E6: Circuit Components

This first group is the foundation for almost everything else in E6. We start with the raw materials, the doped crystals, then build them into the two great families of transistors.

What a semiconductor is, and what "doping" does

A pure semiconductor crystal like silicon barely conducts at all. The trick that makes it useful is doping, deliberately mixing in a tiny amount of a different element (an "impurity") to change how the crystal carries current. Depending on which impurity you add, you get one of two flavors:

• N-type material has excess free electrons. (The "N" is for negative, since electrons carry negative charge.) These extra electrons are free to move and carry current.
• P-type material has a shortage of electrons, which leaves behind empty spots called holes. A hole acts like a positive charge that can move through the crystal. (The "P" is for positive.)

The impurities themselves have names. An impurity atom that adds holes to the crystal is called an acceptor impurity (think of it as a spot that is ready to "accept" an electron, leaving a hole behind). The opposite kind, which donates extra electrons, is a "donor impurity," but the test specifically asks about the acceptor.

Different semiconductor materials for different jobs

Silicon is the everyday workhorse, but it is not the only choice. Gallium arsenide (GaAs) is a different semiconductor compound used in microwave circuits. The reason, which comes up again in group E6E, is that electrons move through gallium arsenide faster (it has higher "electron mobility"), which lets devices made from it work at very high frequencies where silicon would struggle.

The PN junction and the depletion region

Press a piece of P-type material against a piece of N-type material and the boundary between them is called a PN junction. This single junction is the heart of a diode. Right at the boundary, some electrons and holes combine and cancel out, leaving a thin zone with no free charge carriers. That empty zone is the depletion region ("depletion" means it has been emptied of carriers).

Now, why does a diode conduct one way but not the other? It depends on which way you apply the voltage:

• Reverse biased means you connect the voltage the "blocking" way. When reverse biased, a PN-junction diode does not conduct, because the applied voltage pulls the holes in the P-type material and the electrons in the N-type material apart, widening the depletion region. A wider empty zone means no path for current. ("Bias" just means a steady applied voltage that sets up the operating condition.)

Bipolar junction transistors (BJTs)

Stack three doped layers (either N-P-N or P-N-P) and you build a bipolar junction transistor, usually shortened to BJT. Its three connections are the emitter, base, and collector. A small current into the base controls a much larger current from collector to emitter, which is how it amplifies.

• Beta is the measure of that amplifying power. Formally, beta is the change in collector current with respect to the change in base current. In plain terms, it tells you how many times larger the controlled (collector) current is compared to the small controlling (base) current. A higher beta means more current gain.
• Turning it on: for a silicon NPN transistor, you know it is biased on (conducting) when the base-to-emitter voltage is approximately 0.6 to 0.7 volts. That roughly 0.65-volt figure is the signature of a forward-biased silicon junction, so memorize "about 0.6 to 0.7 volts."
• Alpha cutoff frequency: a transistor cannot amplify equally well at every frequency, eventually its gain falls off as the frequency climbs. The alpha cutoff frequency is the frequency at which the grounded-base current gain has dropped to 0.7 of the gain it had at 1 kHz. It is essentially a high-frequency limit marker for the device.

Field-effect transistors (FETs)

The other great family is the field-effect transistor, or FET. Instead of a small current controlling a large one, an FET uses a small voltage on its control terminal (the gate) to control current between its other two terminals (the source and drain). Because the gate controls current with an electric field rather than by drawing current itself, an FET has a key advantage:

• Input impedance: the DC input impedance at the gate of an FET is higher than that of a bipolar transistor. ("Input impedance" is how much the input resists drawing current; a high value means the FET barely loads down whatever drives it. That is often very desirable.)
• Depletion-mode FET: this is a type of FET that conducts current between source and drain even when no voltage is applied to the gate. You apply gate voltage to reduce the current. (The opposite kind, an "enhancement-mode" FET, is off until you apply gate voltage to turn it on.)

A common FET variant is the MOSFET (metal-oxide-semiconductor FET). Its gate is insulated by an extremely thin oxide layer, which is great for high input impedance but makes the gate fragile. A jolt of static electricity can punch right through that thin layer and ruin the part. To prevent that, manufacturers often connect Zener diodes between the MOSFET gate and its source or drain, their purpose is to protect the gate from static damage by safely shunting away dangerous voltage spikes.

Reading the symbols in Figure E6-1

Two questions ask you to identify FET symbols in Figure E6-1. The schematic symbols differ in subtle ways, the direction of an arrow shows whether a junction FET is N-channel or P-channel, and a dual-gate MOSFET shows two separate gate connections. For the exam, just learn the matching answers: in Figure E6-1, the N-channel dual-gate MOSFET is symbol 4, and the P-channel junction FET is symbol 1. When you study, pull up the official figure and trace each symbol so the numbers connect to real pictures rather than rote digits.

A basic diode is a one-way valve for current. But by changing the materials and the geometry, engineers have created a whole zoo of specialized diodes, each tuned for a particular job. This group is mostly about recognizing which diode does what.

Zener diode: a voltage reference

A Zener diode is built to be run backwards (reverse biased) on purpose. When the reverse voltage reaches its rated value, it begins conducting and then holds that voltage steady. Its most useful characteristic is providing a constant voltage drop under conditions of varying current. That makes it perfect as a simple voltage reference or regulator, no matter how the current wiggles, the voltage across it stays put. (This is the same property that lets Zeners protect a MOSFET gate, as we saw in E6A.)

Schottky diode: fast and low-loss

A Schottky diode is built from a metal-semiconductor junction (that metal-to-semiconductor contact, rather than the usual P-to-N junction, is the definition of a "Schottky barrier diode"). It has two standout traits:

• A lower forward voltage drop than an ordinary silicon junction diode. That lower drop wastes less power, which is exactly why a Schottky is a better choice as a power supply rectifier (a "rectifier" is a diode used to convert AC into DC).
• It is fast, which makes it a common choice as a VHF/UHF mixer or detector (a "mixer" combines two signals; a "detector" recovers information from a signal).

In Figure E6-2, the Schottky diode is symbol 6. Its schematic symbol looks like a regular diode but with a distinctive S-shaped or stepped flag on the bar.

LED: the light-emitting diode

A light-emitting diode, or LED, gives off light when current flows through it. The color of light, and the voltage it needs to turn on, are set by a property of its semiconductor material called the band gap. So the LED's forward voltage drop is determined by the band gap of its material. (The "band gap" is the energy step an electron must jump; a bigger gap means higher voltage and more energetic, bluer light.)

Varactor diode: a voltage-controlled capacitor

A varactor diode is a semiconductor device designed for use as a voltage-controlled capacitor. Remember that reverse voltage widens the depletion region (from E6A); changing the width changes the diode's capacitance. By varying the reverse voltage, you electronically tune the capacitance, which is how radios tune themselves without a mechanical knob.

PIN diode: an RF switch and attenuator

A PIN diode has a thin layer of pure ("intrinsic") semiconductor sandwiched between the P and N layers, which gives it low junction capacitance. That low capacitance is what makes it useful as an RF switch (low capacitance means it does not leak the signal through when it is supposed to be off). You control how much it attenuates (weakens) an RF signal by adjusting its forward DC bias current, more bias current, more conduction, less attenuation.

Point-contact diode: an RF detector

A point-contact diode uses a fine wire touching a semiconductor at a tiny point, giving it very low capacitance and good high-frequency behavior. Its common use is as an RF detector.

How diodes die

What kills a junction diode when you push too much current through it? Excessive junction temperature. Current heats the junction, and if it gets too hot, the device is destroyed. (Notice the failure is about heat, not the current number by itself, which is why heat sinking matters.)

Quick reference table

Diode type	Key talent / use
Zener	Constant voltage drop with varying current (voltage reference)
Schottky	Low forward drop; good rectifier and VHF/UHF mixer/detector
LED	Emits light; forward voltage set by band gap
Varactor	Voltage-controlled capacitor (electronic tuning)
PIN	RF switch / attenuator, controlled by forward DC bias current
Point-contact	RF detector

This group moves from individual parts to integrated circuits that contain many components at once, specifically the digital kind that work with the two-state world of ones and zeros. We cover the logic "families" (different ways of building the gates), a few special functions, and the gate symbols in Figure E6-3.

Comparators and hysteresis

A comparator is a circuit that compares an input voltage against a reference (the "threshold voltage") and flips its output depending on which is larger. When the input signal crosses the threshold voltage, the comparator changes its output state, that is its whole job.

The catch: if the input hovers right at the threshold with a little noise on it, the output can flicker rapidly and uselessly. The cure is hysteresis, which means building in a small gap between the turn-on and turn-off thresholds. The function of hysteresis in a comparator is to prevent input noise from causing unstable output signals. (Think of a thermostat that waits until the room is a couple of degrees too cold before turning the heat back on, so it does not click on and off constantly.)

Tri-state logic

Ordinary logic has two output states: 0 (low) and 1 (high). Tri-state logic adds a third option, a high-impedance state. So tri-state logic means logic devices with 0, 1, and high-impedance output states. The high-impedance state is like the output disconnecting itself, which lets many devices share a single wire (a "bus") without fighting each other.

Logic families: CMOS and BiCMOS

A "logic family" is a particular technology for building gates. Two come up here:

• CMOS (complementary metal-oxide-semiconductor) has the lowest power consumption of the common logic families, it draws almost no power except when switching. CMOS also has high immunity to noise on the input or power supply, because its input switching threshold is about half the power supply voltage. A noise spike has to be quite large (about half the supply) to fool it.
• BiCMOS combines bipolar transistors with CMOS on one chip. Its advantage: it has the high input impedance of CMOS and the low output impedance of bipolar transistors, the best of both worlds, easy to drive yet able to deliver current.

Pull-up and pull-down resistors

A pull-up or pull-down resistor is a resistor connected to the positive or negative supply, used to establish a voltage when an input or output is an open circuit. In plain terms, it makes sure a floating (unconnected) line settles to a known 1 or 0 instead of drifting to a random, noise-driven value. A pull-up ties the line toward the positive supply; a pull-down ties it toward ground.

FPGAs and hardware description language

A field-programmable gate array, or FPGA, is a chip full of logic that you configure yourself after it is manufactured (hence "field-programmable", you program it out in the field). You design its configuration using a hardware description language (HDL), a special programming-style language for describing digital hardware.

Gate symbols in Figure E6-3

Three questions ask you to identify gate symbols in Figure E6-3. A logic gate performs a basic logical operation; the little circle (a "bubble") on a symbol means the output is inverted. Memorize these matches:

Gate	Symbol in Figure E6-3
NAND gate	2
NOR gate	4
NOT (inversion)	5

As with the other figures, study the real picture so each number is attached to an actual shape, the D-shape with a bubble for NAND, the curved-back shape with a bubble for NOR, and the triangle with a bubble for NOT.

This group steps away from semiconductors to cover two unrelated topics that share a group: the magnetic cores inside inductors and transformers, and materials that turn pressure into voltage (and back).

Permeability: what sets an inductor's value

An inductor is a coil, often wound on a core of magnetic material. The core property that determines the inductance is its permeability, a measure of how readily the material carries magnetic field. A higher-permeability core concentrates the magnetic field, so you get more inductance from fewer turns of wire.

That directly explains a comparison the test asks: ferrite cores generally require fewer turns to produce a given inductance value than powdered-iron cores, because ferrite has higher permeability. (Ferrite is a ceramic-like magnetic material; powdered iron is exactly what it sounds like, finely ground iron bound together.)

Core trade-offs: stability, eddy currents, and beads

• Temperature stability: of the common core materials, powdered iron has the highest temperature stability of its magnetic characteristics. So when a circuit needs to hold its value steady as temperature drifts, powdered iron is the choice.
• Laminated cores: the cores of many inductors and transformers are built from thin layers (laminations) to reduce power loss from eddy currents in the core. ("Eddy currents" are little circulating currents that the changing magnetic field induces inside a solid metal core; they waste energy as heat. Slicing the core into thin insulated layers breaks up those loops.)
• Ferrite beads: small ferrite beads slipped over a wire are commonly used as VHF and UHF parasitic suppressors at the input and output terminals of a transistor HF amplifier. (A "parasitic" is an unwanted oscillation the amplifier might break into; the bead adds loss at those high frequencies to choke it off.)

Toroidal versus solenoidal cores

A toroidal core is shaped like a doughnut; a solenoidal core is a straight rod or cylinder. The primary advantage of the toroid is that it confines most of the magnetic field within the core material. A closed doughnut loop keeps the field inside, so the inductor neither leaks magnetic field into nearby circuits nor picks up stray field from them.

Making inductance go down, and saturation

• Most cores raise inductance, but inserting a brass core into a coil decreases the inductance. (Brass is non-magnetic and conductive; it actually opposes the field, lowering the inductance, a handy trick for fine adjustment.)
• Inductor saturation is caused by operation at excessive magnetic flux. When the core carries so much magnetic flux that it can hold no more, it "saturates" and the inductance collapses. (Picture a sponge that has soaked up all the water it can hold, pour on more and it just runs off.)

Piezoelectricity and quartz crystals

The second half of this group is about piezoelectricity. That is the characteristic of certain materials that generate a voltage when stressed (squeezed or bent) and that flex when a voltage is applied. It works both directions, which is the key idea. One specific aspect of the piezoelectric effect is the mechanical deformation of a material due to the application of a voltage, apply a voltage and the material physically moves.

The most important piezoelectric device in radio is the quartz crystal, used to set precise frequencies. Electrically, a quartz crystal behaves like a series RLC circuit in parallel with a shunt capacitance that represents the electrodes and stray capacitance. (RLC means resistance, inductance, and capacitance together; the crystal mimics that combination, which is why it resonates so sharply at one frequency.)

This group zooms in on what makes a component work well at very high frequencies, the UHF, microwave, and beyond. Two things matter: the semiconductor material itself, and the physical package that holds it.

Fast materials for high frequencies

• Gallium arsenide (GaAs) is useful for devices operating at UHF and higher because of its higher electron mobility, electrons zip through it faster than through silicon, so it can keep up with very high-frequency signals. (This is the same fact behind GaAs being used in microwave circuits, from group E6A.)
• Gallium nitride (GaN) goes even further: among the materials listed, gallium nitride supports the highest frequency of operation when used in MMICs.

MMICs: monolithic microwave integrated circuits

An MMIC (monolithic microwave integrated circuit, often said "mimic") is a complete RF amplifier or function built onto a single chip for microwave use. Here is what the test wants you to know about them:

• Impedance: the most common input and output impedance of MMICs is 50 ohms. (Fifty ohms is the standard impedance for most RF systems and coaxial cable, so designing MMICs to 50 ohms makes them plug-and-play.)
• Why they are popular: MMICs offer controlled gain, low noise figure, and constant input and output impedance over the specified frequency range. That predictability across VHF through microwave is exactly what RF designers crave.
• Connections: the transmission line often used to connect to MMICs is microstrip, a flat copper trace over a ground plane on a circuit board, which behaves as a controlled-impedance RF line at these frequencies.
• Powering them: the most common type of MMIC gets its power supplied through a resistor and/or RF choke connected to the amplifier output lead. (Cleverly, the same lead carries both the output signal and the DC power; the resistor or choke feeds in the DC while keeping the RF from escaping back into the power supply.)

Noise figure

The noise figure tells you how much noise an amplifier adds to a signal, lower is better. For a good low-noise UHF preamplifier, a typical noise figure is about 0.5 dB. (A preamplifier sits at the front of a receiver to boost weak signals; you want it to add almost no noise of its own.)

Packages: through-hole versus surface-mount

The "package" is the physical housing and the legs (leads) that connect a chip to a circuit board. Two broad styles:

• Through-hole: the leads poke through holes in the board and are soldered on the far side. The classic example is the DIP (dual in-line package), characterized by two rows of connecting pins on opposite sides of the package.
• Surface-mount: tiny parts soldered directly onto pads on the board surface, with very short or no leads.

At RF, lead length is the enemy, because long leads add stray inductance and capacitance (collectively, "parasitic effects") that distort high-frequency signals. So:

• Surface-mount packages have the least parasitic effects at frequencies above the HF range, because their leads are so short.
• Compared with through-hole, surface-mount technology offers several RF advantages at once, so the correct answer to that question is "all these choices are correct."
• DIP through-hole ICs are not typically used at UHF and higher precisely because of their excessive lead length.

The final group of E6 covers components where light and electricity meet, parts that generate electricity from light, change their behavior when lit, or use light internally to do a job. "Electro-optical" simply means electrical-and-optical working together.

Photovoltaic cells: light into electricity

A photovoltaic cell (a solar cell) turns light directly into electrical energy. That conversion, the conversion of light to electrical energy, is called the photovoltaic effect. Here are the details the test asks for:

• What absorbs the energy from the incoming light? The electrons in the cell's material. They soak up the light energy and break free to form a current.
• The efficiency of a photovoltaic cell is the relative fraction of light that is converted to current, how good it is at turning the light it receives into useful electricity.
• The most common material for power-generating photovoltaic cells is silicon.
• A fully illuminated silicon photovoltaic cell produces an open-circuit voltage of about 0.5 volts. (That is per cell, which is why solar panels wire many cells in series to reach useful voltages.)

Photoconductive materials: light changes resistance

A photoconductive material changes how well it conducts depending on light. When light shines on it, its resistance decreases, more light, easier current flow. These devices are most commonly made from a crystalline semiconductor.

Optical shaft encoders

An optical shaft encoder is a device that detects rotation by interrupting a light source with a patterned wheel. As the shaft turns, a slotted or striped wheel chops a light beam into pulses that a sensor counts, that is how a modern radio's tuning knob tells the electronics how far you turned it.

Optoisolators and solid-state relays

An optoisolator (also called an optocoupler) passes a signal from one circuit to another using light, with no direct electrical connection between them. Its most common configuration is an LED and a phototransistor, the LED lights up with the input signal, and the nearby phototransistor sees that light and reproduces the signal on the output side.

Why bother? Because an optoisolator provides electrical isolation between a control circuit and the circuit being switched. That is exactly why optoisolators are often paired with solid-state circuits that control 120 VAC household power, the delicate low-voltage control side never touches the dangerous high-voltage side; only light crosses the gap.

A related part is the solid-state relay, which is a device that uses semiconductors to implement the functions of an electromechanical relay. In other words, it does the job of an old mechanical relay (switching a circuit on and off under control of another circuit) but with no moving parts, just electronics.