The voltage of my pulsed signal is falling when I input into a transistor

Discussion in 'General Electronics Chat' started by emer9, Apr 26, 2016.

  1. emer9

    Thread Starter New Member

    Apr 26, 2016
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    I have a 555 timer outputting a pulsed signal with an amplitude of 4.6V. I am then inputting the signal into a transistor to pulse the LED but when I do this the amplitude drops to .8V. How do I keep the maximum amplitude??
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Schematic?
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Sounds like you are putting the signal directly into a BJT without a series resistor, which is a no-no.
    The base-emitter junction of a BJT looks like a forward biased diode so it requires a resistor in series with base to limit the base current to the desired value (typically about 1/10th of the maximum collector current for it to act as a switch).
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    I agree with crutschow in that you are probably trying to drive a transistor base without a means of absorbing the rest of the voltage -- but whether this is done via a base resistor or an emitter resistor or some other means depends on the circuit. How do you expect people to tell you what is wrong with a circuit when you don't provide the circuit?
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    Did somebody tell you to do that, or did you just figure you'd try it out to see what happens? Inquiring minds want to know.
     
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