EASA Module 4 cheat sheet

N-type silicon is doped with a pentavalent donor (phosphorus, arsenic, antimony) so electrons are the majority carriers; P-type uses a trivalent acceptor (boron, aluminium, gallium) so holes are the majority carriers.

Forward bias lowers the PN junction potential barrier and conduction begins at about 0.7 V for silicon and 0.3 V for germanium; reverse bias widens the depletion layer and leaves only a small leakage current.

In a bipolar transistor in the active region Ic = beta x Ib and Ie = Ic + Ib; the base-emitter junction is forward biased and the collector-base junction is reverse biased.

A transistor used as a switch is driven between cutoff (Ib = 0, Vce approximately Vcc) and saturation (Vce approximately 0.2 V); base current is set above Ic/beta to guarantee saturation.

A Zener diode regulates in reverse breakdown and needs a series resistor sized so that I = (Vin - Vz)/Rs stays between the knee current and the maximum rated Zener current.

A thyristor latches into conduction on a gate pulse and turns off only when anode current falls below the holding current; a triac performs the same function on both half-cycles of an AC supply.

Ripple factor = Vripple(rms)/Vdc, about 1.21 for a half-wave rectifier and 0.482 for a full-wave rectifier; ripple frequency equals supply frequency for half-wave and twice supply frequency for full-wave.

Semiconductor resistivity falls as temperature rises (negative temperature coefficient), which can cause thermal runaway in bipolar devices unless emitter resistance or current limiting is used.

MOSFETs and IGBTs are voltage-controlled with very high gate input impedance and are used for high-frequency power switching; the gate needs a defined pull-down resistor to prevent spurious turn-on.

Copper cladding: 1 oz/ft2 corresponds to 35 micrometres. Track resistance R = rho x L / (w x t), with rho = 1.72 x 10^-8 ohm-metre for annealed copper.

FR-4 woven glass/epoxy has relative permittivity about 4.2 to 4.7 and glass transition temperature typically 130 to 180 degrees Celsius; expansion mismatch between resin and copper is a main cause of plated barrel cracking.

Via types: through-hole (outer to outer), blind (outer to one or more inner layers), buried (inner to inner only) and microvia (laser drilled, typically 0.1 mm or less).

The annular ring is the copper width between the hole wall and the pad edge; insufficient ring width leads to opens after drilling tolerance or thermal cycling.

A continuous ground plane gives a low-impedance return path, minimises loop area and reduces crosstalk and radiated emission compared with individual return tracks.

Surface finishes: HASL (cheap, uneven, good solderability), ENIG (flat, nickel barrier under thin gold, suited to fine pitch), immersion silver or tin (flat, limited shelf life).

Decoupling capacitors are mounted as close as possible to the IC supply pins with a short, wide connection to the ground plane to minimise lead inductance.

Controlled-impedance tracks are set by track width, dielectric thickness and relative permittivity; a microstrip over a plane is typically designed for 50 ohms.

Thermal relief spokes on pads tied to a plane restrict heat conduction so the joint can be soldered; solid connection to a large plane causes cold joints with normal irons.

Creepage and clearance between high-voltage tracks are set by working voltage, pollution degree and the presence of conformal coating.

A servomechanism is a closed-loop system in which error = demand - feedback; the amplified error drives the actuator until the feedback signal equals the demand.

Feedback transducers: potentiometer (cheap, wears, limited resolution), synchro or resolver (accurate and rugged), incremental or absolute encoder, tachogenerator for speed.

A tachogenerator supplies rate feedback that adds damping and reduces overshoot; excessive rate feedback makes the response sluggish.

In a synchro pair the receiver rotor voltage is proportional to the sine of the angle difference; a control transformer is nulled when its rotor is 90 degrees from the transmitter-implied position.

PID action: the proportional term sets the speed of response, integral action removes steady-state error but reduces phase margin, derivative action anticipates the error and adds damping.

Excessive loop gain causes hunting (sustained oscillation about the demanded position); insufficient gain gives a slow response and a large steady-state error.

Backlash in gear trains and dead band in the amplifier or sensor produce a limit cycle, with the load oscillating within the lost-motion band around the demand.

A two-phase AC servo motor has a fixed reference winding and a control winding fed 90 degrees out of phase; reversing the phase of the control voltage reverses rotation.

Stepper motors normally run open loop; step angle = 360 degrees divided by steps per revolution, and steps are lost if the load torque exceeds the pull-out torque.