simple question about transistor datasheet

Discussion in 'Wireless & RF Design' started by baby_1, Mar 25, 2012.

  1. baby_1

    Thread Starter Member

    Jun 3, 2011
    39
    0
    hello i see the 2n3553 NPN power transistor for rf.
    i see these statements

    Minimum Gain: 10dB
    Efficiency: 50%

    what is "Gain: 10dB" meaning?
    why they show us the effeciency of transistor?it doesn't related to circuit that we design? for example class A or class C?

    Thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    The gain of the transistor is depended on the frequency it is used.
    In figure 8 on page 6 of the attached datasheet you will see the relationship,
    You also see that the graphs go down to 50 Mhz, below this frequency the amplifier becomes instable with a great chance of oscillating.
    All circuits are based on a class C amplifier.

    Bertus
     
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  3. Papabravo

    Expert

    Feb 24, 2006
    10,170
    1,797
    Decibels are used as a convenient means of specifying the gain of an amplifier over a wide range. The formula for "Voltage Gain" is:
    Code ( (Unknown Language)):
    1.  
    2. 20 * log_10(Vo/Vi)
    3.  
    In the transistor example we can solve for the ratio of voltage out to voltage in by the following
    Code ( (Unknown Language)):
    1.  
    2. 10 = 20 * log_10(Vo/Vi)
    3. 0.5 = log_10(Vo/Vi)
    4. 10^(0.5) = Vo/Vi
    5. 3.16 = Vo/Vi
    6.  
    So as an RF amplifier if the input from an antenna was 50 microvolts, the output would be 158 microvolts.
     
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  4. baby_1

    Thread Starter Member

    Jun 3, 2011
    39
    0
    Thanks Papabravo
    i want to ask my main question.doesn't have an effect the bias of a transistor on gian?why datasheet show us the gain?
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    The class C amplifier is non-linear.
    You will need a certain amount of power to "activate" the amplifier.
    Above this lower input level the gain will be seen.

    See pages 5 and 6 of the attached PDF for more info.

    Bertus
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,170
    1,797
    There are different types of bias circuits. If I am not mistaken Class C circuits are usually clamped to Vcc and only conduct for a small portion of the cycle. So why do you think the bias will have any effect?
     
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