Transistor theory

Discussion in 'General Electronics Chat' started by Webby, Jun 15, 2008.

  1. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Hi, first time Ive posted here. The website is great and easy to use, got in a bit of a mess with transistor theory. With NPN transistors from what I took in the base is postivley charged which attracts a switching earth i.e collector and emmitter? And vice versa for a PNP transistor except uses a negative base which attracts a positive?

    If some one could just guide me right that would be great and appreciated.

    Secondly what is a transistor measured in i.e current = Amps
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    A transistor is most commonly used as a current amplifier.
    A small current on the base effects a large change in current through the collector and emitter.

    In this configuration, an NPN would have the emitter more negative, the collector more positive; current flow through the base would control a much larger current. The load is between the collector and the positive supply, or Vcc.

    There is also the voltage follower configuration. An NPN transistor can be connected upside-down; ie: collector tied to Vcc, the emitter is the output. The voltage applied to the base is followed by the emitter output, except it is offset by the voltage drop from the base to the emitter; usually around 0.65v, but varies somewhat.

    See the attached demo schematic; the top four are current amplifiers; the bottom four are emitter followers.

    Note that the current output in the current amplifiers varies as a function of the current input to the base. Note that as the current output of the transistor increases, the gain decreases. This is not a linear function.

    Note that in the emitter follower configuration, the output voltage reasonably closely follows the input voltage, albeit with that offset I'd previously mentioned - except the simulation is making a liar out of me; it shows the offset closer to 0.3v! (ETA: added 470 Ohm pull-down resistors on the emitters; now results agree with my former assertions) Really, your mileage may vary - it depends upon what transistor you happen to be using.

    PNP transistor connections are similar - except everything is inverted. Clear as mud, right? :D

    [eta]Thanks, Ron H - for keeping me honest. It's been a long day! Corrections made; diagram added.
    [eta]Updated schematic to include 470 Ohm pull-down resistors on the emitters of the voltage followers
     
    Last edited: Jun 16, 2008
  3. pntrbl

    Active Member

    Apr 21, 2008
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    PNP's and NPN's just recently clicked in my head when I realized the base and collector currents both flow thru the emitter on an NPN. A PNP, being the same only opposite, has the current flowing in the other direction. In a sense the emitter "feeds" the collector and base on a PNP. Then you look at each junction as a diode and know what polarity will be needed to get some action.

    As far as sizing there's a whole slew of parameters, including the "slew rate", but all I know to look at is the max current. You exceed that one and you'll be letting the smoke out ......

    SP
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Actually, the collector still has to be more positive than the base or the emitter. I'll bet you knew that, Sarge.:D
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Noted, and corrected! :rolleyes: Thanks, Ron H! :D

    I spent a good part of the day moving my spousal unit's art collection. She made quite a few LARGE paintings - that was work!
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    My suggestion is to read this section of the AAC eBook...

    http://www.allaboutcircuits.com/vol_3/chpt_4/index.html

    and then ask specific questions. Don't be afraid of asking dumb questions, we don't mind a bit and civility is enforced. We all started somewhere, everyone has stories to tell about themselves. :)
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You're only getting 0.3V because you have no bias current. The emitter follower needs a bias resistor or current sink to a supply more negative than the base voltage. Without it, the stray capacitance at the emitter will charge on positive transitions, but will discharge very slowly.
     
  8. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Thanks for all your replys Ill study the attached documents and research over.

    A NPN transistor uses 2 negatives and a positive to charge the "base"?

    A PNP transistor uses 2 positives and a negative to charge the "base"?
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    An NPN transistor requires a positive voltage relative to the emitter to forward bias the base PN junction and start charges flowing. The significant charge flow is into the emitter and out the collector. The reverse holds for PNP.
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    If people recall more postings here, it is because a number got relocated to the Off-Topic section.
     
  11. mark0908

    New Member

    Feb 18, 2008
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    read the book

    the art of electronic
     
  12. Dave

    Retired Moderator

    Nov 17, 2003
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    Agreed. This will give you a very good conceptual feel for BJTs and their applications.

    Dave
     
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