base of transistor

Discussion in 'General Electronics Chat' started by alexmath, May 19, 2014.

  1. alexmath

    Thread Starter New Member

    May 2, 2014
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    What does "turn on" the transistor? The current applied to the base of the transistor or the voltage?

    Also what happens if you apply constant current through an led with the voltage less then the forward drop voltage?
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    You need both, the current and the voltage.
    The current source will go into saturation, and the current will drop.
     
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
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    1. In the case of a BJT you can't really separate the two. Cannot get current without voltage and neither can you get voltage without current. BUT that said BJTs are basically current driven devices.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Both the LED and the transistor base are diodes, and they will not conduct until the required voltage is reached. This is ~3V for a typical LED and ~0.6V for a silicon diode, including the base of a transistor. Below the threshold voltage, there is only a tiny leakage of current, orders of magnitude less than when conduction starts.
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    how do you propose to put constant current through an led while dropping the voltage? voltage across a junction determines current through the junction. just like a resistor, if you have a 100 ohm resistor with 1 amp of current through it, you have 100 volts across the resistor. if you reduce the voltage, you will have less current. there is no way to hold the curent constant when varying the voltage.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Oh,oh. You've said the trigger words. There have been (nearly endless) discussions on these forums about the nature of a BJT's operation. Solid-state theory basically states that they are voltage operated with the base current being incidental to that, due to the low input impedance of the base-emitter junction. And they are usually considered as voltage operated, low-input-impedance device with transconductance (Gm) gain when used as AC amplifiers. But when operated as switches or calculating the DC bias point of AC amplifiers, it's usually easier to consider them current operated and use the beta or hFE current gain to do the calculations.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    More exactly, solid-state diodes (BJT emitter-base, LEDs, etc.) have a logarithmic relationship between forward voltage and current . If you look at their voltage versus current plot on a semi-log graph you will see a nearly straight line down to a low voltage (see this for example). For most applications, that current is not considered significant until it reaches a practical operating level (typically in the μA to mA region) at which point the voltage is considered to have reached a characteristic (threshold) voltage as determined by the diode material. That's the voltage wayneh is referring too.
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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    In general this is true, since modern devices are optimised to work in this fashion, but actually you can. You can even operate the transistor with the base disconnected!

    http://www.youtube.com/watch?v=NoeuVxYoNio

    If you watch the above video you will see some footage of the original transistors and a simulation of the discovery of the transistor effect. The original effect was light activated.
     
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