Design in a transistor

Discussion in 'General Electronics Chat' started by Tahmod, Feb 25, 2013.

  1. Tahmod

    Thread Starter New Member

    Feb 23, 2013
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    - I just want to ask what are the current or value you need to know before a design of a transistor circuit as a amplifier?
    For example do you need the collector current? You need the current gain?
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    All we need is a supply voltage, what will be our load resistance and from what we have a input signal (signal resistance). And voltage gain.
    And from BJT datasheet we need Hfe_min and sometimes Vbe and Hoe.
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,988
    3,226
    You need:
    required circuit gain,
    frequency response,
    supply voltage,
    desired maximum current draw (power dissipation),
    desired input impedance,
    output load impedance,
    operating temperature range.

    From that you can determine the needed transistor parameters.
     
  4. #12

    Expert

    Nov 30, 2010
    16,255
    6,754
    Designers usually don't worry much about the advertised gain of a transistor because we almost always use them at very much less than the maximum they can do.
     
  5. Tahmod

    Thread Starter New Member

    Feb 23, 2013
    12
    0
    required circuit gain, (help to find the Base current, when you know the value of the collector xcurrent for the design?)
    frequency response, (help to determine the capacitance value? If yes what is the formula?)
    supply voltage, (This is often limited because some component may damage?)
    desired maximum current draw (power dissipation), (this is found when the circuit is build and you can see it in the power supply, how much current the whole circuit takes?)


    desired input impedance, &output load impedance, (did not understand what are these?)
    operating temperature range. (this is useful when finding value in the datasheet, most of the time is 25c because of room temperature?)
     
  6. Tahmod

    Thread Starter New Member

    Feb 23, 2013
    12
    0


    -required circuit gain, (help to find the Base current, when you know the value of the collector xcurrent for the design?)
    -frequency response, (help to determine the capacitance value? If yes what is the formula?)
    -supply voltage, (This is often limited because some component may damage?)
    -desired maximum current draw (power dissipation), (this is found when the circuit is build and you can see it in the power -supply, how much current the whole circuit takes?)


    -desired input impedance, &output load impedance, (did not understand what are these?)
    -operating temperature range. (this is useful when finding value in the datasheet, most of the time is 25c because of room temperature?)


    -Why the Hfe minimum?
    -what is the purpose of the voltage gain ? (we got the current gain which is Hfe)
     
    Last edited: Feb 25, 2013
  7. crutschow

    Expert

    Mar 14, 2008
    12,988
    3,226
    The requirements I posted are the design requirements for the circuit you need to make. You can't do a design without first knowing what the requirements are, before you then calculate the circuit characteristics required to meet the design requirements.

    You seem to want to put the cart before the horse and do the design before you have the requirements.
     
  8. Tahmod

    Thread Starter New Member

    Feb 23, 2013
    12
    0
    required circuit gain, I just need a colector current of 250ma, with the base voltage being 5volts
    supply voltage, is 6volts
    operating temperature range. 25 degree

    Dont know about this
    desired maximum current draw (power dissipation),
    desired input impedance,
    output load impedance,
    frequency response, (I dont know what is best)
     
  9. crutschow

    Expert

    Mar 14, 2008
    12,988
    3,226
    If you are just designing to random requirements then any transistor will do. But normally you want the amplifier to perform a specific function with specific requirements. You haven't stated what those are.

    Why do you need a collector current of 250mA?
    Where does the base voltage of 5V come from?
     
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