to the more negative part of the external circuit.

FIG. 3-11   Symbol for a polarized capacitor. The side with the plus sign (+) should carry a positive DC voltage relative to the other side.

In this chapter, all the capacitors that you’ve seen thus far have a fixed design. In other words, the components have no provision for changing the capacitance value, which the manufacturer determines at the factory. But you can adjust the values of some capacitors at will. They’re called variable capacitors. Some specialized types are known as trimmer capacitors or padder capacitors.

Figure 3-12 shows the most common symbol for a variable capacitor. An arrowed line runs diagonally through it. Figures 3-13A and B show two alternative ways of denoting the same component. All three symbols indicate that you can adjust the capacitance at will, regardless of the physical construction details.

FIG. 3-12   Standard symbol for a variable capacitor. The curved line represents the rotor, and the straight line to its left represents the stator.

FIG. 3-13   Alternate symbols for variable capacitors. At A, the stator is not distinguished from the rotor; at B, the rotor appears as a curved line with an arrow.

An air variable capacitor (one with an air dielectric) allows you to tune many types of RF equipment including antenna matching networks, transmitter output circuits, and old-fashioned radios. A typical “air variable” has interlaced plates connected together alternately to form two distinct contact points. The rotatable set of plates is called the rotor; the stationary set of plates is called the stator. All variable capacitors are nonpolarized components, meaning that you can apply an external DC voltage either way and the performance remains the same.

Sometimes you’ll see multiple variable capacitors connected together or ganged. In a set of ganged variable capacitors, two or more components can control two or more circuits at the same time. The rotors, although physically separate, all share a single shaft. Figure 3-14 shows the schematic symbol for two variable capacitors ganged together. The individual components might have identical minimum and maximum capacitance values, but not necessarily. In any case, they track together. When one capacitor increases in value, the other (or others) also increase in value.

FIG. 3-14   Symbol for two variable capacitors ganged together.

Inductors and Transformers

A basic inductor comprises a coiled-up length of wire that introduces inductance into a circuit. Inductance opposes changes in electric current. With constant DC, an inductor stores electrical energy but offers no opposition to the current itself. Inductors can range in physical size from microscopic to gigantic, depending on the inductance value of the component, and on the amount of current that it can handle.

The standard unit of inductance is called the henry (symbolized H). That’s a large electrical quantity. You’ll find most inductors rated in millihenrys (symbolized mH), where 1 mH = 0.001 H, or in microhenrys (µH), where 1 µH = 0.001 mH = 0.000001 H. Occasionally, you’ll see an inductor rated in nanohenrys (nH), where 1 nH = 0.001 µH = 0.000000001 H.

Figure 3-15 shows the schematic symbol for an air-core inductor. The two leads or terminals are designated by straight lines that merge into the coiled part. An air-core coil has nothing inside the windings that can affect the inductance. Some air-core coils are wound from stiff wire and support themselves mechanically. In other cases, a rigid form made out of plastic or ceramic material supports the coil turns, keeping them in place and enhancing the physical ruggedness of the component without making the inductance any greater.

FIG. 3-15   Symbol for an air-wound (or air-core) inductor.

Figure 3-16 shows the symbol for a tapped air-core inductor; in this case the coil has two tap points along its length. Whereas a fixed inductor has only two leads or terminals (one at either end), a tapped inductor has three or more. When you want to tap a coil, you attach conductors to turns at intermediate points. You get maximum inductance by connecting the end leads or terminals to the external circuit. A tapped arrangement lets you select one or more pairs of points having less inductance than the full coil has.

FIG. 3-16   Symbol for an air-core inductor with two fixed taps.

As an alternative to taps, a coil might have a sliding contact that you can move along the entire length of the windings. The sliding contact, which connects directly to one of the end contacts by means of a shorting wire, lets you vary the inductance almost continuously (actually it happens in little jumps as you slide the contact along, one turn at a time). You can portray this type of variable inductor with either of the symbols shown in Figs. 3-17A or B.

In equipment designed for high-power RF operation, you have an alternative to the sliding-tap method of varying the inductance of an air-core coil. The coil, consisting of solid bare wire, goes around a hollow ceramic cylindrical form, and a shaft attaches to a ceramic disk (or set of disks) inside the cylinder so that you can rotate the coil and the form together. A small wheel-like contact, resembling an automobile tire rim without the tire, travels along the length of the coil as it rotates, allowing smooth, continuous adjustment of the inductance between the “wheel” and either end. Such a component is called a roller inductor. You’ll often encounter roller inductors in radio antenna tuners and matching networks. Figure 3-18 shows a common rendition of its symbol.

FIG. 3-17   Symbols for a continuously variable air-core inductor. At A, arrow above coil symbol; at B, arrow passing diagonally through coil symbol.

FIG. 3-18   Symbol for a three-terminal roller inductor.

An inductor for standard AC applications, such as a 60-Hz choke for use in power-supply filters, can comprise a length of insulated or enameled wire wound around a solid or laminated (layered) iron core. The iron, a ferromagnetic material, replaces the air core. The ferromagnetic core increases the magnetic flux density inside the coil windings, making the inductance thousands of times greater than the inductance of an air-core coil having the same physical dimensions. Figure 3-19 shows the standard schematic symbol for a fixed-value inductor with a solid- or laminated-iron core. It’s the basic coil symbol discussed earlier, along with two parallel straight lines that run for its entire length. Now and then, you’ll see an iron-core inductor rendered as shown in Fig. 3-20, with the straight lines inside the coil turns.

FIG. 3-19   Symbol for an inductor with a solid- or laminated-iron core.

FIG. 3-20   Alternate symbol for an inductor with a solid- or laminated-iron core.

Some iron-core inductors have taps for sampling different inductance values. Once in awhile you’ll encounter an iron-core inductor whose value you can continuously vary by pushing and pulling the core in and out of the coil. The equivalent schematic symbols for these types of inductors appear in Figs. 3-21A and B.

At high frequencies, solid-iron and laminated-iron cores aren’t efficient enough to function in inductors. Engineers would say that they have too much loss. At frequencies above a few kilohertz (kHz), you’ll need a special ferromagnetic core material if you want to increase the inductance over what you can get with nonferromagnetic core materials such as air, plastic, ceramic, or wood. The most common substance for this purpose consists of iron shattered into microscopic fragments, each of which has a layer of sticky, glue-like insulation applied to it. After the fragmentation and insulation process has been completed, the particles get compressed into a “solid” object called a powdered-iron core. Figure 3-22 shows the symbols for three different types of powdered-iron-core inductors.

FIG. 3-21   Symbols for a tapped coil (A) and an adjustable coil (B) with solid- or laminated-iron cores.

FIG. 3-22   Symbols for fixed (A), tapped (B), and continuously adjustable (C) inductors with powdered-iron cores.

A transformer contains two or more coils with the turns interspersed or wound around different parts of a single core. Figure 3-23 shows the symbol for an air-core transformer. It looks like two air-core coil symbols drawn back-to-back. Figure 3-24 shows some transformers that have iron cores. The ones at A and B have solid- or laminated-iron cores; the ones at C and D have powdered-iron cores.

FIG. 3-23   Symbol for a transformer with an air core.

FIG. 3-24   At A, symbol for a transformer with a solid- or laminated-iron core. At B, symbol for a transformer with a solid- or laminated-iron core and tapped windings. At C, symbol for a transformer with a powdered-iron core. At D, symbol for an adjustable transformer with a powdered-iron core.

In a transformer, the output voltage might equal the input voltage, but often the voltages differ. In a step-up transformer, the output voltage is greater than the input voltage. In a step-down transformer, the output voltage is smaller than the input voltage. You’ll find schematic symbols for these transformer types in Appendix A.

Switches and Relays

A switch is a component that can complete or interrupt one or more current paths. Figure 3-25 shows the symbol for a single-pole/single-throw (SPST) switch. It can make or break a contact at only one point in a circuit; it’s a two-position device (on-off or make-break). With the switch “on” or “closed,” current flows. With the switch “off” or “open,” current does not flow.

FIG. 3-25   Symbol for an SPST switch.

Figure 3-26 is the symbol for a single-pole/double-throw (SPDT) switch. The pole coincides with the point of contact at the base of the arrowed line. The throw is the contact to which the arrow points. You can connect the pole to the upper throw or the lower throw, but not to both at once.

FIG. 3-26   Symbol for an SPDT switch.

Some switches have two or more poles. In Fig. 3-27, drawing A shows the symbol for a double-pole/single-throw (DPST) switch, and drawing B shows the symbol for a double-pole/double-throw (DPDT) switch. Some switches have even more elements. The one shown in Fig. 3-28 has five poles. Engineers might call it a five-pole/two-throw (5P2T) switch.

FIG. 3-27   At A, symbol for a DPST switch. At B, symbol for a DPDT switch.

FIG. 3-28   Symbol for a five-pole double-throw switch.

The 5P2T device is a multi-contact switch. This category includes most switches that have at least two throws. For instance, a rotary switch might have a single pole and ten throw positions; Fig. 3-29 shows such a scenario. You can call this thing a one-pole/ten-throw (1P10T) switch!

FIG. 3-29   Symbol for a rotary switch with a single pole and ten throws.

Occasionally, you’ll encounter sets of rotary switches ganged together, much like two or more variable capacitors can rotate in sync with one another. Figure 3-30 is the symbol for a ganged pair of rotary switches. The dashed line tells you that the switches mimic each other’s operations. The arrowed lines indicate the throw positions, which go around in sync with each other. When the left-hand switch pole rests at, say, throw 3 (as shown here), the right-hand switch pole also rests at throw 3.

FIG. 3-30   Symbol for a pair of ganged rotary switches, each of which has a single pole and ten throws.

FIG. 3-31   Symbol for a Morse code key.

You can’t always locate a switch near the circuit or system that it affects. Imagine that you want to switch a radio transmitter/receiver (or transceiver) between two different antennas from a control point 50 meters away. The antennas get their signals through coaxial cables that carry RF current, which must remain confined to the cables if you want the system to work properly. If the cable branch point lies far away from the place where you want to put the control switch, you can use a relay that employs an electromagnet to allow remote-control switching. You install the relay at the cable branch point. You can run a length of “lamp cord,” which carries plain DC, from the relay’s electromagnet to your control switch.

Figure 3-32A is a functional drawing of an SPDT relay, and Fig. 3-32B shows its schematic symbol. A “springy strip” holds a movable lever, called the armature, to one side (which would be all the way up in this case) when no current flows through the electromagnet coil. Under these conditions, terminal X connects to terminal Y but not to Z. When sufficient DC flows in the coil, the armature moves to the other side (which would be all the way down in this case), connecting X to Z rather than to Y.

FIG. 3-32   At A, functional drawing of an SPDT relay. At B, its schematic symbol.

A normally closed relay completes a circuit when the electromagnet coil doesn’t carry current, and breaks the circuit when coil current flows. (“Normal” in this sense means “no current in the coil.”) In contrast, a normally open relay breaks the circuit when the coil doesn’t carry current, and completes the circuit when coil current flows. The relay portrayed in Fig. 3-32 can serve as a normally open or normally closed relay, depending on which contacts you select. The device can also switch a single line between two different circuits.

Conductors and Cables

In a schematic, a solid line commonly symbolizes an electrical conductor. Most circuits contain many conductors. When you draw a schematic of a complicated circuit, you’ll often need to draw lines that cross over each other on your screen or on paper, whether the wires make contact in the real world or not.

Figure 3-33 shows two conductors that cross each other in a diagram, but that don’t connect in the actual circuit. This drawing geometry doesn’t necessarily mean that when you build the real circuit, the conductors come near each other in that vicinity. But when you compose the schematic, you must draw one conductor across the other to avoid confusion and minimize clutter.

FIG. 3-33   Symbol for conductors that cross paths in a schematic but don’t connect in the real world.

Figure 3-34 shows two ways of portraying a point where two wires cross and they do electrically connect there. In the rendition at A, you “split a conductor” so it seems to contact the other one at two different points. This geometry makes it clear that the two conductors (the “split” vertical one and the “solid” horizontal one) connect in the real circuit. Black dots portray electrical contact. In the drawing at B, the two conductors simply cross each other, and you draw a single black dot at the junction. The dot tells your readers that the conductors connect where they cross. The method shown at B might look “cleaner” at first glance, but with this neatness comes a problem: Some readers might overlook the black dot and think that the two conductors do not connect. The method at A avoids such misunderstandings.

FIG. 3-34   At A, preferred symbol for conductors that cross paths in a schematic and actually connect in the real world. At B, alternative symbol for the same scenario.

In some older schematics you’ll see crossed wires shown as in Fig. 3-35. These wires don’t connect in the real circuit. One of the lines has a half loop or “jog” that makes it seem to “jump” over the other line. That trick (which should never have gone out of style, in my opinion) eliminates all doubt as to whether or not the actual wires connect where the lines cross.

FIG. 3-35   Archaic (but clear) representation of conductors that cross paths in a schematic but don’t connect in the real world.

A cable has one or more conductors inside a single insulating jacket. In many cases, unshielded cables are not specifically indicated in a schematic drawing, but appear as two or more lines that run parallel to indicate multiple conductors. Shielded cables require additional symbology along with the conductors. Figures 3-36 A and B show symbols for shielded cable. You’ll see these symbols drawn to indicate coaxial cable, which comprises a single wire called the center conductor surrounded by a cylindrical, conduit-like conductive shield. At A, the shield does not connect to anything in particular, but at B, the shield connects to an earth ground. An insulating layer, called the dielectric, keeps the center conductor isolated from the shield. In most coaxial cables, the dielectric material consists of solid or foamed polyethylene.

FIG. 3-36   At A, symbol for coaxial cable with an ungrounded shield. At B, symbol for coaxial cable with an earth-grounded shield.

FIG. 3-37   Symbol for coaxial cable with a chassis-grounded shield.

In some cables, a single shield surrounds two or more conductors. Figure 3-38 shows the symbol for two-conductor shielded cable whose shield goes to a chassis ground. This symbol looks like the one for single-conductor coaxial cable, except that it has an extra inner conductor. If shielded cable has more than two inner conductors, then the number of straight, parallel lines going through the elliptical part of the symbol tells you how many conductors run inside the shield. For example, if the cable in Fig. 3-38 had five inner conductors, then five horizontal lines would pass through the elliptical part of the symbol.

FIG. 3-38   Symbol for two-conductor cable with a chassis-grounded shield.

Diodes and Transistors

Figure 3-39 is the common symbol for a semiconductor diode. An arrow and a vertical line indicate parts of the diode, and the horizontal lines to the left and right indicate the leads. The arrowed part of the symbol corresponds to the anode, and the short, straight line at the arrow’s tip corresponds to the cathode.

FIG. 3-39   Symbol for a general-purpose semiconductor diode.

An ideal diode conducts current when electrons move against the arrow so the anode has a positive voltage with respect to the cathode. Engineers call that condition forward bias. The ideal diode does not conduct when the cathode has a positive voltage with respect to the anode. Engineers call that a state of reverse bias. But of course, nothing is “ideal” in this imperfect universe.

Figure 3-40 portrays three specialized diode types. Drawing A shows the symbol for a varactor diode, which can act as a variable capacitor when you apply a fluctuating reverse-bias voltage. Drawing B shows the symbol for a Zener diode, which can serve as a voltage regulator in a power supply that converts AC to DC. Drawing C shows the symbol for a Gunn diode, which can generate or amplify radio signals at extremely high and microwave frequencies.

FIG. 3-40   Symbols for a varactor diode (A), a Zener diode (B), and a Gunn diode (C).

A silicon-controlled rectifier (SCR) is, in effect, a semiconductor diode with an extra element and terminal. You’ll see its symbol in Fig. 3-41. In the SCR representation, a circle or ellipse often (but not always) surrounds the diode symbol, and the control element, called the gate, appears as a diagonal line that runs outward from the tip of the arrow. In all cases, the arrow denotes the anode, and the vertical line at the arrow’s tip denotes the cathode.

FIG. 3-41   Symbol for a silicon-controlled rectifier (SCR).

Figure 3-42 shows schematic symbols for bipolar transistors. A so-called PNP transistor appears at A, and an NPN transistor appears at B. In the PNP symbol, the arrow points away from the emitter and toward the base. In the NPN symbol, the arrow points away from the base and toward the emitter. Some engineers leave out the circle that surrounds the combined base, emitter, and collector symbols.

FIG. 3-42   Symbols for a PNP bipolar transistor (A) and an NPN bipolar transistor (B).

FIG. 3-43   At A, symbol for an N-channel JFET. At B, symbol for a P-channel JFET. At C, symbol for an N-channel depletion-mode MOSFET. At D, symbol for a P-channel depletion-mode MOSFET.

Along with the bipolar variety, you’ll encounter other types of transistors. Figure 3-43 shows the symbols for four of these devices, as follows:

•   At A, you see an N-channel junction field-effect transistor (JFET).

•   At B, you see a P-channel JFET.

•   At C, you see an N-channel depletion-mode metal-oxide-semiconductor field-effect transistor (MOSFET).

•   At D, you see a P-channel depletion-mode MOSFET.

A depletion-mode MOSFET has an open (conducting) channel when you don’t apply any bias voltage between the source and the gate; when you do impose a bias voltage, the channel constricts and eventually closes off altogether. Then you have a state of pinchoff. Sometimes you’ll see another type of MOSFET in electronic circuits: the enhancement-mode MOSFET. An enhancement-mode device has a pinched-off channel unless you apply a bias voltage between the source and the gate. Then the channel opens wider and wider as you increase the bias voltage. Figure 3-44A is the symbol for an N-channel enhancement-mode MOSFET. Figure 3-44B shows the symbol for a P-channel enhancement-mode MOSFET.

FIG. 3-44   At A, symbol for an N-channel enhancement-mode MOSFET. At B, symbol for a P-channel enhancement-mode MOSFET.

Operational Amplifiers

An operational amplifier or op amp is a specialized integrated circuit (IC) that comprises bipolar transistors, resistors, diodes, and/or capacitors, all connected together to produce or modify a signal. (Myriad types of ICs exist besides the op amp. Figure 3-45 shows the general symbol for an IC.) Sometimes you’ll find two or more op amps in a single IC package; for example, you might encounter a dual op amp or a quad op amp.

FIG. 3-45   Generic symbol for an integrated circuit (IC).

Figure 3-46 is the schematic symbol for an op amp. The device has two inputs, one non-inverting, indicated by a plus (+) sign, and the other inverting, as shown by the minus (−) sign. When a signal enters the non-inverting input, the output wave emerges in phase coincidence (“right-side-up”) with respect to the the input wave. When a signal enters the inverting input, the output wave appears in phase opposition (“upside-down”) with respect to the input wave. The device has two power-supply connections, one for the emitters of the internal bipolar transistors (V_ee) and one for the collectors (V_cc).

FIG. 3-46   Symbol for an operational amplifier (op amp).

When a signal comes into either input for amplification, you can place a resistor between the output and the inverting input to cause negative feedback that reduces or controls the gain. As you reduce the value of the resistor, the gain decreases because the negative feedback increases. This state of affairs is called the closed-loop configuration.

If you install a resistance-capacitance (RC) combination in the inverting-feedback loop of an op amp, the gain depends on the frequency of the signal that enters the device. Using specific values of resistance and capacitance, you can make a frequency-sensitive filter that provides any of four different characteristics as shown in Fig. 3-47:

FIG. 3-47   Gain-versus-frequency response curves. At A, lowpass; at B, highpass; at C, resonant peak; at D, resonant notch.

•   A lowpass response that favors low frequencies (A).

•   A highpass response that favors high frequencies (B).

•   A resonant peak with maximum gain at a single frequency (C).

•   A resonant notch with minimum gain at a single frequency (D).

Electron Tubes

Although you won’t encounter electron tubes (often simply called tubes) as frequently as you would have a few decades ago, plenty of circuits and systems still use them. When you want to create the symbol for a tube, you should draw a circle and then add the necessary symbols inside the circle to portray the type of tube involved. Figure 3-48 shows the symbols for various internal tube elements.

FIG. 3-48   Symbols for electron-tube elements and characteristics. A: Filament or directly heated cathode. B: Indirectly heated cathode. C: Cold cathode. D: Photocathode. E: Grid. F: Anode (plate). G: Beam-deflection plate. H: Beam-focusing plates. I: Envelope (enclosure) for a vacuum tube. J: Envelope for a gas-filled tube.

Figure 3-49 shows the schematic symbol for a diode vacuum tube. This two-element component contains an anode (also called a plate) and a cathode. As with a semiconductor diode, the anode normally carries a more positive voltage than the cathode when the device conducts current. When the anode has a more negative voltage than the cathode, the device generally does not conduct. The cathode emits electrons that travel through the vacuum to the anode. A hot-wire filament, something like a miniature incandescent bulb, heats the cathode to help drive electrons from it. In Fig. 3-49, the filament has been omitted for simplicity, a common practice in vacuum tube symbols when the filament and cathode are physically separate, an arrangement known as an indirectly heated cathode.

FIG. 3-49   Schematic symbol for a diode vacuum tube with an indirectly heated cathode. Although a filament exists, it’s often omitted to reduce clutter.

Figure 3-50 shows two versions of a triode vacuum tube, which consists of the same elements as the diode previously discussed, with the addition of a dashed line to indicate the grid. But another difference exists in Fig. 3-50A compared with Fig. 3-49. Look closely at the cathode. The tube shown in Fig. 3-50A has a directly heated cathode, in which the filament and the cathode are the same physical object! You apply the negative cathode voltage directly to the filament. Figure 3-50B shows the symbol for a triode tube with an indirectly heated cathode. In this case the filament resides inside the cathode, which physically takes the form of a metal cylinder running along the central vertical axis of the tube, surrounding the filament.

FIG. 3-50   Symbols for triode tubes with a directly heated cathode (A) and an indirectly heated cathode (B).

Tetrode vacuum tubes have two grids, so you must draw an additional dashed line as shown in Figs. 3-51 A and B. In the tetrode, the upper grid, closer to the anode, is called the screen grid (or simply the screen). Figure 3-52 shows symbols for a pentode tube, which has three grids and a total of five elements. In the pentode, the middle grid is the screen, and the top grid is called the suppressor grid (or simply the suppressor).

FIG. 3-51   Symbols for tetrode tubes with a directly heated cathode (A) and an indirectly heated cathode (B).

FIG. 3-52   Symbols for pentode tubes with a directly heated cathode (A) and an indirectly heated cathode (B).

Figures 3-51A and 3-52A portray tubes with directly heated cathodes, while Figs. 3-51B and 3-52B symbolize tubes with indirectly heated cathodes.

Some tubes consist of two separate, independent sets of electrodes housed in a single envelope. These components are called dual tubes. If the two sets of electrodes are identical, the entire component is called a dual diode, dual triode, dual tetrode, or dual pentode. Figure 3-53 shows the schematic symbol for a dual triode vacuum tube with indirectly heated cathodes.

FIG. 3-53   Symbol for a dual triode tube with indirectly heated cathodes.

In old radio and television receivers, you’ll sometimes find tubes with four or five grids. These tubes have six and seven elements, respectively, and are called a hexode and a heptode. They work for mixing, a process in which two RF signals having different frequencies combine to produce new signals at the sum and difference frequencies. Figure 3-54A shows the schematic symbol for a hexode; Fig. 3-54B shows the symbol for a heptode. Some engineers call the heptode a pentagrid converter.

FIG. 3-54   At A, symbol for a hexode tube. At B, symbol for a heptode tube, also called a pentagrid converter. Both symbols show tubes with indirectly heated cathodes.

Electrochemical Cells and Batteries

You’ll often see a cell or battery employed as the power source for a circuit or system. Figure 3-55 shows the schematic symbol for a single electrochemical cell, such as the sort that you’ll find in a flashlight. Most such cells produce approximately 1.5 V DC.

FIG. 3-55   Symbol for a single electrochemical cell.

Electrochemical batteries with higher voltage outputs comprise multiple cells connected in series (negative-to-positive in a chain or string); the symbol for a multicell battery takes this design into account as shown in Fig. 3-56.

FIG. 3-56   Symmbol for a self-contained multicell electrochemical battery.

If a circuit calls for a multicell battery in the form of two or more discrete single cells in series, you can draw the single-cell symbols individually with wire conductor symbols between them. Figure 3-57 shows an example with three cells.

FIG. 3-57   Symbol for three single electrochemical cells connected in series to form a battery.

Standard practice calls for polarity signs to go with the symbols for cells or batteries. Unfortunately, some people neglect this detail. Then when you look at the schematic, you’ll have to infer the cell or battery polarity by scrutinizing the rest of the circuit. (The longer end line usually goes with the positive terminal, but not always.)

Logic Gates

All digital electronic devices employ switches that perform specific logical operations. These switches, called logic gates, can have from one to several inputs and a single output. Logic devices have two states, represented by the digits 0 and 1. In most cases, 0 represents the low state and 1 represents the high state.

•   A logical inverter, also called a NOT gate, has one input and one output. It reverses, or inverts, the state of the input. If the input equals 1, then the output equals 0. If the input equals 0, then the output equals 1. Figure 3-58A shows its symbol.

•   An OR gate can have two or more inputs (although it usually has only two). If both, or all, of the inputs equal 0, then the output equals 0. If any input equals 1, then the output equals 1. Mathematicians would say that such a gate performs an inclusive-OR operation. Figure 3-58B shows its symbol.

•   An AND gate can have two or more inputs (although it usually has only two). If both, or all, of the inputs equal 1, then the output equals 1. If any input equals 0, then the output equals 0. Figure 3-58C shows its symbol.

•   An OR gate can be followed by a NOT gate. This combination gives you a NOT-OR gate, more often called a NOR gate. If both, or all, of the inputs equal 0, then the output equals 1. If any inputs equals 1, then the output equals 0. Figure 3-58D shows its symbol.

FIG. 3-58   Symbols for a logical inverter or NOT gate (A), an OR gate (B), an AND gate (C), a NOR gate (D), a NAND gate (E), and an XOR gate (F).

•   An AND gate can be followed by a NOT gate. This combination gives you a NOT-AND gate, more often called a NAND gate. If both, or all, of the inputs equal 1, then the output equals 0. If any input equals 0, then the output equals 1. Figure 3-58E shows its symbol.

•   An exclusive OR gate, also called an XOR gate, has two inputs and one output. If the two inputs have the same state (both 1 or both 0), then the output equals 0. If the two inputs have different states, then the output equals 1. Mathematicians would say that such a gate performs the exclusive-OR operation. Figure 3-58F shows its symbol.

TABLE 3-1   Logic gates and their characteristics.

Summary

This chapter identifies most of the symbols that you’ll see in schematics, but plenty of symbols exist that are not included here. You’ll find a comprehensive table of schematic symbols in Appendix A. You’ll see symbols for jacks and plugs, piezoelectric crystals, lamps, microphones, meters, antennas, and many other components.

Do you expect to have a difficult time memorizing all these symbols? Well, don’t worry; you’ll get to know them over time. Look at some simple schematics after you’ve read this chapter. Refer to Appendix A whenever you see a symbol that you can’t identify. Within a few hours you’ll want to move on to more complex schematics, again referencing the unknown symbols. After a few weekends of practice, you’ll recognize most symbols without even thinking.

In electricity and electronics, most symbols derive from the structures of the components they represent. Schematic symbols often appear in groups, each of which bears some relationship to the others. For example, you’ll encounter many different types of transistors, but they all look somewhat alike. The same rule applies to the symbols for diodes, resistors, capacitors, inductors, transformers, meters, lamps, and most other electronic components. Most, but not all. A few “renegade” symbols exist that defy reason. All you can do with these things is memorize them, scratch your head, and laugh.