Transistor as amplifier

Discussion in 'General Electronics Chat' started by srn, Jan 11, 2011.

  1. srn

    Thread Starter New Member

    Jan 5, 2011
    2
    0
    Hi,
    I'm just starting to learn basic electronics and I have a question regarding transistors and their use as current amplifiers: If the maximum collector current is given by
    Ib x hFE then why don't we want to use this current as the output current and we want the transistor to be in "active" mode where the output current is less than Ib x hFE? Or in other words why don't we want the transistor to be saturated allowing the maximum current amplification?

    Thank you for your time
     
    Last edited: Jan 11, 2011
  2. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    It all depends upon what you are amplifying. If you need variability of amplitude, such as audio, a transistor has no ability to follow the signal when it reaches saturation, which would appear as clipping. On the other hand, if you are using something like PWM, saturation would be desired to reduce heat buildup in the transistor. Just my thoughts. Others may disagree or have additional thoughts.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Generally, from the point of view of biasing, Ic is hFE* Ib, where hFE is the large-signal current gain. In the saturated condition, hFE(SAT) applies: this is a gain value which must be assumed to guarantee Vce below some specified value at a particular value of Ic.

    Small-signal h-parameter calculations on signal currents (ib, ic) use the small-signal current gain hfe.

    The saturated condition relates to switching operation, or to some cases of large-signal amplification. Saturation is a maximum collector current condition. It does not represent maximum gain, in fact hFE(SAT) is generally less than the normal hFE.

    In many applications, linear or proportional amplification is needed. In this case saturation represents one of the limits of operation. Once saturation is reached the output cannot change any further in that direction. - this should be avoided if it is required to minimise distortion.

    The Wikipedia article entitled "Bipolar Junction Transistor" may help, although some of the material may be too complex at this stage.
    http://en.wikipedia.org/wiki/Bipolar_junction_transistor
     
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    Looking at the data sheet for a 2N3904 a common NPN transistor, at saturation its hFE is 10. In the active region it is 30-300. Saturated transistors make good switches because they will have a very low C-E voltage when "on". This minimizes the power dissipated in the transistor and puts most of the available voltage across the load. When used as amplifiers, most transistor circuits are designed to make the collector voltage 1/2 of the supply to allow for maximum output voltage swing. Most circuits also are designed to minimize the effect of different gains among "identical" transistors.
     
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