1.6 Amplifiers

1. Classification of Output Stages: Class A, Class B, and Class AB Stages

Amplifiers are classified based on their output stages and efficiency. The output stage of an amplifier determines the linearity, efficiency, and power handling of the amplifier. The common classes of amplifier output stages are Class A, Class B, and Class AB.

1. Class A Amplifier

In a Class A amplifier, the output transistor conducts for the entire cycle (360°) of the input signal. This means the transistor is always on, regardless of the input signal's magnitude.

• Advantages:

  • High linearity: The output is a faithful reproduction of the input signal, with minimal distortion.

  • Low harmonic distortion.

• Disadvantages:

  • Low efficiency: Due to continuous conduction, Class A amplifiers are not power-efficient (around 25-30%).

  • Heat generation: High power loss results in significant heat dissipation.

• Application:

  • Used in high-fidelity audio systems and situations where minimal distortion is crucial.

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1. Class B Amplifier

In a Class B amplifier, the output transistor conducts for half (180°) of the input signal cycle. Two transistors are used, each amplifying one half of the waveform (positive or negative).

• Advantages:

  • Higher efficiency: Class B amplifiers are more efficient than Class A (around 50-60%).

• Disadvantages:

  • Crossover distortion: At the point where the two transistors switch between conducting and non-conducting states, distortion can occur, leading to non-linearities.

• Application:

  • Used in power amplifiers for radio frequency (RF) systems and audio systems where efficiency is important.

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1. Class AB Amplifier

Class AB amplifiers combine the advantages of Class A and Class B amplifiers. The output transistors conduct for more than half (180°) but less than the entire input signal cycle (less than 360°). This reduces crossover distortion while maintaining better efficiency than Class A.

• Advantages:

  • Better efficiency: Class AB amplifiers are more efficient than Class A but provide less distortion than Class B (around 50-70%).

  • Reduced crossover distortion: Through careful biasing, the distortion at the crossover point can be minimized.

• Disadvantages:

  • Slightly higher distortion than Class A.

• Application:

  • Widely used in audio power amplifiers, such as in car audio systems, home theater systems, and professional audio equipment.

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2. Biasing, Power BJTs, Transformer-Coupled Push-Pull Stages, Tuned Amplifiers, and Op-Amps

1. Biasing

Biasing in amplifiers refers to setting the operating point of the transistor to ensure it operates in the desired region of the output characteristic curve. Proper biasing is crucial for linear amplification and to avoid distortion.

• Types of Biasing:

  • Fixed bias: The base bias voltage is applied through a resistor.

  • Self-bias (or emitter-bias): The biasing resistor is placed in the emitter leg of the transistor to stabilize the operating point.

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1. Power BJTs (Bipolar Junction Transistors)

Power BJTs are used in high-power applications, where large current and voltage handling are required. These BJTs are designed to amplify large signals and deliver significant power to the load (e.g., in audio or RF amplifiers).

• Key Parameters: The key parameters for power BJTs are current gain, saturation voltage, and power dissipation.

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1. Transformer-Coupled Push-Pull Stages

A push-pull amplifier is a type of amplifier circuit that uses two transistors (or vacuum tubes) to amplify both halves of an AC signal. One transistor handles the positive half-cycle, while the other handles the negative half-cycle, resulting in a more efficient and powerful output. To combine these two amplified signals into a single output, a transformer is used as a coupling device. This method of combining signals is known as transformer coupling and is widely used in high-power audio and RF applications.

• Advantages:

  • High efficiency.

  • Reduced distortion (no crossover distortion as in Class B).

  • Suitable for high-power applications.

• Application:

  • Common in audio power amplifiers and RF amplifiers.

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1. Tuned Amplifiers

Tuned Amplifiers are amplifiers designed to work at a specific frequency or range of frequencies, achieved by using resonant circuits (LC circuits) in the amplifier's feedback or load.

• Application: Used in radio frequency (RF) applications such as radio transmitters and receivers, where only a specific frequency needs to be amplified.

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1. Op-Amps (Operational Amplifiers)

Op-Amps are versatile, high-gain electronic voltage amplifiers with differential inputs. They are used in a wide variety of applications, including signal processing, control systems, and filters.

• Ideal Characteristics: Infinite open-loop gain, infinite input impedance, and zero output impedance (in ideal cases).

• Applications:

  • Used in audio amplification, filtering, and signal conditioning.

  • Active filters, oscillators, and buffers.

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3. Common Emitter, Common Base, and Common Collector Amplifiers

Overview of Amplifier Configurations

Parameter	Common Emitter (CE)	Common Base (CB)	Common Collector (CC)
Voltage Gain $A_v$	$A_v = -\frac{\beta R_C}{r_e}$	$A_v = \frac{R_C}{r_e}$	$A_v \approx 1$
Current Gain $A_i$	$A_i = \beta$	$A_i < 1$	$A_i = \beta + 1$
Input Impedance $Z_{in}$	$Z_{in} = \frac{\beta}{g_m}$, where $g_m = \frac{I_C}{V_T}$	$Z_{in} = r_e$	$Z_{in} = \beta R_E$
Output Impedance $Z_{out}$	$Z_{out} = R_C$	$Z_{out} = R_C$	$Z_{out} \approx \frac{1}{g_m}$
Phase Relationship	Inverted (180° phase shift)	No phase shift	No phase shift
Primary Use	Voltage amplification	Current amplification	Impedance matching

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Key Points for Each Configuration

1. Common Emitter (CE) Amplifier

• Characteristics:

  • Provides both voltage gain and current gain, making it ideal for amplification.

  • Produces a 180° phase inversion between input and output signals.

  • Has a moderate input impedance and output impedance.

• Applications:

  • Used as a voltage amplifier in audio, radio, and other signal-processing circuits.

• Key Formulas:

  • Voltage gain:

    • $A_v = -\frac{\beta R_C}{r_e}$

  • Input impedance:

    • $Z_{in} = \frac{\beta}{g_m}, \text{ where } g_m = \frac{I_C}{V_T}$

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1. Common Base (CB) Amplifier

• Characteristics:

  • Provides voltage gain but no current gain $A_i < 1$.

  • Input impedance is low, and output impedance is high.

  • Suitable for circuits requiring current amplification with stable voltage.

• Applications:

  • Used in high-frequency applications, such as RF amplifiers.

• Key Formulas:

  • Voltage gain:

    • $A_v = \frac{R_C}{r_e}$

  • Input impedance:

    • $Z_{in} = r_e$

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1. Common Collector (CC) Amplifier (Emitter Follower)

• Characteristics:

  • Provides current gain but very little voltage gain $A_v \approx 1$.

  • Input impedance is high, and output impedance is low, making it ideal for impedance matching.

  • No phase inversion occurs.

• Applications:

  • Used as a buffer to connect high-impedance sources to low-impedance loads.

• Key Formulas:

  • Voltage gain:

    • $A_v \approx 1$

  • Input impedance:

    • $Z_{in} = \beta R_E$

  • Output impedance:

    • $Z_{out} \approx \frac{1}{g_m}$

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Common Emitter:

• High voltage and current gain.

• Phase inversion (180° shift).

• Best for general-purpose voltage amplification.

Common Base:

• High voltage gain but low current gain.

• Low input impedance and high output impedance.

• Used in high-frequency and RF circuits.

Common Collector:

• High current gain but unity voltage gain.

• High input impedance and low output impedance.

• Ideal for impedance matching.

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Conclusion

• Amplifier output stages (Class A, Class B, and Class AB) each have distinct advantages and disadvantages in terms of linearity, efficiency, and distortion, with Class A offering high fidelity but low efficiency, Class B offering higher efficiency but distortion, and Class AB balancing both factors for practical applications.

• Biasing is crucial in ensuring that amplifiers, especially in power BJTs and push-pull configurations, operate in the desired region for linear amplification, avoiding distortion and ensuring optimal performance.

• Op-Amps, tuned amplifiers, and transformer-coupled push-pull stages offer versatile solutions for specialized amplification needs, such as high-power amplification, frequency-specific signal boosting, and low-distortion audio amplification.