ASVAB Electronics — Components and Magnetism

Transistor: semiconductor device that AMPLIFIES or SWITCHES electronic signals. Invented at Bell Labs in 1947 (Bardeen, Brattain, Shockley — Nobel Prize 1956). Foundation of all modern electronics. Two main types: (1) BJT (Bipolar Junction Transistor) — three terminals: Base, Collector, Emitter; small current at base controls larger current between collector and emitter; comes in NPN and PNP polarities; (2) FET (Field-Effect Transistor) — three terminals: Gate, Drain, Source; voltage at gate controls current between drain and source; very high input impedance (doesn't draw current at gate); MOSFETs are most common — used in nearly all digital electronics. As AMPLIFIER: small input signal (e.g., audio from microphone, signal from antenna) varies the larger current flowing through the transistor; output is amplified version of input. Used in audio amplifiers, radio receivers/transmitters, RF circuits. As SWITCH: small base current (BJT) or gate voltage (FET) turns the transistor fully ON or fully OFF; replaces mechanical switches in electronic circuits; very fast (billions of switches per second); essential for digital electronics. Integrated circuits (ICs) pack millions to billions of transistors on a single silicon chip: microprocessor (modern CPU) has tens of billions of transistors; memory chips have similar counts. Moore's Law (Gordon Moore, 1965): transistor density on chips doubles approximately every 2 years — held remarkably true for decades. Modern transistor gate sizes: 3-5 nanometers (about 10-20 atoms wide). Transistor configurations: COMMON EMITTER (amplifies both current and voltage), COMMON BASE (amplifies voltage), COMMON COLLECTOR / EMITTER FOLLOWER (amplifies current). Identification: schematic symbol shows arrow indicating emitter direction (NPN arrow out, PNP arrow in). Transistor ratings: maximum voltage, maximum current, maximum power dissipation, gain (hFE for BJT, transconductance for FET). Common BJTs: 2N2222, 2N3904 (general purpose), 2N3055 (power). Common MOSFETs: 2N7000, IRF series (power switching).

Frequency = number of cycles per second of a periodic wave. Unit: HERTZ (Hz), named after Heinrich Hertz who demonstrated electromagnetic waves in 1886. 1 Hz = 1 cycle/second. Multiples: 1 kHz = 1,000 Hz; 1 MHz = 1,000,000 Hz; 1 GHz = 1,000,000,000 Hz; 1 THz = 10¹² Hz. Wavelength: λ = c/f, where c is the wave's speed and f is frequency. For electromagnetic waves in vacuum: c = 3 × 10⁸ m/s; for sound in air at room temperature: ~340 m/s. Higher frequency = shorter wavelength. Common frequencies: (1) HOUSEHOLD AC POWER: 60 Hz (US), 50 Hz (much of world); (2) HUMAN HEARING: 20 Hz - 20 kHz (declines with age); audio frequencies; (3) AM RADIO: 535-1605 kHz (medium wave); (4) FM RADIO: 88-108 MHz; (5) TV: VHF and UHF bands; (6) WIFI: 2.4 GHz, 5 GHz, 6 GHz (Wi-Fi 6E); (7) CELL PHONES: 700 MHz - 3.5 GHz typical; (8) GPS: ~1.5 GHz; (9) BLUETOOTH: 2.4 GHz; (10) MICROWAVES (oven): 2.45 GHz; (11) X-RAYS: 10¹⁶-10¹⁹ Hz; (12) GAMMA RAYS: above 10¹⁹ Hz. The ELECTROMAGNETIC SPECTRUM (lowest to highest frequency): radio waves → microwaves → infrared → visible light (red to violet) → ultraviolet → X-rays → gamma rays. Visible light is a tiny slice of the spectrum: ~430-770 THz, corresponding to wavelengths 400-700 nm. ASVAB EI may ask about RADIO bands and applications (AM, FM, VHF, UHF, microwave). AM vs FM: AM (Amplitude Modulation) varies wave amplitude with signal — older, longer range, more susceptible to noise; FM (Frequency Modulation) varies wave frequency with signal — higher quality, less noise, shorter range. SIDEBAND: in AM, audio signal creates upper and lower sidebands around the carrier frequency; SSB (single-sideband) uses only one for efficiency. Bandwidth: range of frequencies a signal or system occupies.

DIRECT CURRENT (DC): electrons flow in one direction only. Constant polarity. Examples: batteries (chemical reactions provide constant voltage), solar cells, DC generators, USB power, automotive 12V system, electronic devices internally. ALTERNATING CURRENT (AC): electrons reverse direction periodically. Voltage cycles between positive and negative peaks. In US household wiring, AC alternates 60 times per second (60 Hz). Most of the world uses 50 Hz. Why AC dominates electrical distribution: (1) TRANSFORMERS work with AC, allowing efficient voltage transformation; (2) HIGH VOLTAGE TRANSMISSION at low current minimizes I²R losses in wires; (3) Stepping down to safer voltages near use; (4) Easier to generate with rotating machinery (generators naturally produce AC). 'War of currents' (1880s-90s): Edison championed DC; Tesla/Westinghouse championed AC; AC won due to transformer-enabled distribution. Modern situation: AC for transmission and distribution; DC for electronics and increasingly for HVDC long-distance transmission (less loss, easier underwater/underground). Conversion: RECTIFIER (diodes) converts AC to DC — used in power supplies, chargers; INVERTER converts DC to AC — used in solar systems (panels produce DC, grid uses AC), UPS, electric vehicle drives. AC waveforms: typically SINUSOIDAL (smooth sine wave); also SQUARE WAVE (digital, switches between high and low); SAWTOOTH; triangular. AC voltage specifications: PEAK voltage (maximum positive or negative); PEAK-TO-PEAK (peak positive to peak negative); RMS (root mean square — equivalent DC voltage that delivers same power; for sine wave, RMS = peak / √2 ≈ 0.707 × peak); 120V US household is RMS — peak is about 170V. AC frequency in different countries: 60 Hz (US, Canada, much of Americas, parts of Asia); 50 Hz (Europe, Africa, most of Asia, Australia). Voltage: 120V (US, Canada, Mexico); 230V (Europe, much of world); 100V (Japan). International travelers need adapters and possibly voltage converters.

Integrated Circuit (IC): an electronic circuit fabricated on a single piece of semiconductor material (usually silicon). Contains anywhere from a few to billions of transistors plus other components — all interconnected in a single piece. Invented by Jack Kilby (Texas Instruments) and Robert Noyce (Fairchild Semiconductor) in 1958-1959. Common form factors: DIP (Dual In-line Package — through-hole), SOIC (Small Outline IC — surface mount), QFP (Quad Flat Pack), BGA (Ball Grid Array), and many specialty packages. Types of ICs: (1) ANALOG/LINEAR — process continuous signals: op-amps (operational amplifiers like 741, LM358), voltage regulators (7805, LM317), audio amplifiers, comparators; (2) DIGITAL — process binary signals: logic gates (74-series, 4000-series CMOS), flip-flops, counters; (3) MIXED-SIGNAL — combine analog and digital: ADCs (analog-to-digital converters), DACs, signal processors; (4) MICROPROCESSORS (CPUs) — programmable processors: Intel, AMD, ARM cores; modern CPUs have 10-50 billion transistors; (5) MICROCONTROLLERS — CPU + memory + I/O in one chip: Arduino's ATmega328, ESP32, STM32; (6) MEMORY — store data: DRAM (main computer memory), SRAM (cache), flash (SSDs, USB drives, smartphones), EEPROM; (7) ASIC (Application-Specific) — custom chips for specific functions: cryptocurrency miners, AI accelerators, video codecs; (8) FPGA (Field-Programmable Gate Array) — reconfigurable hardware. Manufacturing: photolithography — light patterns on photoresist coated silicon define structures; multiple layers built up; can be thousands of process steps; cleanroom environment; multi-billion-dollar fabrication facilities. Moore's Law: transistor count doubles ~every 2 years (held since 1965, slowing recently). Current process nodes: 3-5 nm (TSMC, Samsung). One modern smartphone has more computing power than NASA's entire infrastructure used during Apollo. ASVAB EI tests basic IC recognition, common types (741 op-amp, 555 timer, 7400-series logic), and concept of integration.

Antennas are bidirectional transducers between guided (wire) and unguided (free space) electromagnetic waves. RECEIVING: incoming radio waves induce voltages in the antenna; amplifier extracts and processes the signal. TRANSMITTING: oscillating current in the antenna radiates electromagnetic waves into space. Antenna types: (1) DIPOLE — simplest, two conductive elements; length typically 1/2 wavelength of operating frequency; common in radio; (2) MONOPOLE — single vertical element above ground plane (which acts as the second element via reflection); typical 1/4 wavelength; whip antennas on vehicles, AM tower antennas; (3) YAGI-UDA — directional antenna with driven element and parasitic reflectors/directors; high gain in one direction; classic TV antennas, ham radio; (4) PARABOLIC DISH — focuses signals from a focal point reflector; very high gain in one direction; satellite TV, radio telescopes, deep space communications; (5) LOOP — circular conductor; small loops are magnetic antennas; (6) PATCH/MICROSTRIP — flat antennas printed on circuit boards; common in GPS, WiFi, cell phones; (7) HELICAL — wound in helix shape; satellite communications; (8) PHASED ARRAY — multiple antenna elements with controlled phasing; can electronically steer beam direction; radar, modern 5G base stations. Antenna characteristics: (1) GAIN — directional concentration of energy compared to omnidirectional reference; (2) POLARIZATION — direction of the electric field (vertical, horizontal, circular); transmit and receive antennas should match; (3) BEAMWIDTH — angular width of main lobe; (4) IMPEDANCE — typically 50 or 75 ohms; must match feed line for efficient transfer; (5) BANDWIDTH — range of frequencies the antenna efficiently handles. Antenna length: roughly proportional to wavelength; lower frequencies need bigger antennas (AM radio antennas are huge, GHz cell phone antennas are tiny). Cell phones have multiple antennas: cellular, WiFi, Bluetooth, GPS, NFC. Modern smartphones cleverly use the device frame and internal structures as antennas. Antenna theory: an electron oscillating produces electromagnetic waves at the oscillation frequency; the wire 'pumps' electrons; the changing electric/magnetic fields propagate outward at light speed.

The TRANSISTOR (invented 1947, Bell Labs) is arguably the most important invention of the 20th century. TWO KEY FUNCTIONS: SWITCH: A small base current turns the collector-emitter current ON or OFF — this is how billions of transistors in microprocessors perform digital logic (0 or 1); AMPLIFIER: A small base signal controls a proportionally larger collector current — this is how audio amplifiers, radio receivers, and signal processing work. Modern CPUs contain billions of transistors on a single chip. Types: BJT (Bipolar Junction Transistor) — current-controlled; MOSFET (Metal-Oxide-Semiconductor FET) — voltage-controlled, used in digital circuits.

DC (Direct Current): Electrons flow in one consistent direction — positive terminal to negative terminal (conventional current). Source: batteries, solar cells, fuel cells. Used in: all electronics, electric vehicles, USB devices. AC (Alternating Current): Direction of electron flow reverses periodically. US standard: 60 Hz (60 cycles per second = 120 direction reversals/second); European standard: 50 Hz. Used in: power grid transmission (more efficient for long-distance), motors, household appliances. CONVERSION: DC → AC: Inverter; AC → DC: Rectifier (diodes). The power adapter for your laptop converts AC wall power to DC for the device.

An INDUCTOR (also called a coil or choke) is a coil of wire that stores energy in a magnetic field. KEY PROPERTY: Inductors OPPOSE CHANGES IN CURRENT (Lenz's Law) — this is called inductive reactance. When current increases: the magnetic field builds, inducing a back-EMF that opposes the increase; When current decreases: the collapsing field releases energy trying to maintain current. APPLICATIONS: Power supply filtering (smooths ripple); radio tuning circuits (with capacitors, forms LC resonant circuits); transformers (mutual inductance between coils); electric motors and generators. COMPARE with CAPACITOR: Capacitors oppose changes in VOLTAGE and store energy in an electric field; inductors oppose changes in CURRENT and store energy in a magnetic field.

A CAPACITOR STORES electrical energy (charge) in an electric field and releases it when needed. ASVAB Electronics Information tests components. Capacitors store charge on two plates separated by an insulator (dielectric); capacitance is measured in FARADS. Uses: smoothing/filtering power supplies, timing circuits, storing energy, blocking DC while passing AC. Unlike a resistor (which limits current) or battery (chemical storage), a capacitor stores energy electrostatically and can charge/discharge quickly. Knowing the capacitor stores and releases electrical energy is commonly tested.