Electronic Fundamentals
Core concepts
Semiconductor theory begins with intrinsic and extrinsic (n-type and p-type) materials, then the pn junction. You must understand diode operation and its applications: half-wave and full-wave rectification, smoothing, zener regulation, light-emitting diodes and photodiodes. Bipolar transistors as switches and amplifiers, the meaning of gain, and the difference between NPN and PNP configurations are examined, along with field-effect devices. Expect also: integrated circuits and their packaging, printed circuit boards and soldering, filters, oscillators, and the fundamentals of servomechanisms - open and closed loop, feedback, error signal, and the roles of the synchro, resolver, amplifier and motor in an aircraft position-indicating system.
Traps in questions
- Forward versus reverse bias. Forward bias on a silicon diode needs about 0.6–0.7 V; in reverse it blocks until breakdown.
- Full-wave rectification doubles the ripple frequency compared with the supply; half-wave leaves the supply frequency.
- NPN versus PNP arrow direction on the emitter - arrow points out for NPN, in for PNP.
- Closed-loop versus open-loop servos - only closed loop uses feedback to null the error signal.
- Confusing gain in decibels (20 log V ratio) with power gain (10 log P ratio).
Worked example
A transformer secondary delivers 12 V rms to a bridge rectifier. The peak voltage is 12 × 1.414 ≈ 17 V. Two diodes conduct in each half cycle, so subtract roughly 1.4 V: the unsmoothed peak DC is about 15.6 V. With a reservoir capacitor the output approaches this peak at light load; at heavier load it droops towards the average value. For a 50 Hz supply, the ripple frequency after a bridge is 100 Hz.
Rule of thumb
Silicon diode drop ≈ 0.7 V; germanium ≈ 0.3 V. A bridge rectifier loses about 1.4 V because two diodes are always in the path.