Power Amplifiers Questions and Answers

Q: Which class would you choose for a battery-powered portable speaker?

Class D reaches above ninety per cent efficiency, so battery life is long and the heatsink is small - which is why essentially every portable speaker made now uses it. Class A would flatten a battery at idle, class B is better but still wastes a fifth of the power, and class C cannot reproduce an audio waveform at all. Where the energy source is the binding constraint, efficiency outweighs every other consideration.

Q: Which class would you choose where absolute linearity matters more than efficiency?

Class A never hands over between devices, so it has no crossover step and its single device works over a smooth part of its characteristic. The price is an efficiency of twenty-five per cent and a large standing dissipation. Class D introduces switching artefacts, class C is grossly non-linear, and unbiased class B is the worst case for crossover. Choosing the least efficient class deliberately is defensible only when the power involved is small enough for the waste not to matter.

Q: Which of the following is not a thermal resistance in the path to ambient? Emitter to base within the device Junction to case within the package Case to heatsink across the interface Heatsink to ambient air

Emitter to base is an electrical junction rather than a step in the thermal path; heat flows from the junction outward through the package, the mounting and the sink. The other three are the standard three terms that add in series. Confusing an electrical structure with a thermal one is easy when both are described as resistances, and keeping the two circuits separate is what makes the analogy useful rather than misleading. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Q: Small resistors placed in the emitters of paralleled output devices to equalise their currents are called ______ resistors.

These are ballast resistors, and they work because a device taking more current develops more voltage across its own resistor, which reduces its drive. A bleeder discharges a capacitor, a swamping resistor stabilises a gain, and a pull-up establishes a logic level. Each name describes what the resistor is there to do, and using the right one makes a schematic self-explanatory to whoever reads it next.

Q: What is a power amplifier's job?

A power stage supplies the current a low-impedance load demands at the voltage the preceding stages have already established, so its figure of merit is watts delivered rather than voltage gain. Voltage gain is the small-signal stages' business, matching is rarely wanted in a voltage-source amplifier, and filtering is a separate function. That the final stage often has a voltage gain near one, and is still the most demanding part of the design, is worth understanding before any of its details make sense.

Q: Which quantity does a loudspeaker present to an amplifier?

A loudspeaker is a few ohms, and its impedance rises at its resonance and again at high frequencies because of the voice coil's inductance, so a nominal eight ohms may be four at one frequency and thirty at another. It is neither high nor purely capacitive nor constant. Because the load varies, an amplifier's power is quoted into a stated nominal impedance and its safe operating area must be checked against a reactive load rather than a resistive one. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Q: Why does an amplifier deliver more power into four ohms than into eight?

For the same voltage swing, halving the load impedance doubles the current and so doubles the power - which is why a power figure means nothing without the load it was measured into. The supply is unchanged, the devices dissipate more rather than less, and the loop gain is unaffected. Whether an amplifier can actually deliver that doubled power depends on its output devices and its supply, which is why the four-ohm figure is often less than twice the eight-ohm one. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Q: What is a decibel of power ratio?

For powers the decibel is ten times the base-ten logarithm of the ratio, while for voltages and other amplitudes it is twenty times. The factor of two between the two conventions comes from power going as the square of amplitude. Getting it wrong squares or square-roots the answer, which is a large error, so establishing whether a quantity is a power or an amplitude is the first step in any decibel calculation.

Q: An amplifier's output rises from \( 10 \ \mathrm{W} \) to \( 100 \ \mathrm{W} \). That is a rise of

Applying the power convention:\[ 10 \log_{10}\left(\frac{100}{10}\right) = 10 \ \mathrm{dB} \]giving 10 dB. Twenty decibels would follow from the amplitude convention, which does not apply to powers. A tenfold increase in power is ten decibels and a tenfold increase in voltage is twenty, and holding that pair in mind settles which factor to use without having to recall the reason each time.

Q: Doubling an amplifier's output power is a rise of about

Applying the same convention:\[ 10 \log_{10} 2 \approx 3 \ \mathrm{dB} \]giving about 3 dB, which is barely perceptible as a change in loudness. Six decibels would be a doubling of voltage, which is a quadrupling of power. That a doubling of amplifier power buys so little audible difference is why loudspeaker sensitivity usually matters more than amplifier rating in a practical system.

71.

Which class would you choose for a battery-powered portable speaker?

A.Class B with a large heatsink
B.Class C with an output filter
C.Class D
D.Class A with transformer coupling

Answer: C

Class D reaches above ninety per cent efficiency, so battery life is long and the heatsink is small - which is why essentially every portable speaker made now uses it. Class A would flatten a battery at idle, class B is better but still wastes a fifth of the power, and class C cannot reproduce an audio waveform at all. Where the energy source is the binding constraint, efficiency outweighs every other consideration.

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72.

Which class would you choose where absolute linearity matters more than efficiency?

A.Class D with a high switching rate
B.Class C with a tuned output circuit
C.Class B without any quiescent bias
D.Class A

Answer: D

Class A never hands over between devices, so it has no crossover step and its single device works over a smooth part of its characteristic. The price is an efficiency of twenty-five per cent and a large standing dissipation. Class D introduces switching artefacts, class C is grossly non-linear, and unbiased class B is the worst case for crossover. Choosing the least efficient class deliberately is defensible only when the power involved is small enough for the waste not to matter.

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73.

Which of the following is not a thermal resistance in the path to ambient?

Emitter to base within the device

Junction to case within the package

Case to heatsink across the interface

Heatsink to ambient air

A.Emitter to base within the device
B.Junction to case within the package
C.Case to heatsink across the interface
D.Heatsink to ambient air

Answer: A

Emitter to base is an electrical junction rather than a step in the thermal path; heat flows from the junction outward through the package, the mounting and the sink. The other three are the standard three terms that add in series. Confusing an electrical structure with a thermal one is easy when both are described as resistances, and keeping the two circuits separate is what makes the analogy useful rather than misleading. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Discuss in Forum

74.

Small resistors placed in the emitters of paralleled output devices to equalise their currents are called ______ resistors.

A.pull-up
B.ballast
C.bleeder
D.swamping

Answer: B

These are ballast resistors, and they work because a device taking more current develops more voltage across its own resistor, which reduces its drive. A bleeder discharges a capacitor, a swamping resistor stabilises a gain, and a pull-up establishes a logic level. Each name describes what the resistor is there to do, and using the right one makes a schematic self-explanatory to whoever reads it next.

Discuss in Forum

75.

What is a power amplifier's job?

A.To match the source impedance to the load impedance
B.To filter unwanted frequencies from the signal
C.To deliver power to a load rather than voltage gain
D.To raise a signal's voltage as far as possible

Answer: C

A power stage supplies the current a low-impedance load demands at the voltage the preceding stages have already established, so its figure of merit is watts delivered rather than voltage gain. Voltage gain is the small-signal stages' business, matching is rarely wanted in a voltage-source amplifier, and filtering is a separate function. That the final stage often has a voltage gain near one, and is still the most demanding part of the design, is worth understanding before any of its details make sense.

Discuss in Forum

76.

Which quantity does a loudspeaker present to an amplifier?

A.A high impedance that is essentially resistive
B.A purely capacitive load at all frequencies
C.A constant impedance independent of frequency
D.A low impedance that varies with frequency

Answer: D

A loudspeaker is a few ohms, and its impedance rises at its resonance and again at high frequencies because of the voice coil's inductance, so a nominal eight ohms may be four at one frequency and thirty at another. It is neither high nor purely capacitive nor constant. Because the load varies, an amplifier's power is quoted into a stated nominal impedance and its safe operating area must be checked against a reactive load rather than a resistive one. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Discuss in Forum

77.

Why does an amplifier deliver more power into four ohms than into eight?

A.The same voltage drives twice the current
B.The supply voltage rises with the lower impedance
C.The output devices dissipate less at lower impedance
D.The feedback loop increases its gain automatically

Answer: A

For the same voltage swing, halving the load impedance doubles the current and so doubles the power - which is why a power figure means nothing without the load it was measured into. The supply is unchanged, the devices dissipate more rather than less, and the loop gain is unaffected. Whether an amplifier can actually deliver that doubled power depends on its output devices and its supply, which is why the four-ohm figure is often less than twice the eight-ohm one. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Discuss in Forum

78.

What is a decibel of power ratio?

A.The square root of the ratio
B.Ten times the logarithm of the ratio
C.Twenty times the logarithm of the ratio
D.The logarithm of the ratio itself

Answer: B

For powers the decibel is ten times the base-ten logarithm of the ratio, while for voltages and other amplitudes it is twenty times. The factor of two between the two conventions comes from power going as the square of amplitude. Getting it wrong squares or square-roots the answer, which is a large error, so establishing whether a quantity is a power or an amplitude is the first step in any decibel calculation.

Discuss in Forum

79.

An amplifier's output rises from \( 10 \ \mathrm{W} \) to \( 100 \ \mathrm{W} \). That is a rise of

A.100 dB
B.3 dB
C.10 dB
D.20 dB

Answer: C

Applying the power convention:

\[ 10 \log_{10}\left(\frac{100}{10}\right) = 10 \ \mathrm{dB} \]

giving 10 dB. Twenty decibels would follow from the amplitude convention, which does not apply to powers. A tenfold increase in power is ten decibels and a tenfold increase in voltage is twenty, and holding that pair in mind settles which factor to use without having to recall the reason each time.

Discuss in Forum

80.

Doubling an amplifier's output power is a rise of about

A.6 dB
B.2 dB
C.10 dB
D.3 dB

Answer: D

Applying the same convention:

\[ 10 \log_{10} 2 \approx 3 \ \mathrm{dB} \]

giving about 3 dB, which is barely perceptible as a change in loudness. Six decibels would be a doubling of voltage, which is a quadrupling of power. That a doubling of amplifier power buys so little audible difference is why loudspeaker sensitivity usually matters more than amplifier rating in a practical system.

Discuss in Forum

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