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Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Problem: Crossover distortion in Class B.
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Solution: Small quiescent current in Class AB.
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Implementation: Diodes provide the necessary stable forward bias.
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Diode Polarity: Crucial for correct operation.
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Modification: Builds upon the existing Class B circuit.
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Examples
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Biasing Network: Imagine the bases of your NPN (Q1) and PNP (Q2) transistors are points 'A' and 'B' respectively. You connect the anode of Diode 1 to point 'A', its cathode to the anode of Diode 2. The cathode of Diode 2 connects to point 'B'. This series combination of diodes is then typically connected between a voltage divider from your main power rails to set the overall bias point.
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Observed Effect: When you apply a small sinusoidal input to the Class AB amplifier, you will observe a smooth output waveform, particularly around the zero-crossing point, in stark contrast to the notched waveform of the Class B amplifier at similar low input levels.
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Flashcards
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Term: Class AB Biasing
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Definition: Providing a small, continuous forward bias to push-pull transistors to maintain slight conduction and eliminate crossover distortion.
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Term: Diode Biasing Network
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Definition: A common method for Class AB biasing, using one or more forward-biased diodes in series between the transistor bases to provide a stable voltage drop.
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Term: 1N4001 / 1N4148
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Definition: Common silicon diodes suitable for providing \~0.7V voltage drop each, used in Class AB biasing.
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Term: Crossover Distortion Elimination
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Definition: The primary benefit of Class AB biasing, resulting in a cleaner output waveform by ensuring both transistors are never simultaneously off.
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Memory Aids
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"Diodes to Destroy Distortion": The diodes are key to getting rid of the crossover distortion.
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"AB is A Better B": Class AB is an improvement over Class B by fixing its main issue.
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"Just a Little Overlap": Remember that Class AB means the transistors conduct for just a little bit more than half a cycle, causing their conduction regions to overlap slightly.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
-
Biasing Network: Imagine the bases of your NPN (Q1) and PNP (Q2) transistors are points 'A' and 'B' respectively. You connect the anode of Diode 1 to point 'A', its cathode to the anode of Diode 2. The cathode of Diode 2 connects to point 'B'. This series combination of diodes is then typically connected between a voltage divider from your main power rails to set the overall bias point.
-
Observed Effect: When you apply a small sinusoidal input to the Class AB amplifier, you will observe a smooth output waveform, particularly around the zero-crossing point, in stark contrast to the notched waveform of the Class B amplifier at similar low input levels.
-
-
Flashcards
-
Term: Class AB Biasing
-
Definition: Providing a small, continuous forward bias to push-pull transistors to maintain slight conduction and eliminate crossover distortion.
-
Term: Diode Biasing Network
-
Definition: A common method for Class AB biasing, using one or more forward-biased diodes in series between the transistor bases to provide a stable voltage drop.
-
Term: 1N4001 / 1N4148
-
Definition: Common silicon diodes suitable for providing \~0.7V voltage drop each, used in Class AB biasing.
-
Term: Crossover Distortion Elimination
-
Definition: The primary benefit of Class AB biasing, resulting in a cleaner output waveform by ensuring both transistors are never simultaneously off.
-
-
Memory Aids
-
"Diodes to Destroy Distortion": The diodes are key to getting rid of the crossover distortion.
-
"AB is A Better B": Class AB is an improvement over Class B by fixing its main issue.
-
"Just a Little Overlap": Remember that Class AB means the transistors conduct for just a little bit more than half a cycle, causing their conduction regions to overlap slightly.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🧠 Other Memory Gems
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The diodes are key to getting rid of the crossover distortion.
* "AB is A Better B"
🧠 Other Memory Gems
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Remember that Class AB means the transistors conduct for just a little bit more than half a cycle, causing their conduction regions to overlap slightly.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: PNP Transistor
Definition:
A type of bipolar junction transistor where the N-type material is between two P-type materials. Conducts when base-emitter is negatively biased.
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Term: Modification
Definition:
Builds upon the existing Class B circuit.
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Term: Observed Effect
Definition:
When you apply a small sinusoidal input to the Class AB amplifier, you will observe a smooth output waveform, particularly around the zero-crossing point, in stark contrast to the notched waveform of the Class B amplifier at similar low input levels.
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Term: Definition
Definition:
The primary benefit of Class AB biasing, resulting in a cleaner output waveform by ensuring both transistors are never simultaneously off.
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Term: "Just a Little Overlap"
Definition:
Remember that Class AB means the transistors conduct for just a little bit more than half a cycle, causing their conduction regions to overlap slightly.
Detailed Explanation
The transition from a Class B to a Class AB power amplifier is a crucial modification aimed at eliminating the undesirable crossover distortion inherent in Class B designs. This distortion occurs because Class B transistors are biased exactly at cutoff, meaning there's a small "dead zone" around the zero-crossing of the input signal where neither transistor is sufficiently turned on. Class AB solves this by biasing each transistor slightly above cutoff, ensuring a small quiescent current flows even with no input signal, and creating a slight overlap in their conduction.
The primary conceptual modification for circuit construction involves adding a small, stable forward bias to the base-emitter junctions of both the NPN and PNP transistors in the push-pull stage.
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Starting Point: Existing Class B Circuit:
- You begin with the Class B push-pull amplifier circuit already assembled (from Part B). This circuit typically has an NPN transistor handling the positive half of the signal and a PNP transistor handling the negative half. Their bases receive the input signal.
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Introducing the Biasing Network (Diodes):
- The most common and effective way to achieve the slight forward bias for Class AB is by incorporating two forward-biased silicon diodes in series.
- Placement: These two diodes (e.g., 1N4001 or 1N4148) are placed directly between the bases of the NPN and PNP transistors.
- One diode's anode connects to the NPN transistor's base, and its cathode connects to the next diode's anode.
- The second diode's cathode connects to the PNP transistor's base.
- Functionality: When forward-biased, each silicon diode drops approximately 0.7V. Therefore, two diodes in series provide a total voltage drop of about 1.4V. This voltage directly applies a slight forward bias across the base-emitter junctions of both the NPN and PNP transistors.
- For the NPN, its base will be \~0.7V higher than its emitter.
- For the PNP, its base will be \~0.7V lower than its emitter.
- Result: This small, constant bias ensures that both transistors are just on the verge of conduction, or slightly conducting, for a small portion of the opposite half-cycle, eliminating the dead zone.
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Powering the Biasing Network:
- The series diodes typically derive their current from the main power supply rails through current-limiting resistors (not explicitly detailed in the provided text, but implied in a practical circuit). For example, a resistor from the positive supply to the NPN diode's anode, and another from the PNP diode's cathode to the negative supply (or ground if single supply with appropriate voltage division). This establishes a small quiescent current through the diodes, providing the stable bias voltage.
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Careful Connection and Polarity:
- Diode Orientation: It is absolutely critical to connect the diodes in the correct forward-biased direction. Current must flow through them from the positive supply side (or NPN base side) to the negative supply side (or PNP base side). If reversed, they will block the bias voltage, and the circuit will behave like Class B or not function at all.
- Secure Connections: Ensure the diode leads are firmly inserted into the breadboard holes and make good contact with the transistor bases. Loose connections can lead to intermittent operation or unwanted distortion.
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Re-verification:
- After incorporating the diodes, visually inspect the entire Class AB circuit.
- Double-check:
- The polarity of the diodes.
- The connections to the transistor bases.
- All other existing Class B connections remain correct.
- This final check before applying power is vital to prevent damage.
By making this modification, the amplifier's quiescent current increases slightly (from nearly zero in Class B to a small, measurable value), but the significant benefit is the elimination of crossover distortion, leading to a much cleaner and higher-fidelity output waveform, particularly at low signal levels.