Power Amplifiers Questions and Answers

Q: Why do MOSFETs need no emitter ballast when paralleled?

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.

Q: What does a totem pole output stage require that a complementary one does not?

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.

Q: What does a device's junction-to-case thermal resistance describe?

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.

Q: What makes up the thermal path from junction to ambient?

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.

Q: A device dissipates \( 5 \ \mathrm{W} \) with resistances of \( 3 \), \( 0.5 \) and \( 2 \ \mathrm{^\circ C/W} \) in a \( 40 \ \mathrm{^\circ C} \) ambient. Its junction reaches

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.

Q: Which term in the thermal chain does a larger heatsink reduce?

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.

Q: What does thermal compound at the mounting interface do?

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.

Q: Why is an insulating washer often needed under a power device?

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.

Q: What does a safe operating area diagram bound?

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.

21.

Why do MOSFETs need no emitter ballast when paralleled?

A.Their on-resistance rises with temperature
B.Their threshold voltages match closely between devices
C.Their gates draw no current from the driver
D.Their safe operating areas are larger than bipolar devices'

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.

Discuss in Forum

22.

What does a totem pole output stage require that a complementary one does not?

A.Emitter ballast resistors in each device
B.A phase splitter to drive the two devices
C.A larger supply voltage for the same output
D.A transformer to couple to the load

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.

Discuss in Forum

23.

What does a device's junction-to-case thermal resistance describe?

A.The time the junction takes to reach equilibrium
B.The temperature at which the device fails
C.The temperature rise per watt inside the package
D.The maximum power the device may dissipate

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.

Discuss in Forum

24.

What makes up the thermal path from junction to ambient?

A.Junction to air alone, in a free-standing package
B.Case to sink and sink to air, with the junction excluded
C.Junction to case only, the rest being negligible
D.Junction to case, case to sink, sink to air

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.

Discuss in Forum

25.

A device dissipates \( 5 \ \mathrm{W} \) with resistances of \( 3 \), \( 0.5 \) and \( 2 \ \mathrm{^\circ C/W} \) in a \( 40 \ \mathrm{^\circ C} \) ambient. Its junction reaches

A.67.5 degrees
B.55 degrees
C.40 degrees
D.82.5 degrees

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.

Discuss in Forum

26.

Which term in the thermal chain does a larger heatsink reduce?

A.All three equally
B.Sink to ambient
C.Junction to case
D.Case to sink

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.

Discuss in Forum

27.

What does thermal compound at the mounting interface do?

A.Reduces the device's internal thermal resistance
B.Prevents corrosion of the mating surfaces
C.Fills the air gaps between two imperfect surfaces
D.Insulates the device electrically from the heatsink

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.

Discuss in Forum

28.

Why is an insulating washer often needed under a power device?

A.It would otherwise vibrate against the heatsink
B.The heatsink would corrode in contact with the case
C.The device would run too cool without one
D.Its case is connected to one of its terminals

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.

Discuss in Forum

29.

What does a safe operating area diagram bound?

A.The permitted combinations of voltage and current
B.The temperature range over which gain is specified
C.The frequency range in which the device is usable
D.The supply voltages the device may be operated from

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.

Discuss in Forum

30.

Why does a reactive load make the safe operating area harder to satisfy?

A.The supply voltage must be raised to reach the same output
B.Current and voltage peak at different instants
C.The load draws more average power than a resistive one
D.The output devices must be biased further into class A

Answer: B

With a resistive load the device's current is largest when its voltage is smallest, but a reactive load shifts the phase so that substantial current flows while substantial voltage is present, and the operating point swings out into the high-dissipation region. Average power is actually lower, and neither biasing nor supply voltage changes. A loudspeaker is markedly reactive, which is why audio output stages are specified against reactive loads rather than resistive ones.

Discuss in Forum

Loading…

Topics

Other Subjects

Tags: Power Amplifiers Questions and Answers || Power Amplifiers MCQ Questions and Answers || Power Amplifiers GK Questions and Answers || Power Amplifiers GK MCQ Questions || Power Amplifiers Multiple Choice Questions and Answers || Power Amplifiers GK || GK on Power Amplifiers || Analog Electronics Questions and Answers || Analog Electronics MCQ Questions and Answers || Analog Electronics GK Questions and Answers || GK on Analog Electronics

Power Amplifiers Questions and Answers

Topics

Other Subjects

About Us

Stay informed about all important Dates & Notifications