RRB JE Electronics Engineering — Complete CBT-2 Study Guide

• Number systems: Binary (base 2), Octal (base 8), Decimal (base 10), Hexadecimal (base 16, digits 0-9 then A-F for 10-15) — conversions between them are a near-guaranteed exam topic. • Logic gates: AND (output 1 only if all inputs 1), OR (output 1 if any input 1), NOT (inverts input), NAND and NOR are "universal gates" — any logic circuit can be built using only NAND gates or only NOR gates. • Boolean algebra & De Morgan's theorems: (A·B)' = A'+B' and (A+B)' = A'·B' — used to simplify/convert logic expressions between AND-OR and NAND/NOR forms. • Flip-flops: SR (Set-Reset, has an invalid/forbidden state when both inputs are 1), D (Data, avoids the SR forbidden state, output simply follows input at clock edge), JK (resolves SR's forbidden state — toggles output when both J,K=1), T (Toggle, output flips every clock pulse when T=1). • Combinational vs Sequential circuits: Combinational circuits (adders, multiplexers, decoders) have output depending ONLY on current input, no memory; Sequential circuits (counters, registers, flip-flop based) have output depending on current input AND past state (memory element required, typically clock-driven).

• Diode: conducts in forward bias (P-side to +), blocks in reverse bias; Silicon diode forward voltage drop ≈ 0.7V, Germanium ≈ 0.3V. Zener diode operates in reverse breakdown region deliberately, used for voltage regulation at a fixed reference voltage. • Rectifiers: Half-wave (1 diode, output ripple frequency = supply frequency, efficiency ~40.6%), Full-wave/Bridge (2 or 4 diodes, ripple frequency = 2× supply frequency, efficiency ~81.2%, much smoother DC output). • Transistor (BJT) configurations: Common Emitter (CE — highest power gain, most widely used, 180° phase inversion between input and output), Common Base (CB — high voltage gain, low current gain, no phase inversion), Common Collector (CC — voltage follower/buffer, high current gain, no voltage gain, no phase inversion). • Transistor as switch vs amplifier: operating in Saturation and Cutoff regions = switch (fully ON/OFF, digital circuits); operating in the Active region = amplifier (linear amplification, analog circuits). • Operational Amplifier (Op-Amp): ideal op-amp has infinite input impedance, zero output impedance, and infinite open-loop gain; used as inverting/non-inverting amplifier, summing amplifier, comparator, and integrator/differentiator circuits with appropriate feedback.

• Modulation embeds a low-frequency message signal onto a high-frequency carrier for efficient transmission; AM (Amplitude Modulation) varies carrier amplitude, FM (Frequency Modulation) varies carrier frequency — FM gives better noise immunity than AM since noise typically affects amplitude, not frequency. • Sampling theorem (Nyquist): a signal must be sampled at a rate at least TWICE its highest frequency component to be reconstructed without loss (aliasing) — sampling rate < 2×fmax causes aliasing distortion. • Multiplexing: TDM (Time Division Multiplexing — different signals share the same channel by taking turns in time slots) vs FDM (Frequency Division Multiplexing — different signals occupy different frequency bands simultaneously on the same medium). • Transmission media: guided (copper cable, coaxial cable, optical fiber — signal travels along a physical path) vs unguided/wireless (radio waves, microwave, satellite — signal radiates through free space); optical fiber offers the highest bandwidth and immunity to electromagnetic interference. • Antenna basics: a half-wave dipole antenna has a length of λ/2 (half the wavelength of the transmitted signal) — antenna dimensions must be matched to the operating frequency for efficient radiation.

• Basic architecture: ALU (Arithmetic Logic Unit — performs arithmetic/logical operations), Control Unit (generates timing/control signals, decodes instructions), Registers (small, very fast internal memory — accumulator, program counter, instruction register). • Program Counter (PC) always holds the address of the NEXT instruction to be fetched — it increments automatically after each fetch unless a jump/branch instruction changes it directly. • Bus types: Address bus (unidirectional, carries memory/IO address — its width determines maximum addressable memory, e.g. a 16-bit address bus addresses 2^16 = 65,536 locations), Data bus (bidirectional, carries actual data), Control bus (carries control signals like read/write, clock). • Interrupt: a signal that temporarily suspends normal program execution to service a higher-priority event (e.g. a peripheral needing attention), then returns to the exact point of suspension — essential for real-time I/O handling without constant CPU polling. • Instruction cycle stages: Fetch (get instruction from memory, pointed to by PC) → Decode (Control Unit interprets the instruction) → Execute (ALU/registers perform the operation) — this fetch-decode-execute cycle repeats continuously.