G6: Circuit Components

This group introduces the classic, everyday components. We will go one family at a time. For each, the test wants a specific fact or two, so watch for the bold numbers and phrases.

Batteries: how low you can go, and why resistance matters

A battery stores chemical energy and releases it as electricity. The most common big battery in a ham shack is the lead-acid type, the same chemistry as a car battery, named for the lead plates and acid inside. A standard one is labeled "12 volts," but that label is a nickname; the real voltage drifts up and down as the battery charges and drains.

Here is the key safety number: if you drain a 12-volt lead-acid battery too far, you permanently damage it. For the longest life, you should not let it fall below 10.5 volts. That is the floor. ("Discharge" just means using up the stored energy; the "minimum discharge voltage" is the lowest you should let it sink to.) So picture a gas gauge where the "empty" warning light comes on at 10.5 volts, not at zero, stop there to keep the battery healthy.

The other battery idea is internal resistance. Every battery has a tiny bit of resistance built into it, an unavoidable speed bump that the current has to push through on its way out. A battery with low internal resistance has a smaller speed bump, so it can shove out a lot of current quickly when something demands it. The advantage of low internal resistance is therefore high discharge current, the ability to deliver a big gulp of current when needed (think of starting a car or keying up a high-power transmitter). It is not mainly about long life or high voltage; it is about how hard the battery can push current right now.

Resistors: why wire-wound ones avoid RF

A resistor deliberately resists the flow of current, like a narrow spot in a hose. One way to build a resistor is to wind a long, thin resistance wire into a coil, that is a wire-wound resistor. It is rugged and handles a lot of heat, but it has a hidden flaw at radio frequencies.

Any coil of wire is also, by its very shape, an inductor (a part that resists changes in current, more on those below). So a wire-wound resistor is secretly part resistor and part coil. At RF (radio frequency, the fast-wiggling signals a radio works with) that accidental coiliness, its inductance, starts to matter and can throw the circuit off. That is exactly why wire-wound resistors should not be used in RF circuits: the resistor's inductance could make circuit performance unpredictable. The hidden coil behaves in ways the designer did not plan for.

Inductors: the part that becomes a capacitor

An inductor is a coil of wire that stores energy in a magnetic field and resists sudden changes in current. By itself it is simple, but here is the surprising fact the test loves. Every real inductor also has a little bit of stray capacitance (the ability to store charge) between its windings. The combination of the coil's inductance and that stray capacitance forms a natural tuned circuit with its own special frequency, called the self-resonant frequency.

What happens when you operate an inductor above its self-resonant frequency? The stray capacitance takes over and the part stops acting like a coil: it becomes capacitive. In other words, push the frequency high enough and an inductor flips its personality and behaves like a capacitor instead. (You do not need to calculate this for the test, just remember the flip: above self-resonance, an inductor goes capacitive.)

Capacitors: picking the right type for the job

A capacitor stores electric charge between two plates and is great at passing fast-changing (AC) signals while blocking steady (DC) ones. There are many flavors, each with strengths and weaknesses, and the test asks you to match the type to its trait. ("Tolerance" means how close a part is to its marked value; "tight tolerance" means very accurate. "Leakage" means a little unwanted current sneaking through. "High capacitance for a given volume" means it packs a lot of storage into a small physical size.)

• Electrolytic capacitor: its standout trait is high capacitance for a given volume. It crams a large amount of storage into a small can, which is why you see them in power supplies. The trade-off is loose tolerance and more leakage, so they are not for precision RF work.
• Low-voltage ceramic capacitor: its standout trait is being comparatively low cost. They are the cheap, tiny everyday capacitors found all over a circuit board. (Do not confuse these inexpensive ones with the special high-stability ceramics; the test's plain "low voltage ceramic" answer is "low cost.")

Diodes: the one-way valves and their turn-on voltage

A diode is a one-way valve for current: it lets current flow easily in one direction and blocks it in the other. But the valve does not open at zero, it takes a small push to get current flowing forward. That push is called the forward threshold voltage (also called the turn-on or "knee" voltage). "Forward" means the direction the diode allows; "threshold" means the level you must reach before it conducts. The exact number depends on what the diode is made of:

Diode material	Forward threshold voltage
Silicon junction diode (the common kind)	about 0.7 volts
Germanium diode (an older material)	about 0.3 volts

Memory trick: Silicon is the bigger, more common word, give it the bigger number, 0.7. Germanium is the older, smaller-use material, give it the smaller number, 0.3.

Transistors: switches, and how a MOSFET is built

A transistor is a tiny part that uses a small signal to control a much larger current, so it can act as an amplifier (making signals bigger) or as a switch (turning current fully on or off). One common family is the bipolar transistor (often called a BJT, for "bipolar junction transistor").

When a bipolar transistor is used as a switch, it works at two extreme operating points: saturation and cutoff. "Cutoff" means fully off, no current flows, like an open light switch. "Saturation" means fully on, conducting as hard as it can, like a closed switch. A switch only ever wants to be all-the-way-on or all-the-way-off, so those two points, saturation and cutoff, are exactly right. (The in-between "active region" is where you would run it as an amplifier, not a switch.)

Another important family is the MOSFET, which stands for Metal-Oxide-Semiconductor Field-Effect Transistor. That mouthful describes how it is built, and the test asks about one structural detail. In a MOSFET, the control terminal, called the gate, is separated from the channel by a thin insulating layer. ("Channel" is the path the main current flows through; the gate sits just above it.) That thin insulator is the defining feature of a MOSFET: the gate does not touch the channel at all, it controls it from across a tiny insulating gap, like a magnet moving something through a pane of glass.

Vacuum tubes: the grandparents, still around

Before transistors, radios used vacuum tubes, glass bulbs with electrodes inside and the air pumped out. You still find big tubes in high-power amplifiers. Inside a tube, electrons boil off a hot piece called the cathode and fly across to a positive plate called the plate (or "anode"). Between them sit one or more wire mesh screens called grids, and the test asks what two of them do:

• The control grid is the one that regulates the flow of electrons between cathode and plate. A small voltage on this grid throttles the big electron stream, exactly like a transistor's gate or base. It is the "volume knob" inside the tube.
• The screen grid's primary purpose is to reduce grid-to-plate capacitance. Without it, stray capacitance between the control grid and the plate would let the signal feed back and cause instability; the screen grid sits in between and shields them from each other.

Memory trick: Control grid controls the electron flow. Screen grid acts as a screen (a shield) that cuts grid-to-plate capacitance.

This group jumps from single parts to integrated circuits (whole circuits shrunk onto one tiny chip), plus the lights and displays we read, the connectors on our cables, and the magic magnetic donuts called ferrites. Let's take them in order.

Integrated circuits: analog vs. digital

An integrated circuit, or IC (also nicknamed a "chip"), packs many components, sometimes millions, onto one small sliver of silicon. ICs come in two big families, and the test wants you to tell them apart:

• Analog ICs work with smoothly varying signals (any value in a range, like a dimmer knob).
• Digital ICs work with on/off, 1-or-0 signals (like a light switch).

The star example is the operational amplifier, almost always called an op-amp. An op-amp is an extremely flexible amplifier building block. The fact to lock in: an integrated-circuit operational amplifier is an analog device. It handles those smooth, continuously varying signals, so "analog" is the answer.

CMOS vs. TTL: a power-saving showdown

Digital chips come in different internal technologies. Two old rivals are TTL (Transistor-Transistor Logic) and CMOS (Complementary Metal-Oxide-Semiconductor, related to the MOSFETs from G6A). You do not need the chemistry, just one comparison: the big advantage of CMOS over TTL is low power consumption. CMOS chips sip power instead of guzzling it, which is why they took over battery-powered gadgets. So whenever the test pits CMOS against TTL, the CMOS advantage is "uses less power."

MMIC: a chip for microwaves

Up at very high frequencies (microwaves), designers use a special kind of chip called an MMIC, which stands for Monolithic Microwave Integrated Circuit. Let's unpack that: "Monolithic" means built as one solid piece; "Microwave" means it works at those very high frequencies; "Integrated Circuit" means it is a chip. So an MMIC is a single-chip building block, often a small amplifier, made to work at microwave frequencies. Just remember the words behind the letters: Monolithic Microwave Integrated Circuit.

Display devices: the LED

An LED (Light-Emitting Diode) is a diode that glows when current passes through it, the little indicator lights and modern display elements on your gear. Because it is a diode (a one-way valve, just like in G6A), it only conducts and lights up when current flows the direction it allows. So an LED emits light when it is forward biased. ("Bias" means the steady voltage you apply to set a part's operating condition; "forward biased" means voltage applied in the direction the diode conducts.) Reverse-bias it and it stays dark. Remember: an LED lights up only when forward biased.

RF connectors: matching the plug to the job

The plugs on the ends of your coaxial cables are RF connectors, and different ones are good up to different frequencies. ("SWR," standing wave ratio, is a measure of how well a connector or cable matches the system; "low SWR" means a clean, efficient connection. "GHz" means gigahertz, a thousand megahertz, used for very high frequencies.) Here are the ones the test asks about:

Connector	What to remember
BNC	A small twist-lock connector; good for low SWR up to about 4 GHz.
Type N	A moisture-resistant RF connector useful to 10 GHz, a rugged, weatherproof choice for higher frequencies.
SMA	A small threaded connector suitable for signals up to several GHz, the tiny screw-on type seen on handhelds and test gear.
RCA Phono	The everyday audio-style plug; commonly used for low-frequency or DC signal connections to a transceiver.

Memory trick: BNC = "Bayonet, to 4 GHz." Type N = "Nature-proof (moisture-resistant), to 10 GHz." SMA = "Small, threaded, several GHz." RCA = "the cheap audio plug for DC and low-frequency stuff."

Ferrites: the magnetic donuts that tame stray RF

A ferrite is a special magnetic ceramic material, usually shaped like a ring or bead, that you slip wire or cable through. Ferrites do two big jobs for hams, and the test covers both.

First, as the heart of a toroidal inductor. A "toroid" is just a donut shape; a toroidal inductor is a coil wound around a ferrite donut. Why use one? All of these reasons at once: large values of inductance may be obtained, the magnetic properties of the core may be optimized for a specific range of frequencies, and most of the magnetic field is contained in the core (so it does not spray out and interfere with nearby parts). Because every one of those is true, the test answer is "all these choices are correct."

That middle point leads to a separate question: what actually decides how a ferrite core performs at different frequencies? It is the composition, or "mix," of materials used. Manufacturers blend different recipes ("mixes") of ferrite, and the chosen mix is what makes a core good for, say, HF versus VHF. So performance comes down to the material mix, not its thickness or its size.

Second, ferrites are used as a bead or core to choke off unwanted RF. A common problem is common-mode current, stray RF riding along the outside shield of a coaxial cable where it does not belong (it can cause interference). Slip a ferrite over the cable and it stops that stray current. How? By creating an impedance in the current's path. ("Impedance" is opposition to AC current; the ferrite makes it hard for the stray RF to flow.) The ferrite simply throws up a roadblock the unwanted current cannot easily get through. Remember: ferrite bead reduces common-mode current by creating an impedance in the current's path.