transistor selection?

Discussion in 'General Electronics Chat' started by aj_silverthunder, Feb 17, 2010.

  1. aj_silverthunder

    Thread Starter Active Member

    Jun 6, 2009
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    my friend later said that one can use a transistor which has got greater Vceo,Vcbo,Vebo,Ic,Hfe,Ft,PD watts,
    and also said it will affect the circuit.
    but the thing i didn't understand correctly is that " will the new higher power transistor affect in a bad way or in a good way,like the new high power transistor will it amplify even more than the previous low rating transistor,or will the amplification will be distorted or in other words gives a bad output"
    please explain me ,in simple word friends
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Depends on the original circuit. If it was properly designed to accomodate transistors with a wide spread of parameters, then substituting a similar type, but different part number should not matter. If the design is naive or marginal, it could be exquisitely sensitive to a specific transistor, requiring a hand-selected replacement.
     
  3. aj_silverthunder

    Thread Starter Active Member

    Jun 6, 2009
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    so better i can use a Smith chart to make the input and output network for the transistor,
    would u think its the best way to design a RF Amp
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    RF design is a different story. What I posted applies to use of transistors for switching/audio.
     
  5. aj_silverthunder

    Thread Starter Active Member

    Jun 6, 2009
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    sorry but i was talking about RF amplification
     
  6. aj_silverthunder

    Thread Starter Active Member

    Jun 6, 2009
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    guys as per my knowledge that the amplified RF output from the first transistor ,should be filter with a output matching network(50ohm)and then the 50ohm output should be connected to the base of the next transistor amplifier stage.As,50Ω is normally used for RF, 75Ω for video, 100Ω for data and 600Ω for audio.And the output from the second transistor should go through output matching network(50ohm) and finally connected to the 50ohm Antenna.
    this type of technique will have good amplification because 1)there will be good gain ,because of good matching of impedances.
    2)for example the first stage transistor amplifies 2V RF to 5V,then the second RF transistor will amplify that 5V to maybe 10V.
    this type is cascading is better than using a cap,which has low reactance for the particular frequency of signal.for ex,a 39pf cap has reactance of 40ohms for 100Mhz frequency.

    so please tell me is my theory is correct,because i have never tried in practical before.please correct me.
     
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