Electronics: push-pull amplifiers

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Discussion Overview

The discussion focuses on push-pull amplifiers, specifically Classes A and B, exploring their characteristics, advantages, disadvantages, and applications. Participants seek clarity on the distinctions between these amplifier classes and delve into related concepts such as Class C and Class D amplifiers.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants describe Class A amplifiers as having both upper and lower transistors conducting at all times, varying the current they conduct.
  • Others explain Class B amplifiers as having the upper transistor cut off when the lower conducts and vice versa, noting the difficulty in creating a smooth transition between these states.
  • Class AB amplifiers are mentioned as a popular compromise where both transistors conduct around the zero output point.
  • Class C amplifiers are characterized by only one transistor conducting at any time, with no conduction during zero output, making them suitable for RF applications.
  • Class D amplifiers are described as "digital" amplifiers that use pulse width modulation, with loudspeakers acting as low pass filters to recreate the signal.
  • One participant raises a question about the efficiency and distortion of Class B amplifiers, seeking an explanation for the observed trade-off.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the distinctions between amplifier classes, and while some points are clarified, the discussion remains unresolved regarding the reasons behind the efficiency and distortion characteristics of Class B amplifiers.

Contextual Notes

The discussion includes references to the conduction angle and efficiency differences among amplifier classes, but lacks detailed mathematical explanations or definitions that might clarify these concepts further.

kopinator
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I am writing a paper on Classes A and B push-pull amplifiers and was wondering if anyone would be willing to share some valuable insight into this type of circuit, i.e. advantages, disadvantages, when and where they can be used, etc.. Also, I have been doing a lot of my own research into this, but I'm struggling to find a distinction between Class A and Class B that is clear and easy to understand. Would anyone be willing to enlighten me on those differences?
Thank you!
 
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kopinator said:
I'm struggling to find a distinction between Class A and Class B that is clear and easy to understand. Would anyone be willing to enlighten me on those differences?
I will use transistor amplifiers in my examples.
  • Class A amplifiers: Both upper and lower transistors conduct the whole time, just varying the amount of current they conduct.
  • Class B amplifiers: Upper transistor cuts off when the lower conducts and vice versa. Creating a smooth transition between the two states is difficult, therefore class AB is more popular (both transistors conduct when the output goes through zero and some way beyond)
  • Class C amplifiers: Only one transistor conducts at any time. No transistor conducts when the output goes through zero (only useful for RF amplifiers)
  • Class D amplifiers: "Digital" amplifiers - the signal is converted to pulse width modulation and fed to the loudspeakers more or less directly. The loudspeakers act as a low pass filter and recreates the signal.
 
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The conduction angle is related to how long during the entire waveform the output stage transistor(s) behaves as a variable resistance (using power) in a divider with the load resistance and how long it operates switched off with 'no' power dissipated in the transistor. In the case of digital modes the 'ON' power used by the transistor is related to the usually small about of voltage drop across the device when saturated so it's not a linear resistance following the signal and efficiency is much higher because of that.
 
Wow, that's great! Thank you for your help. I can turn this into another double-spaced page of info. Thanks!
 
Actually, I have one more question: The Class B is more efficient, but has more distortion. Why is that?
 
kopinator said:
Actually, I have one more question: The Class B is more efficient, but has more distortion. Why is that?
Svein has the answer in that post.
 

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