how to connect output to drive multiple transistors?

Discussion in 'General Electronics Chat' started by tpny, Oct 31, 2012.

  1. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    If I want my output to drive two transistors, is (a) or (b) below better?

    Code ( (Unknown Language)):
    1.  
    2. a) output ---resistor-----transistor base1
    3.                       |
    4.                       |
    5.                       -----transistor base2
    6.  
    7. or
    8.  
    9. b) output-----resistor-----transistor base1
    10.          |
    11.          |
    12.          ---resistor-----transistor base2
    13.  
     
    Last edited: Oct 31, 2012
  2. crutschow

    Expert

    Mar 14, 2008
    13,013
    3,233
    b) is the proper choice.

    a) will allow the transistor with the slightly lower base-emitter voltage to hog a majority of the base current and the other transistor may not turn on. Never do a)
     
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  3. Austin Clark

    Member

    Dec 28, 2011
    409
    44
    Aye. Basically, the current capable of flowing through the resistor in example A will be split to both transistors, and unevenly at that. This is true for driving LEDs and other devices as well. Imagine trying to light a thousand LEDs this way, or driving a thousand transistors... The sum of current required will far exceed the maximum possible current capable of passing through the single series resistor given it's resistance and source voltage. As a consequence, you'll get a heavy voltage drop, and the resistor may even become overloaded.
     
  4. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    assuming i'm using a resistor value in (a) that is the parallel equivalent of the 2 resistors in (b); and assuming both transistors are the same type. Are (a) and (b) equivalent?
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,441
    3,361
    No. No two transistors are the same.
     
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