Chapter 5: Power Amplifiers Quiz

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Questions and Answers

What is the conduction angle for a Class A power amplifier?

  • Less than 180°
  • 360° (correct)
  • 90°
  • 180°

What is the maximum efficiency of a Class A power amplifier?

  • 100%
  • 50%
  • 25% (correct)
  • 78.5%

What characteristic of a Class A amplifier allows it to conduct over the entire input signal cycle?

  • It operates with a conduction angle of 90°.
  • It only conducts during positive cycles.
  • It is biased at half of the supply voltage. (correct)
  • It has high maximum efficiency.

What is the maximum efficiency of a Class A amplifier?

<p>25% (D)</p> Signup and view all the answers

Which class of amplifier has an output wave that is 180° out of phase with the input?

<p>Class A (C)</p> Signup and view all the answers

In which amplifier class does the transistor conduct over only half of the input signal cycle?

<p>Class B (A)</p> Signup and view all the answers

What is the operating cycle for a Class B amplifier?

<p>180° (B)</p> Signup and view all the answers

What is the relationship between the load line and the Q-point in a Class A amplifier?

<p>The Q-point must be centered on the load line (B)</p> Signup and view all the answers

What is the primary basis for classifying amplifiers into different classes?

<p>Construction and operating characteristics (B)</p> Signup and view all the answers

What conduction angle does a Class B amplifier operate at?

<p>180° (D)</p> Signup and view all the answers

Which class of amplifier typically has a power efficiency greater than 90%?

<p>Class C (A)</p> Signup and view all the answers

In terms of power efficiency, which amplifier class falls between 25% and 78.5%?

<p>Class AB (B)</p> Signup and view all the answers

Which statement accurately describes a Class A amplifier?

<p>It transmits both positive and negative cycles equally. (D)</p> Signup and view all the answers

What is the conduction angle of a Class C amplifier?

<p>Less than 180° (A)</p> Signup and view all the answers

How does the Q-point relate to Class A amplifiers?

<p>It is biased to ensure $I_{CQ} &gt; I_{s max}$. (D)</p> Signup and view all the answers

What is a key benefit of Class B amplifiers compared to Class A amplifiers?

<p>Higher power efficiency. (B)</p> Signup and view all the answers

What is the purpose of biasing a transistor in a series-fed Class A amplifier?

<p>To allow for maximum output voltage swing (C)</p> Signup and view all the answers

What does the collector current in a Class A amplifier depend on?

<p>Base-bias current and the transistor's beta (C)</p> Signup and view all the answers

In a Class A amplifier, what is the relationship between collector-emitter voltage (VCE) and collector current (IC)?

<p>VCE decreases as IC increases (C)</p> Signup and view all the answers

What is the overall efficiency of a Class A amplifier compared to other classes of amplifiers?

<p>Generally lower than other amplifier classes (B)</p> Signup and view all the answers

Why is it important to allow the output to vary through a full VCC volts peak-to-peak in a Class A amplifier?

<p>To allow for a full cycle of input sine wave operation (C)</p> Signup and view all the answers

What formula represents the DC base-bias current in a Class A amplifier?

<p>$I_B = V_{CC} - 0.7 ΤR_B$ (D)</p> Signup and view all the answers

What does the power drawn from the supply in a Class A amplifier represent?

<p>Active power consumed by the amplifier (B)</p> Signup and view all the answers

What impact does a larger collector resistor (RC) have on a Class A amplifier's performance?

<p>Results in lower output power (A)</p> Signup and view all the answers

What type of transistors are used in a Class B push-pull amplifier?

<p>One npn and one pnp transistor (B)</p> Signup and view all the answers

What issue does crossover distortion cause in a Class B amplifier?

<p>Distortion in the output waveform (B)</p> Signup and view all the answers

How is a Class AB amplifier biased compared to a Class B amplifier?

<p>Slightly above cutoff (C)</p> Signup and view all the answers

What is a primary advantage of Class AB amplifiers over Class B amplifiers?

<p>Elimination of crossover distortion (A)</p> Signup and view all the answers

What happens when the DC base voltage is zero in a Class B amplifier?

<p>Both transistors are off (C)</p> Signup and view all the answers

What is the effect of adjusting the biasing in a Class AB amplifier?

<p>Allows slight conduction even with no input (C)</p> Signup and view all the answers

In a Class B push-pull configuration, what is meant by 'transformer coupling'?

<p>Using transformers to connect the stages (A)</p> Signup and view all the answers

What defines the operation of complementary symmetry transistors in a Class B amplifier?

<p>They conduct on opposite alternations of the input cycle (C)</p> Signup and view all the answers

What is the primary characteristic of Class C amplifiers regarding conduction?

<p>Conduction occurs for much less than 180 degrees. (D)</p> Signup and view all the answers

Why are Class C amplifiers not suitable for linear amplification?

<p>Their output amplitude is a nonlinear function of the input. (D)</p> Signup and view all the answers

What type of load is a Class C amplifier typically operated with?

<p>Resonant circuit load (B)</p> Signup and view all the answers

During which event does the transistor in a Class C amplifier turn on?

<p>When the peak value exceeds the barrier potential. (A)</p> Signup and view all the answers

What does the power dissipation equation for a Class C amplifier represent in terms of operation?

<p>The average power dissipation during the entire cycle. (B)</p> Signup and view all the answers

In tuned Class C operation, how is the collector voltage characterized?

<p>It is not a replica of the input signal. (C)</p> Signup and view all the answers

How does the efficiency of Class C amplifiers compare to Class A, B, or AB amplifiers?

<p>It is more efficient than all classes. (B)</p> Signup and view all the answers

For what purpose is the resistive load used in a Class C amplifier's operation?

<p>To illustrate the operational concept. (D)</p> Signup and view all the answers

What is the primary function of using equal values of R1 and R2 in the voltage-divider arrangement?

<p>To force the voltage at point A to equal 0 V (C)</p> Signup and view all the answers

In a Class B amplifier, what happens to the transistors when there is no input signal?

<p>They have only a very small current (A)</p> Signup and view all the answers

How does the Q-point of a Class A amplifier differ from that of a Class B amplifier?

<p>It is near the middle of the output characteristics (D)</p> Signup and view all the answers

What is the ideal maximum peak output voltage estimated to be in a Class B amplifier?

<p>$V_{CC}$ (B)</p> Signup and view all the answers

What condition allows the current in the diodes D1 and D2 to match the current in the transistor BE junctions?

<p>Having closely matched diode characteristics (B)</p> Signup and view all the answers

What does the ac saturation current ($I_{c(sat)}$) expression imply in a class B amplifier?

<p>$I_{c(sat)} = C imes R_L$ (A)</p> Signup and view all the answers

What is a significant advantage of a Class B amplifier compared to a Class A amplifier?

<p>Higher efficiency (D)</p> Signup and view all the answers

Which voltage represents the threshold for ac cutoff in a Class B amplifier?

<p>$V_{CC}$ (D)</p> Signup and view all the answers

Flashcards

Class A Amplifier

An amplifier where the output signal varies for a full 360° of the input signal. The transistor conducts for the entire input cycle.

Class B Amplifier

An amplifier that provides an output signal varying over one-half the input cycle or 180° of signal. The transistor conducts only for one-half the input signal.

Class AB Amplifier

An amplifier that operates between class A and class B, outputting signal close to one half of input cycle. The transistors conduct for more than 180° of the signal.

Class C Amplifier

An amplifier where the output signal varies for less than 180° of the input cycle. The transistor conducts for parts of one input cycle only.

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Amplifier Classes

Amplifiers are categorized based on their construction and operating characteristics, determining how much of the input signal they amplify.

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Class A Amplifier Operation

The transistor conducts for the full input cycle, operating in the linear region of its characteristic curves.

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Class A Operating Cycle

The amplifier's operating cycle is 360 degrees.

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Class A Power Efficiency

The maximum efficiency is 25% to 50%.

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Power Amplifier Efficiency

Measure of how much input power is converted to output power.

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Class A Output/Input Phase

The output is 180 degrees out of phase with the input.

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Class AB Amplifier Output Cycle

Amplifier operates between 180 and 360 degrees, producing output for more than half the input cycle.

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Class AB Power Efficiency

Class AB Amplifier efficiency is between 25% and 78.5%

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Class B Operating Cycle

Class B amplifier operates over 180°. Output signal varies for a half input cycle.

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Class B Power Efficiency

Class B Power Amplifier efficiency ranges from 25% to 78.5%.

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Class C Operating Cycle

Amplifier operates for less than 180° of the input cycle.

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Class C Power Efficiency

Efficiency is greater than 90%.

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Class A Amplifier

An amplifier where the transistor is active for the entire input cycle (360 degrees).

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DC Bias Operation

Calculating the DC bias current (IB), collector current (IC), and collector-emitter voltage (VCE) in Class A amplifier.

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Base Bias Current (IB)

Calculated as (VCC - 0.7) / RB.

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Collector Current (IC)

Calculated as β * IB, where β is the current gain.

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Collector-Emitter Voltage (VCE)

Calculated as VCC - IC * RC.

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AC Operation

Analysis of the amplifier's behavior under AC conditions.

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Power Considerations

Evaluation of power consumption and output in a Class A amplifier.

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Maximum Efficiency

Highest possible percentage of power conversion from input to output.

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Class B Push-Pull Operation

A power amplifier configuration using two transistors (e.g., npn and pnp) to amplify an AC signal. Each transistor conducts for half of the input cycle, creating an output with a 180-degree phase shift.

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Complementary Symmetry Transistors

Using transistors of different types (npn and pnp) in a push-pull amplifier configuration. This arrangement allows for current flow in both direction of an AC cycle.

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Crossover Distortion

Distortion in the output waveform of a Class B amplifier caused by a brief period where neither transistor conducts (because base voltage is too low).

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Class AB Amplifiers

A power amplifier design that combines aspects of class A and class B to reduce crossover distortion by biasing transistors slightly to operate in their linear region.

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Biasing for Class AB Operation

Adjusting the bias voltage to slightly turn on the transistors in a push-pull amplifier configuration. Preventing the crossover distortion present in Class B.

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Class AB Amplifier Q-point

Slightly above cutoff, unlike class B amplifiers which are at cutoff.

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Class AB Cutoff Voltage

The voltage at which the transistor stops conducting.

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Class AB Saturation Current

Maximum current an amplifier can produce when fully saturated.

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Class A Q-point Location

Near the middle of the load-line.

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Class B Amplifier Q-point

At cutoff

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Class AB Amplifier's Output Cycle

Operates between 180 and 360 degrees,producing output for more than half the input cycle.

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Class AB Diode Current

Equal to the transistor collector current(ICQ).

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Class AB Amplifier Efficiency

Lies between 25% and 78.5%.

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Class C Amplifier Conduction Time

Conducts for less than 180 degrees of the input signal's cycle.

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Class C Amplifier Efficiency

Higher efficiency compared to Class A, B, and AB.

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Class C Amplifier Linearity

Output isn't a direct copy (linear) of the input signal.

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Class C Amplifier Application

Used primarily in radio frequency (RF) circuits and resonant loads.

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Class C Amplifier On-Time Power Dissipation

Calculated as Ic(sat) * Vcc(sat).

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Class C Amplifier Average Power Dissipation

Calculated by averaging the dissipation over a complete input cycle.

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Tuned Class C Operation

Output is not a direct replica of the input in a purely resistive load, making it unsuitable for linear applications.

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Study Notes

Chapter 5: Power Amplifiers

  • Power amplifiers are used in the final stage of communication receivers or transmitters to deliver signal power to devices like speakers or antennas.
  • Bipolar Junction Transistors (BJTs) are used to illustrate power amplifier principles.
  • Efficiency is a critical factor in power amplifiers to prevent excessive junction temperature or prolong battery life. Optimizing temperature range typically is 150-200°C for silicon junctions.
  • Linearity (or Total Harmonic Distortion (THD)) is important to maintain signal quality.
  • Amplifiers are classified by frequency capabilities (audio or radio frequency) and coupling methods (R-C, transformer, direct).
  • Voltage amplifiers primarily increase the input voltage with minimal current output.
  • Power amplifiers increase the input power with minimal change in voltage, used in applications requiring high power delivery (e.g. audio and radio frequency).
  • Power amplifiers and voltage amplifiers differ in current gain, voltage gain, heat dissipation, cooling, transistor size, base width and beta. (refer to relevant tables).
  • Large-signal amplifiers (power amplifiers) feature high efficiency, high power handling capability and proper impedance matching with output devices.
  • Amplifier classes represent the amount the output signal varies over one cycle of operation for a full cycle of input signal (class A, AB, B, C).
  • Class A amplifier operates in the linear region of the transistor's characteristic curves throughout the input cycle, with an efficiency of 25% to 50%. The output is 180° out of phase with the input.
  • Class B amplifier conducts for only half the input cycle.  The efficiency is up to 78.5%.
  • Class AB amplifier is biased slightly above cutoff and operates in the linear region for slightly more than 180° of the input cycle.  It eliminates crossover distortion found in pure class B amplifiers.
  • Class C amplifier is biased for operation at less than 180° of the input signal cycle, usually with a tuned (resonant) circuit, for high efficiency (greater than 90%).

Transformer-Coupled Class-A Power Amplifiers

  • Voltage transformation: V₂/V₁ = N₂/N₁

  • Current transformation: I₂/I₁ = N₁/N₂

  • Impedance transformation: R₁/R₂ = (N₁/N₂)² = α²

  • The reflected impedance (R₁) is directly related to the square of the turns ratio (N₁/N₂).

  • Efficiency depends on both the load resistance (RL) and the collector resistance (Rc).

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