answer my questions about transistor

Discussion in 'General Electronics Chat' started by mahmoodsani, Jun 7, 2010.

  1. mahmoodsani

    Thread Starter Member

    Jun 3, 2010
    43
    0
    please answer my questions about transistor

    i have skimmed google but there are some v basc question that disturb me and still i dont have any answer i want to discuss it with u ..start from NPN transistor

    i see in transistor circuit that there are always volt supplied on collector.why are 2 supplies used 1 on base and one on collector i dont understand
    and i also read on google that the input voltage have must to be converted into current with voltage to current converter and suggesting a base resistor as voltage to current converter.how does it do it ............is it the same with collector
    and one more we know that base and collector are reverse biased then how the output is collected from collecter.why does it drop only .6 volts whats the reason behind that........plz discuss with me and answer my questions
     
    Last edited: Jun 7, 2010
  2. mahmoodsani

    Thread Starter Member

    Jun 3, 2010
    43
    0
    hello friends anyone there..........
     
  3. cmancuso

    New Member

    Jun 7, 2010
    4
    0
    One supply is needed to turn the transistor on for an npn transistor, and to turn off the pnp transistor.

    The input must be transformed into current because a Bipolar Junction Transistor (BJT) is a current multiplier, unlike a MOSFET ( Metal Oxide Semiconductor Field Effect Transistor ) multiplies voltage.

    The BJT transistor drops about 0.6V because the transistor from the base to the emitter is a pn junction which can also be looked upon as a diode, and therefore, drops the voltage coming in about 0.6V, which is the same as a pn junction diode.
     
  4. mahmoodsani

    Thread Starter Member

    Jun 3, 2010
    43
    0
    how the input is turned itno current......one other thing voltage is it self current.i mean voltage is only pressure with makes currnt to flow....so what the relation to convert voltage into current......
     
  5. eng1ne

    Member

    Dec 4, 2009
    97
    3
    You are referring to 'Ohm's Law', http://lmgtfy.com/?q=ohm's+law.

    Voltage is a potential energy per unit of charge, a force; Joules/Coulomb

    Current is the rate at which the voltage makes the charge flow; Coulomb/Second
     
  6. mahmoodsani

    Thread Starter Member

    Jun 3, 2010
    43
    0
    Ye i knw the definitions,and camansuso made me to understand tht bjt multiplies the current there fre v shd be cnvertd into curent.bt my questinn is tht how cn 1 change v in current and how is it dne by using the base resistor can sme 1 explain it
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Most transistors actually use a single power supply, but to understand theory they simply it (and it becomes two power supplies).

    A modern silicon BJT transistor had a drop of 0.6, just like a diode. In many ways it is a diode, but the rest of the device makes a difference.

    You take the 0.6V into account as a constant, it has to be there for the transistor to work. Anything over that voltage is what calculates into current.
     
  8. mahmoodsani

    Thread Starter Member

    Jun 3, 2010
    43
    0
    If .6 v are neccesary than can we do it with 1.5 v batery,if not Y
    and why z 2nd supply use at collector while we also collect o/p frm colector.in tht case when same the collector is usd for o/p and suply v then what would be relation btween them and y is it necessary to use collector suply
     
  9. mahmoodsani

    Thread Starter Member

    Jun 3, 2010
    43
    0
    If .6 v are neccesary than can we do it with 1.5 v batery,if not then Y
    and why z 2nd supply use at collector while we also collect o/p frm colector.in tht case when same the collector is usd for o/p and suply v then what would be relation btween them and y is it necessary to use collector suply
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    The 0.6 base emitter drop is part of the transistor, a built in requirement, same as with a silicon diode.

    Silicon diodes drop around 0.6 Volts, Germanium diodes drop around 0.3V. Silicon and Germanium transistors drop the same amounts. You probably need to review PN junctions for a full understanding.

    Your questions would be a lot more clear if you could point to illustrations. Schematics in electronics are worth a thousand words.

    Here is the AAC book chapter on transistors.

    Chapter 4: BIPOLAR JUNCTION TRANSISTORS


    I suggest you review it, then ask specific questions on parts you don't understand.

    If I understand your question correctly, yes, you can bias a transistor with a 1½V battery.
     
  11. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    A BJT is LIKE two diodes back to back. The Base to Collector is normally reverse biased, and Collector Emitter is almost completely blocked MegaOhms equivalent resistance until the Base Emitter is forward biased and conducting.

    Look at a BJT as almost like a current controlled resistor in the collector layer. If the BE circuit has .1mA current the collector can't see the base emittor circuit and becomes a resistor until if reaches about 25mA collector current (base current multiplied by 250 gain for this transistor). Forward base emitter current lets the Collector behave like a semiconductor resistor of variable value with conduction controlled by the Base.

    At that point the collector current reaches its limit it sees the boundary again when it starts to overflow at the CB layer causing the Collector Base reverse bias diode to kicks back in to shut down more collector current.

    If you don`t completely understand then start setting up circuits and learn how to use them. A few transistors and resistors in a battery circuit can teach you more about what you need to know.

    I think you also asked why they normally take output from the collector.

    The Emitter and base are both parts of the input circuit.

    You can use the emitter as the input for a Low impedance signal source like when you are also using the speaker in a PA system as a microphone for the room to respond. That speaker is too low an impedance to power the base of a transistor so it would work better in a common base circuit.

    You can use the emitter for an output of sorts in an emitter follower
    circuit. There will be no voltage gain but it will provide a good current gain and low impedance because the load on the emitter provides positive feedback.

    You might also see an RC network between the emitter and the ground or rail in circuits as a bias stabilization support. The capacitors move the signal from the emitter to ground and prevent loss of voltage gain. The resistor provides a feedback for thermal conduction to raise the DC emitter level and reduce the base emitter forward bias.
     
  12. mahmoodsani

    Thread Starter Member

    Jun 3, 2010
    43
    0
    Ok i would read it...but my main question is that in many circuits supply is gven to collector.i undrstnd suply it base but the reason for suplyng volt at colector
     
  13. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    You are focused on volts, but BJT transistors are current devices. It is the base emitter current that is important, not the voltage. You have only to meet the basic requirement of 0.6 volts, then it is unimportant.

    BJT use a simple formula, Ice=β Ibe

    Where:
    Ice is Collector Emitter Current
    Ibe is Base Emitter Current
    β is the gain of the transistor, approximately the same as hfe.

    You need to read, then ask.

    [​IMG]
     
  14. mahmoodsani

    Thread Starter Member

    Jun 3, 2010
    43
    0
    Nice reply.it helpd me
     
Loading...