Power Amplifiers Questions and Answers

Answer: D

A bipolar device's forward drop falls as it heats, so the hottest device would take more current and run away; a fraction of an ohm in each emitter develops an opposing voltage and equalises the sharing. They raise the output impedance slightly rather than lowering it, and provide no gain or filtering. Any device whose conduction improves as it heats needs a ballast element when paralleled, which is exactly the property MOSFETs lack and why they parallel freely. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: A

A hot MOSFET conducts less well, so it sheds current to its cooler partners and the sharing self-corrects - negative feedback where a bipolar device has positive. Threshold matching is poor, gate current is irrelevant to sharing, and the safe operating area is a separate advantage. Whether a device's principal imperfection acts with or against an imbalance decides whether it can be paralleled without help.

Answer: B

With both devices of the same polarity, one must be driven in antiphase to the other, so a phase splitter or an inverting driver is needed. Supply voltage, transformer coupling and ballast resistors are not what distinguishes the arrangements. The complementary pair's advantage is precisely that opposite-polarity devices respond naturally to the two halves of one signal, which removes a whole stage.

Answer: C

Thermal resistance is degrees of rise per watt dissipated, so it converts power into a temperature difference exactly as an electrical resistance converts current into a voltage difference. The maximum power, the thermal time constant and the failure temperature are separate specifications. Treating a thermal path as a series circuit with power in place of current and temperature in place of voltage makes every such calculation routine. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: D

Three resistances in series carry the heat outward: the device's internal path, the mounting interface with its insulating washer and compound, and the heatsink's own path to the air. Omitting any of them understates the junction temperature, and the interface is the one most often forgotten despite being easily the most variable. A series chain is only as good as its largest term, which is usually the heatsink but can be a badly made interface. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: A

The three resistances add and the rise is added to ambient:

\[ \begin{aligned} R_{th} &= 3 + 0.5 + 2 = 5.5 \\ T_{j} &= 40 + 5(5.5) = 67.5 \ \mathrm{^\circ C} \end{aligned} \]

giving 67.5 degrees, with useful margin below a 150 degree limit. Omitting the interface gives 65, and omitting the heatsink gives 57.5 - both optimistic. Adding every term in the chain before multiplying is what makes the answer trustworthy.

Answer: B

A bigger or better-ventilated sink reduces only its own resistance to the surrounding air. The device's internal path is fixed at manufacture and the interface depends on the mounting rather than on the sink's size. Since the sink term is usually the largest, attacking it is usually right - but once it has been reduced below the other two, further enlargement buys almost nothing, which is a limit worth checking before ordering a larger extrusion. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: C

Two nominally flat surfaces touch only at a few points, and air is a poor conductor, so a thin film of compound displaces it and cuts the interface resistance substantially. Electrical insulation is the washer's job, not the compound's, the internal path is unaffected, and corrosion is incidental. A thin film is essential and a thick one is worse than none, since the compound itself conducts far less well than metal - which is why it is spread rather than applied generously.

Answer: D

In most power packages the metal tab is the collector or the drain, so bolting it directly to a grounded heatsink would short that terminal. The washer insulates electrically while conducting heat as well as it can, which is a compromise that adds to the interface resistance. Mica, silicone rubber and anodised aluminium all trade insulation quality against thermal resistance differently, and choosing between them is a real design decision rather than a detail. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

The diagram bounds which pairs of collector voltage and collector current the device may occupy, limited by maximum current, maximum voltage, maximum power and secondary breakdown. Gain, frequency and supply range are specified elsewhere. Four separate limits drawn on one pair of axes is far more useful than four numbers, because an operating point can then be checked against all of them at a glance - and a reactive load's trajectory can be plotted on it. Most errors here come from carrying a result across from a neighbouring case where it does not hold.