Electronics and Communication Systems
What this domain tests
Electronics and Communication Systems covers semiconductors, diodes, transistors, amplifiers, digital logic, modulation and communication. Questions test biasing, device characteristics, gain and signal bandwidth.
Core rules
- A PN junction diode conducts when forward-biased and blocks when reverse-biased; a rectifier converts AC to DC.
- BJT relations: I_E = I_B + I_C; β = I_C/I_B; α = I_C/I_E; β = α/(1 − α).
- Transistor configurations: common emitter gives voltage and current gain; common base gives voltage gain but current gain below 1; common collector is a voltage follower.
- Logic gates: AND outputs 1 only when all inputs are 1; OR outputs 1 when any input is 1; NOT inverts.
- Modulation: AM varies amplitude; FM varies frequency. AM bandwidth is approximately 2f_m; FM has constant amplitude and better noise immunity.
Worked example
A transistor has β = 100 and base current I_B = 20 μA. Then I_C = βI_B = 100 × 20 μA = 2,000 μA = 2 mA. Emitter current I_E = I_B + I_C = 20 μA + 2,000 μA = 2,020 μA = 2.02 mA.
Traps
Do not confuse α and β; α is always less than 1, while β can be much larger. Do not reverse forward and reverse bias. In decibels, power gain uses 10 log₁₀(P₂/P₁), while voltage gain uses 20 log₁₀(V₂/V₁). In AM, the sidebands carry information and the bandwidth is twice the modulating frequency.
Exam focus
Practise transistor current relations, logic truth tables and modulation bandwidth calculations.