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Understanding class A & B amplifiers

  1. Oct 18, 2007 #1
    I have read some texts myself but the explainations i read still leave me some queries. Below I have copied some text from wiki. From the text is in bold and my queries are in italic. Can someone please help me with my understanding, THANKS!!

    Class A amplifiers are the usual means of implementing small-signal amplifiers. They are not very efficient; a theoretical maximum of 50% is obtainable with inductive output coupling and only 25% with capacitive coupling.
    1. Does this mean Class A can only amplify small signals, and what range of volts would it be?
    2. What does it mean by 50% efficient? how is efficiency calculated??

    Class B amplifiers only amplify half of the input wave cycle. As such they create a large amount of distortion, but their efficiency is greatly improved and is much better than Class A. Class B has a maximum theoretical efficiency of 78.5%. This is because the amplifying element is switched off altogether half of the time, and so cannot dissipate power.
    3. They say that the efficiency is increased because the amplifying element is switched off half of time!! Then whats the point!! Isnt this just the same as Class A except that it is only amplifying half of the signal, how can this be efficient??
  2. jcsd
  3. Oct 18, 2007 #2
  4. Oct 18, 2007 #3


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    Class A amplifiers are usually mid point biased. This means that the output can vary widely about its midpoint. Biasing is in any other way can result in distortion and the output swing will decrease accordingly. Because of their poor efficiency, they are used as small signal or low power amplifiers.

    The efficiency refers the percentage of power drawn from the DC supply that is actually delivered to the load. You can refer to here for more talk on efficiency.

    The efficiency for a class B amp comes with the push pull output stage. This simply means that only one transistor is conducting for a given half of the output signal. The fact that they don't conduct fully at the same time makes it efficient. The key thing to note when analyzing this amplifier for efficiency is to assume that no base current flows and the transistor is biased just inside the cutoff region.
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