Circuit Construction - 6.3.2
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 & Applications
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
Interactive tools to help you remember key concepts
Memory Tools
The diodes are key to getting rid of the crossover distortion.
* "AB is A Better B"
Memory Tools
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
Glossary
- PNP Transistor
A type of bipolar junction transistor where the N-type material is between two P-type materials. Conducts when base-emitter is negatively biased.
- Modification
Builds upon the existing Class B circuit.
- 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.
- Definition
The primary benefit of Class AB biasing, resulting in a cleaner output waveform by ensuring both transistors are never simultaneously off.
- "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.