1What is the primary function of a power amplifier in an electronic system?
A.To amplify the voltage of the signal significantly
B.To deliver maximum power to the load
C.To introduce harmonic distortion for effect
D.To filter high-frequency noise
Correct Answer: To deliver maximum power to the load
Explanation:A power amplifier is the last stage of an amplifier chain, designed to deliver sufficient power (current and voltage) to drive the load, such as a loudspeaker or motor.
Incorrect! Try again.
2In a Class A amplifier, the conduction angle of the collector current is:
A.
B.Between and
C.
D.Less than
Correct Answer:
Explanation:In Class A operation, the transistor is biased such that it conducts current during the entire cycle of the input signal, resulting in a conduction angle of .
Incorrect! Try again.
3What is the maximum theoretical efficiency of a series-fed Class A amplifier?
A.
B.
C.
D.
Correct Answer:
Explanation:For a series-fed Class A amplifier (resistive load), the maximum efficiency is . A large amount of power is wasted as heat in the load resistor even when no signal is applied.
Incorrect! Try again.
4Which of the following improves the efficiency of a Class A amplifier to a theoretical maximum of 50%?
A.Using a Darlington pair
B.Using transformer coupling
C.Increasing the input signal frequency
D.Using a bypass capacitor
Correct Answer: Using transformer coupling
Explanation:Transformer coupling eliminates DC power loss in the load resistance (since the transformer primary has negligible DC resistance), doubling the theoretical maximum efficiency to .
Incorrect! Try again.
5In a Class B amplifier, the Q-point is located at:
A.The center of the load line
B.The saturation region
C.The cutoff voltage on the load line
D.Ideally at
Correct Answer: The cutoff voltage on the load line
Explanation:Class B amplifiers are biased at cutoff (, ). Conduction only occurs when the input signal drives the transistor out of cutoff.
Incorrect! Try again.
6What is the conduction angle for a Class B amplifier?
A.
B.
C.
D.
Correct Answer:
Explanation:A Class B amplifier conducts for only one half-cycle of the input signal, meaning the conduction angle is .
Incorrect! Try again.
7The primary cause of crossover distortion in Class B push-pull amplifiers is:
A.Saturation of the transistors
B.The turn-on voltage () of the transistors
C.Impedance mismatching
D.Excessive input signal amplitude
Correct Answer: The turn-on voltage () of the transistors
Explanation:Crossover distortion occurs because transistors require a base-emitter voltage (approx 0.7V for Si) to start conducting. There is a 'dead zone' where neither transistor conducts when the signal crosses zero.
Incorrect! Try again.
8How is crossover distortion eliminated in power amplifiers?
A.By using Class C operation
B.By increasing the load resistance
C.By using Class AB configuration with trickle bias
D.By using a larger transformer
Correct Answer: By using Class AB configuration with trickle bias
Explanation:Class AB applies a small 'trickle' bias voltage to the base-emitter junctions to keep the transistors slightly conducting even with zero signal, overcoming the threshold.
Incorrect! Try again.
9What is the maximum theoretical efficiency of a Class B push-pull amplifier?
A.
B.
C.
D.
Correct Answer:
Explanation:The maximum efficiency is derived as , which equals .
Incorrect! Try again.
10The conduction angle for a Class AB amplifier is:
A.Exactly
B.Between and
C.Exactly
D.Less than
Correct Answer: Between and
Explanation:Class AB conducts for slightly more than half a cycle () but less than the full cycle () to prevent crossover distortion while maintaining high efficiency.
Incorrect! Try again.
11In a push-pull amplifier, even harmonic components of the signal:
A.Are amplified significantly
B.Cancel out in the output transformer
C.Cause thermal runaway
D.Shift the Q-point
Correct Answer: Cancel out in the output transformer
Explanation:Due to the phase relationship in a push-pull configuration, even harmonics (2nd, 4th, etc.) flow in opposite directions in the primary winding and cancel each other out.
Incorrect! Try again.
12Second harmonic distortion () is defined as:
A.
B.
C.
D.
Correct Answer:
Explanation: is the ratio of the amplitude of the second harmonic component () to the amplitude of the fundamental component ().
Incorrect! Try again.
13If the distortion components are , the Total Harmonic Distortion (THD) is calculated as:
A.
B.
C.
D.
Correct Answer:
Explanation:THD is the square root of the sum of the squares of the individual harmonic distortion percentages.
Incorrect! Try again.
14In a transformer-coupled amplifier, the relationship between primary impedance () and secondary load () with turns ratio is:
A.
B.
C.
D.
Correct Answer:
Explanation:The reflected impedance from the secondary to the primary is proportional to the square of the turns ratio.
Incorrect! Try again.
15The collector efficiency () is defined as the ratio of:
A.DC output power to AC input power
B.AC output power to DC input power
C.AC input power to DC output power
D.Dissipated power to Input power
Correct Answer: AC output power to DC input power
Explanation:Efficiency is the ratio of useful AC power delivered to the load () to the DC power supplied by the source ().
Incorrect! Try again.
16In a Class A amplifier, maximum power dissipation in the transistor occurs when:
A.Maximum signal is applied
B.Zero signal is applied
C.Signal is half the maximum amplitude
D.The load is shorted
Correct Answer: Zero signal is applied
Explanation:In Class A, the DC bias current flows constantly. When there is no AC signal to transfer power to the load, all the DC power is dissipated as heat in the transistor.
Incorrect! Try again.
17For a Class B amplifier using a single power supply , the DC input power is calculated as:
A.
B.
C.
D.
Correct Answer:
Explanation:The average current drawn by a full-wave rectified signal (which is what the supply sees from a push-pull Class B) is .
Incorrect! Try again.
18Which power amplifier class introduces the least distortion (highest linearity)?
A.Class A
B.Class B
C.Class C
D.Class D
Correct Answer: Class A
Explanation:Class A amplifiers operate in the linear region of the load line for the full cycle, offering the highest linearity and lowest distortion, albeit at low efficiency.
Incorrect! Try again.
19What is the purpose of a heat sink in a power amplifier circuit?
A.To increase voltage gain
B.To reduce the output impedance
C.To dissipate heat generated by the transistor to the ambient air
D.To provide electrical insulation
Correct Answer: To dissipate heat generated by the transistor to the ambient air
Explanation:Power transistors generate significant heat. A heat sink increases the surface area to transfer this heat to the air, preventing the junction temperature from exceeding safe limits.
Incorrect! Try again.
20The 2N3055 is a popular power transistor. What is its semiconductor material and polarity?
A.Germanium PNP
B.Silicon NPN
C.Silicon PNP
D.Germanium NPN
Correct Answer: Silicon NPN
Explanation:The 2N3055 is a classic Silicon NPN power BJT.
Incorrect! Try again.
21According to the datasheet, what is the typical package type for a 2N3055 transistor?
A.TO-92
B.TO-220
C.TO-3 (Metal Can)
D.DIP-8
Correct Answer: TO-3 (Metal Can)
Explanation:The 2N3055 is historically and typically housed in a TO-3 metal can package, which allows for excellent thermal transfer.
Incorrect! Try again.
22What is the maximum Collector-Emitter Voltage () rating for a standard 2N3055?
A.12 V
B.30 V
C.60 V
D.200 V
Correct Answer: 60 V
Explanation:The maximum for a 2N3055 is typically 60V.
Incorrect! Try again.
23What is the maximum continuous collector current () rating for a 2N3055?
A.1 A
B.5 A
C.15 A
D.50 A
Correct Answer: 15 A
Explanation:The 2N3055 is rated for a continuous collector current of up to 15A.
Incorrect! Try again.
24What is the maximum power dissipation () of a 2N3055 at a case temperature of ?
A.15 W
B.60 W
C.115 W
D.200 W
Correct Answer: 115 W
Explanation:The total device dissipation is rated at 115W at .
Incorrect! Try again.
25In the context of the 2N3055 datasheet, what does represent?
A.Input impedance
B.DC Current Gain
C.Output Admittance
D.Thermal Resistance
Correct Answer: DC Current Gain
Explanation: is the DC current gain (Beta) of the transistor.
C.One NPN and one PNP transistor with matched characteristics
D.Two Darlington pairs of the same polarity
Correct Answer: One NPN and one PNP transistor with matched characteristics
Explanation:Complementary-symmetry amplifiers use one NPN and one PNP transistor to conduct opposite halves of the waveform without needing a center-tapped transformer.
Incorrect! Try again.
27What is the function of the driver stage before the power amplifier stage?
A.To provide voltage amplification to drive the power stage
B.To filter the DC component
C.To cool down the circuit
D.To convert AC to DC
Correct Answer: To provide voltage amplification to drive the power stage
Explanation:Power amplifiers usually have low voltage gain (often unity in emitter followers). The driver stage provides the necessary voltage swing to drive the power transistors.
Incorrect! Try again.
28In a transformer-coupled Class A amplifier, the DC load line is:
A.A horizontal line passing through
B.A vertical line passing through
C.A line with slope
D.Almost vertical (very steep slope)
Correct Answer: Almost vertical (very steep slope)
Explanation:The DC resistance of the transformer primary is very low, making the DC load line almost vertical compared to the AC load line.
Incorrect! Try again.
29The maximum AC power output of a Class A transformer-coupled amplifier is given by:
A.
B.
C.
D.
Correct Answer:
Explanation:Using RMS values (), . At max swing, and , so .
Incorrect! Try again.
30Which phenomenon limits the safe operating area (SOA) of a power transistor primarily due to hot spots?
A.Early Effect
B.Second Breakdown
C.Miller Effect
D.Thermal Runaway
Correct Answer: Second Breakdown
Explanation:Second breakdown is a failure mode in BJTs where localized heating causes current concentration (hot spots), leading to a rapid drop in voltage and destruction of the device.
Incorrect! Try again.
31Calculate the efficiency of a power amp delivering 10W to a load while drawing 20W from the DC supply.
A.
B.
C.
D.
Correct Answer:
Explanation:.
Incorrect! Try again.
32In a Class B amplifier, if the peak output voltage is and load is , the AC output power is:
A.
B.
C.
D.
Correct Answer:
Explanation:. Since , . Therefore .
Incorrect! Try again.
33Which type of distortion results in the output waveform being 'clipped' at the peaks?
Explanation:When the input signal is too large, the transistor is driven into saturation or cutoff, flattening (clipping) the peaks of the waveform.
Incorrect! Try again.
34In a push-pull amplifier, if one transistor fails (opens), the output will:
A.Remain the same
B.Become zero
C.Become a half-wave rectified signal
D.Become a DC signal
Correct Answer: Become a half-wave rectified signal
Explanation:Since each transistor amplifies one half of the waveform, if one fails open, only the other half appears at the output, resembling half-wave rectification.
Incorrect! Try again.
35The 'figure of merit' for a power amplifier usually refers to:
A.Gain-Bandwidth product
B.Collector Efficiency
C.Max Power Dissipation / Max Power Output ratio
D.Signal to Noise Ratio
Correct Answer: Max Power Dissipation / Max Power Output ratio
Explanation:The figure of merit often compares the rating of the transistor required () to the useful power output (). Ideally, we want a low ratio (smaller transistor for more output).
Incorrect! Try again.
36Quasi-complementary push-pull amplifiers are used to:
A.Avoid using PNP power transistors
B.Increase crossover distortion
C.Decrease input impedance
D.Eliminate the power supply
Correct Answer: Avoid using PNP power transistors
Explanation:High-power PNP transistors were historically more expensive or harder to match than NPNs. Quasi-complementary designs use low-power driver pairs (NPN/PNP) to drive high-power NPN output transistors for both halves of the cycle.
Incorrect! Try again.
37For a 2N3055, the Base-Emitter On Voltage () is typically around:
A.0.2 V
B.1.0 - 1.5 V
C.5 V
D.10 V
Correct Answer: 1.0 - 1.5 V
Explanation:For high-current power transistors like the 2N3055, is often higher than the standard small-signal 0.7V, typically ranging up to 1.5V at high currents.
Incorrect! Try again.
38The thermal resistance in a datasheet relates:
A.Junction temperature to Ambient temperature
B.Junction temperature to Case temperature
C.Case temperature to Sink temperature
D.Sink temperature to Ambient temperature
Correct Answer: Junction temperature to Case temperature
Explanation: represents the thermal resistance between the semiconductor junction and the external case of the package.
Incorrect! Try again.
39If a Class A amplifier has a collector efficiency of 30%, what percentage of DC power is wasted as heat?
A.
B.
C.
D.
Correct Answer:
Explanation:Power conservation dictates that . If 30% is useful output, 70% is dissipated as heat.
Incorrect! Try again.
40In harmonic distortion analysis, if the fundamental amplitude is 5V and the second harmonic amplitude is 0.2V, the percentage second harmonic distortion is:
A.
B.
C.
D.
Correct Answer:
Explanation:.
Incorrect! Try again.
41Why are transformers used in audio power amplifiers (especially tube or early transistor designs)?
A.To increase frequency response
B.To match the high output impedance of the amplifier to the low impedance of the speaker
C.To reduce the size of the amplifier
D.To filter DC from the input
Correct Answer: To match the high output impedance of the amplifier to the low impedance of the speaker
Explanation:Impedance matching maximizes power transfer. Speakers are low impedance (4-8), while amplifier stages (especially Class A) often have higher output impedances.
Incorrect! Try again.
42When reading the 2N3055 datasheet, the parameter (Transition Frequency) indicates:
A.The frequency where gain becomes 0
B.The frequency where the current gain () drops to unity (1)
C.The maximum switching frequency
D.The thermal oscillation frequency
Correct Answer: The frequency where the current gain () drops to unity (1)
Explanation: is the frequency at which the small-signal current gain decreases to 1. For a 2N3055, this is typically relatively low (e.g., 2.5 MHz or less).
Incorrect! Try again.
43In Class B operation, the DC current drawn from the supply varies with:
A.The temperature only
B.The amplitude of the input signal
C.The supply voltage only
D.It is constant regardless of signal
Correct Answer: The amplitude of the input signal
Explanation:Unlike Class A where average current is constant, Class B draws current proportional to the signal amplitude. Zero signal = zero current (ideally).
Incorrect! Try again.
44The maximum efficiency of a Class A amplifier with a direct-coupled resistive load is limited because:
A.The transistor cannot saturate
B.DC power is continuously dissipated in the load resistor
C.The transformer is inefficient
D.The power supply ripples
Correct Answer: DC power is continuously dissipated in the load resistor
Explanation:In series-fed Class A, the quiescent current flows through the load resistor constantly, dissipating significant power even with no signal.
Incorrect! Try again.
45A key advantage of Class AB over Class A is:
A.Simpler circuitry
B.Zero crossover distortion
C.Higher efficiency and lower quiescent power dissipation
D.No need for complementary transistors
Correct Answer: Higher efficiency and lower quiescent power dissipation
Explanation:Class AB rests near cutoff, drawing very little current when idle, whereas Class A draws full current when idle. This makes Class AB much more efficient.
Incorrect! Try again.
46Which parameter in the 2N3055 datasheet would you check to ensure the transistor does not break down when the base is open?
A.
B.
C.
D.
Correct Answer:
Explanation: stands for Collector-Emitter voltage with Base Open. This is the breakdown voltage limit in the common-emitter configuration.
Incorrect! Try again.
47In a harmonic distortion analysis using the three-point method, the waveform is analyzed at which points of the AC cycle?
A.Maximum, minimum, and zero crossing
B.Only at zero crossings
C.At every
D.At the RMS value
Correct Answer: Maximum, minimum, and zero crossing
Explanation:The three-point method usually uses the quiescent point (zero signal), the positive peak, and the negative peak currents/voltages to estimate distortion.
Incorrect! Try again.
48What is the typical value of thermal resistance () for a TO-3 packaged power transistor like the 2N3055?
A.
B.
C.
D.
Correct Answer:
Explanation:Power packages like TO-3 have low thermal resistance to the case, typically around , to facilitate heat transfer to a heatsink.
Incorrect! Try again.
49In a Class B push-pull amplifier, if the supply is and load , what is the peak current ?
A.1.25 A
B.2.5 A
C.5.0 A
D.0.8 A
Correct Answer: 2.5 A
Explanation:.
Incorrect! Try again.
50The 'derating factor' in a power transistor datasheet tells you:
A.How much the gain decreases with frequency
B.How much the maximum power dissipation rating decreases as temperature increases
C.The reduction in voltage rating with altitude
D.The decrease in price over time
Correct Answer: How much the maximum power dissipation rating decreases as temperature increases
Explanation:Above , the ability of the transistor to dissipate power decreases. The derating factor (e.g., ) is used to calculate the new max power limit.
Incorrect! Try again.
Give Feedback
Help us improve by sharing your thoughts or reporting issues.