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Transistor amplification related doubts

  1. Feb 17, 2014 #1
    I understand the basic concept. A small change in the base current will lead to large change in the collector current and will lead to amplification.

    But the question is, the small change in base current is brought by input signal or the biasing? Also the coupling capacitors allow only the ac signal to be amplified, so whats the use of dc biasing? It will be wasted.

    Also whats the effect of ac changing its polarity on the transistor?

    Some doubts...But i will love to have them cleared. Thank you :]

    Edit: Small not large change in base current.
    Last edited: Feb 17, 2014
  2. jcsd
  3. Feb 17, 2014 #2
    I don't understand this. What large change in the base current?
  4. Feb 17, 2014 #3
    Sorry, I meant a small change in base current leading to a large change in collector current is the basis of amplification. This small change is the change in input signal right? The biasing is not changed throughout the amplification process?

    For example, in a speaker while adjusting its volume do we change the input signal or the biasing? Quite confusing :S
    Last edited: Feb 17, 2014
  5. Feb 17, 2014 #4
  6. Feb 17, 2014 #5


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    Gold Member

    None of this is constant for all applications, but generally the bias sets the DC operating point such that the small signal is being amplified by the transistors at an optimum point in their characteristics. The gain is generally changed by a resistor divider or resistor ratios somewhere in the amplifier. Generally the bias remains constant.
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