JTO Fundamentals — Electronics & Communication
Electronic Devices — Quick Reference
BJT vs MOSFET
| Parameter | BJT | MOSFET |
|---|
|-----------|-----|--------|
| Control | Current-controlled | Voltage-controlled |
|---|---|---|
| Input impedance | Low | Very high |
| Switching speed | Moderate | Very fast |
| Power dissipation | Higher | Lower |
| Application | Analog amplifiers | Digital ICs, power electronics |
Op-Amp Configurations (Ideal Op-Amp: A = ∞, Rin = ∞, Rout = 0)
Inverting amplifier: Gain = -Rf/R1
Non-inverting amplifier: Gain = 1 + Rf/R1
Voltage follower: Gain = 1 (buffer)
Differentiator: Output = -RC × d(Vin)/dt
Integrator: Output = -(1/RC) × ∫Vin dt
Summing amplifier: Vout = -(Rf/R1 × V1 + Rf/R2 × V2)
Digital Electronics — Key Concepts
Number Systems
•Binary to Decimal: 1101₂ = 8+4+0+1 = 13₁₀
•BCD: Each decimal digit encoded in 4-bit binary. 9 = 1001, max valid BCD = 1001
•2's complement: For negative numbers. -5 in 8-bit = 11111011
K-Map Rules
•Group sizes: 1, 2, 4, 8, 16 (powers of 2 only)
•Groups must be rectangular, can wrap around
•Essential prime implicants must be covered
•Minimize terms
Flip-Flops
•SR: Set-Reset. Invalid state: S=R=1
•D: Data/Delay. Q(t+1) = D
•JK: Master-slave. J=K=1 → Toggle. No invalid state
•T: Toggle. T=0 → Hold, T=1 → Toggle
Microprocessor 8085 — Key for BSNL JTO
•8-bit data bus, 16-bit address bus
•Registers: A (accumulator), B, C, D, E, H, L, PC, SP
•Addressing modes: Register, Register indirect, Immediate, Direct, Implicit
•Interrupts: TRAP (highest), RST 7.5, RST 6.5, RST 5.5, INTR (lowest)
Communication Systems — Modulation
Amplitude Modulation (AM)
•DSB-FC: Modulation index m = Am/Ac. Bandwidth = 2fm
•DSB-SC: No carrier. Better power efficiency
•SSB: Half bandwidth of DSB. Used in shortwave radio
•Power: Total = Pc(1 + m²/2). Efficiency = m²/(2 + m²)
Frequency Modulation (FM)
•Modulation index β = Δf/fm (Δf = max freq deviation)
•Carson's rule: BW = 2(Δf + fm) = 2fm(1 + β)
•Capture effect: FM locks onto stronger signal
•Pre-emphasis/de-emphasis: Boost/reduce high frequencies to reduce noise
Digital Modulation (for 4G/5G exams)
•BPSK: 1 bit/symbol. Most robust.
•QPSK: 2 bits/symbol.
•16-QAM: 4 bits/symbol.
•64-QAM: 6 bits/symbol.
•256-QAM: 8 bits/symbol. Used in 5G. Requires best SNR.
•LTE uses OFDM: divides channel into many narrow subcarriers (15 kHz each)
Transmission Lines
Key parameters:
•Characteristic impedance Z₀ = √(L/C) for lossless line
•Reflection coefficient Γ = (ZL - Z₀)/(ZL + Z₀)
•VSWR = (1 + |Γ|)/(1 - |Γ|)
•Perfect match: ZL = Z₀, Γ = 0, VSWR = 1
•Short circuit: ZL = 0, Γ = -1, VSWR = ∞
•Open circuit: ZL = ∞, Γ = +1, VSWR = ∞
Quarter-wave transformer: Input impedance = Z₀²/ZL. Used to match impedances.
Smith Chart: Graphical tool for transmission line calculations. Normalized impedance plotted. Used to find matching networks.
Antenna Theory
Fundamental parameters:
•Gain: Ratio of power radiated in best direction vs isotropic radiator (dBi)
•Directivity: Ratio of max radiation intensity vs average (for lossless antenna = gain)
•Radiation resistance: Power radiated / I² (for half-wave dipole = 73.2 Ω)
•Bandwidth: Range of frequencies over which antenna performs acceptably (VSWR < 2)
•Polarization: Orientation of E-field. Linear, circular, elliptical
Common antennas:
| Antenna | Gain | Application |
|---|
|---------|------|-------------|
| Isotropic | 0 dBi | Reference |
|---|---|---|
| Half-wave dipole | 2.15 dBi | Reference for dBd |
| Yagi-Uda | 7–20 dBi | TV reception, point-to-point |
| Parabolic dish | 25–45 dBi | Satellite, microwave backhaul |
| Patch/Microstrip | 5–9 dBi | Mobile base stations, WiFi |
| Phased array | 15–30+ dBi | 5G massive MIMO, radar |