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Transistor darlington pair question

  1. Dec 7, 2008 #1
    A bit confused about how it works.
    Link - http://www.technologystudent.com/elec1/transis2.htm (3rd fig from top)

    The way I understand, transistor TR1's Base-emitter conducts, which fwd biases B-E junction of TR2. This in turn will turn on the lamp, but it won't be really bright.
    Once its on, the collector of TR1 is at +Vcc minus the drop across the lamp. This will increase emitter current of TR1, which in turn increases the base current going to TR2, which is amplified and makes the lamp glow even more.

    Also, why is TR1's collector connected to lamp instead of Vcc?
  2. jcsd
  3. Dec 8, 2008 #2
    please observe that the darlington pair is being operated in saturation mode/cutoff mode in quiescent point. The gain of the transistor pair is a fixed value : hfe^2.
    when base of TR1 is driven high VBE is > 0.7V hence TR1 enters sat mode, gushing out emitter current. which inturn drives TR2.
    The lamp is the load here, it is turned on when current flows through it.
    this is possible only if the Darllington pair in saturation to provide a path to ground.
    Hope this explaination helps.
  4. Dec 8, 2008 #3


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    I'm puzzled by this too. If TR1's collector were connected to Vcc, then TR2's collector could be brought to about a diode's drop away from ground. As it is, TR1's collector can't go closer than about two diode's drops from ground without taking the transistors out of saturation.
  5. Dec 8, 2008 #4
    Initially when TR1 has no base current, its collector is open. Just base current is enough to drive it into saturation? No need for BC junction to be reverse biased?
    Now, I can't figure out how a transistor switch works. How can collector current flow when there is no potential at Collector terminal.
  6. Dec 8, 2008 #5


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    Figure the wattage dissipated by TR1 when its collector is tied to +9V. It will unnecessarily be dissipating well over a watt.
  7. Dec 8, 2008 #6


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    Good point.
  8. Dec 8, 2008 #7
    likephysics , I would suggest you to go through transistor operation @ tpub.com
    I have found the explanations simple and accurate.
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