common emitter amplifier

Discussion in 'The Projects Forum' started by hensle, Feb 9, 2012.

  1. hensle

    Thread Starter Member

    Dec 31, 2011
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    I am having a problem building a simple amplifier using a TIP31C transistor.

    I actually succeeded before with different transistors, but I can't seem to get this to work.

    I am trying to amplify a signal of about 1V up to 50V, but if I could just get an amplification of 10x I would be happy as I could always add another stage to get the final result.

    The source voltage is 100V, but I do not want more than 10mA of current, preferably less. So I start with a collector resistance of 10k.

    I need to know the emitter resistance and the two resistors to the base (for biasing). I have a book and went through the process given, but it didn't work. I also tried some ideas from the internet, but they didn't work either.

    I played around with it and got some amplification, but with lower collector resistance and it drew too much current.

    Does anyone know if this could work and what resistance there should be in the other three resistors (when the collector resistance is 10k)?

    I am sorry to ask as this is probably really simple, but I just can't seem to get it.
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What is the range of frequencies that you want to amplify? What is the input signal coming from?
     
  3. DickCappels

    Moderator

    Aug 21, 2008
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    Please show us the schematic. One picture is worth...
     
  4. hensle

    Thread Starter Member

    Dec 31, 2011
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    http://www.electronics-tutorials.ws/amplifier/amp5.gif

    Exactly like this except no capacitor C2. Vcc=100V and RL=10k.
    I can't seem to get the right R1 R2 and RE. The transistor is TIP31C
     
  5. hensle

    Thread Starter Member

    Dec 31, 2011
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    The input signal is a Colpitts Oscillator with a variable frequency from 900kHz to 2.5MHz.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    TIP31C will not work. Its unity gain-bandwidth product is only 3MHz, meaning you will get little gain at 900kHz-2.5MHz. Most high-current power transistors have this "problem".
     
    Last edited: Feb 10, 2012
  7. Audioguru

    New Member

    Dec 20, 2007
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    The frequencies are radio frequencies so a high frequency power transistor should be used. A BD139 is a high frequency (190MHz) power transistor has an absolute maximum collector voltage of 100V. Its datasheet does not show its safe operating area but your current is very low so it should work. It will get warm but does not need a heatsink.

    The schematic you posted uses a bypassed emitter resistor so its voltage gain without a load will be about 180 and its distortion at high levels will be extremely high.
     
  8. hensle

    Thread Starter Member

    Dec 31, 2011
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    Hmm..that explains how my lower frequency circuits worked, but not this one.

    Thanks much.
     
  9. hensle

    Thread Starter Member

    Dec 31, 2011
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    I missed that I was getting into frequencies where this would be a problem. Thanks for pointing me to the solution.
     
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