Tramsistor Question

Discussion in 'General Electronics Chat' started by Mazaag, Dec 1, 2004.

  1. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    255
    0
    Hey guys,
    Newbie here..

    Just a quick question..

    On this circuit Diagram, what is the purpose of the 1K ohm resistor connected underneath the 3K ohm resistor next to the base of the transistor?
    don't you just require a current to enter the base, and it will switch the collector on ? ( ie : can't you just have it connecting straight from the 3kohm resistor to the base ? without the 1 k ohm ? )

    thanks guys

    http://skyscraper.fortunecity.com/plug/587/learn97.htm
     
  2. vhneumann

    New Member

    Dec 1, 2004
    4
    0
    Even when the output of the comparator goes low the voltage at the transistor's base it is not 0V because of Icbo (leakage current from collector to base). This current causes a voltage to appear across the base-emitter junction, causing greater current through collector. Build the circuit, leave the 1K resistor open and you will see the LED dimly lit when the ouput from the comparator is 0V. Connecting 1K resistor in place lowers the impedance seen from the base; Vbe gets lower and Ic reaches cutoff (Icbo is still there). The same circuit applies if you use a MOSFET, though the physics are different.

    Best regards
     
  3. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    255
    0
    Wouldn't we fix the problem if we put a much higher resistor instead of the 3k ohm ? I mean woudln't that lower the current into the base , and there make the LED dimmer ?

    Thanks
     
  4. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    255
    0
    I'm kinda lost here heheh
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    A higher value resistor just makes the problem worse. Transistors are current amplifiers. The beta, or Hfe figure gives the potential amplification of the transistor. Limiting current into the base also affects the current through the collector circuit, and dims the led. And without the other resistor to ground, the leakage problem is still there, so the led never goes out.
     
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