Part-66 Module 4: diodes, transistors, op amps and servos

For category B1, Module 4 is 20 multiple-choice questions in 25 minutes, three choices each, with a 75% pass mark. The paper is short, so every question counts. Most of it covers diodes and transistors, with smaller parts on integrated circuits, printed circuit boards and servomechanisms.

Diodes

A PN junction conducts when forward biased (anode positive). A silicon diode drops about 0.6–0.7 V; germanium about 0.3 V. Under reverse bias the depletion layer widens and only a small leakage current flows, which rises with temperature.

• P-type material is doped with a trivalent impurity (holes are the majority carriers); N-type with a pentavalent impurity (electrons).
• A single-phase bridge rectifier uses four diodes and gives a ripple at twice the supply frequency; a three-phase full-wave bridge uses six.
• A Zener diode works reverse biased at its breakdown voltage to regulate voltage. Its series resistor drops the difference between supply and Zener voltage.
• An LED needs forward bias; a photodiode works reverse biased; a varactor is a voltage-controlled capacitor; a Schottky diode has a low forward drop and switches fast.
• A thyristor (SCR) has anode, cathode and gate. Once triggered it stays on until the current falls below its holding current. A triac conducts both ways.
• An ohmmeter reading low in both directions means a shorted diode; high both ways means open.

Transistors

A bipolar transistor has emitter, base and collector. On an NPN symbol the emitter arrow points out. A small base current controls a large collector current: Ic = β × Ib, and Ie = Ib + Ic.

• Common emitter: voltage and current gain with 180° phase inversion.
• Common collector (emitter follower): gain of about 1, high input impedance, low output impedance.
• Common base: current gain of about 1, used at high frequency.
• As a switch, a transistor works between cut-off and saturation.
• FETs are voltage controlled with a very high input impedance; MOSFET gates are easily damaged by static.

Integrated circuits and op amps

An ideal op amp has very high gain and input impedance and very low output impedance. Inverting gain = −Rf / Rin; non-inverting gain = 1 + Rf / Rin. A voltage follower has unity gain; a comparator has no feedback; a capacitor in the feedback path makes an integrator.

PCBs and servomechanisms

Handle circuit boards by their edges with ESD precautions. A closed-loop servo compares command and feedback to produce an error signal. Hunting is continuous oscillation about the command; rate (tachogenerator) feedback damps it. An LVDT measures linear position and gives zero output at null; a resolver gives sine and cosine outputs; synchros transmit angles over three stator wires.