Circuit Components
Passive components
Know resistor color codes and tolerances, capacitor types and their uses, and inductor behavior. Capacitors block DC and pass AC; inductors pass DC and block AC. Electrolytic capacitors are polarized and can fail if reversed. Know that capacitors in parallel add (C = C1 + C2) while capacitors in series combine like parallel resistors. Inductors in series add, and in parallel combine like parallel resistors.
Semiconductors
Know the basics of diodes, LEDs, and transistors. A diode conducts when forward biased (anode positive relative to cathode) and blocks in reverse. A zener diode regulates voltage when reverse biased. A bipolar transistor has base, collector, and emitter; a small base current controls a larger collector current. A field-effect transistor is controlled by gate voltage. Know that a varactor diode acts as a voltage-variable capacitor, useful in tuning circuits.
Transformers and filters
Transformers change impedance by the square of the turns ratio: a 4:1 turns ratio gives a 16:1 impedance transformation. Know that a balun converts between balanced and unbalanced lines. Filters: low-pass passes low frequencies, high-pass passes high, band-pass passes a band, and notch rejects a narrow band.
Worked example
A transformer has 100 turns on the primary and 50 on the secondary. Turns ratio = 2:1, so impedance ratio = 4:1. A 200 Ω primary load reflects as 50 Ω on the secondary side.
Common traps
Forgetting that capacitors combine opposite to resistors. Confusing a zener with a regular diode. Mixing up turns ratio and impedance ratio - impedance goes as the square of turns.