Transistor question

Discussion in 'General Electronics Chat' started by dthx, May 25, 2013.

  1. dthx

    Thread Starter Member

    May 2, 2013
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    I used the search function and couldnt find anything....
    Could someone explain Generally how a transistor amplifies voltage.
    When I come off my board to run a motor or something.....dont I need to use a transistor to drive it....?
    And how does that transistor do that.
    Maybe I'm wrong.
    D.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,238
    Read this for starters.
     
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  3. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    A transistor don't actually amplify a voltage, but it can control a larger voltage, like a relay. You can have a small signal (3.3 or 5 V) control a larger voltage source (12 V or more). The best transistor to use for controlling a motor would be a MOSFET.
     
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  4. dthx

    Thread Starter Member

    May 2, 2013
    194
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    amplifier [edit]
    [​IMG]

    Ok....a transistor is a electronic pilot operated flow contro....a little gate in which the base is the gate master....
    But in the above circuit from the Wikapedia site you suggested....
    Why wouldnt power go from V+ to Vout...thru the resistor and capacitor...
    It doesnt need to go thru the transistor.....does it...
    It has a parallel pathway .....
     
  5. dthx

    Thread Starter Member

    May 2, 2013
    194
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    Wait a minute....whats the diff between V+ and Vin....."?
    Are they both supply and just labled differently to confuse me...
    D.
     
  6. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    It may seem that way, but no. V+ is the positive power supply.
    Vin indicates the input. In this case, a changing voltage.
    Current cannot flow through a capacitor, so only a changing voltage is seen on the other side. This means that there could be a voltage potential on the capacitor. A constant voltage is not passed through, but a changing voltage is seen by all the devices attached to it.
    In the diagram, R1 and R2 set the voltage of the gate on the transistor so it is halfway on. A changing voltage on the input passes through the capacitor and changes the voltage on the gate.
    The change in voltage causes a change in current at the base, which the transistor amplifies and different amounts of current flow through the transistor from collector to emitter. The different amount of current flowing shows up as a change in voltage at the ends of the resistors, Rc and Re. Again, the changing voltage is passed through the capacitor and shows up as a change in voltage on the output.
     
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  7. dthx

    Thread Starter Member

    May 2, 2013
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    thank you....so this is little variable voltage regulator?
     
  8. dthx

    Thread Starter Member

    May 2, 2013
    194
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    Im still not getting it 100% yet....how you can have a voltage potential with no flow of electrons...ie, no current...
    Is there no current at Vout?
     
  9. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    The diagram shows a signal amplifier. A small changing voltage is amplified to a larger changing voltage.
    I said that the capacitor blocks a steady DC voltage where there is no current flowing.
    When the voltage changes on the input, it creates a different voltage potential on the capacitor (Cin). This voltage potential causes some electrons to either be attracted or repelled to/from the capacitor. This small change is noticed by the base on the transistor and the flow from the collector to emitter is changed. The flow of electrons changes the potential on Cout, which produces the amplified signal.
     
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  10. Shagas

    Active Member

    May 13, 2013
    802
    74
    Yes . A transistor is a variable resistor . You can use a transistor in many ways .
    One of the way is using a transistor in series with a motor for example . When the transistor is off then the resistance is very high (20 Mohm ++ ) so no current will flow. When the transistor is on then the resistance is < 1ohm so current will flow (the transistor isn't limited to on/off !! , you can have any value inbetween). Basically you are using your transistor as a variable switch.

    Another way of using it is as you said 'variable voltage regulator ' in a sense .
    where you arrange the transistor in a voltage divider configuration .
    A resistor (1k for example) is R1 and your transistor will be R2 . And when you send a variable current to the base of the transistor then the output between the collector and R1 will bounce relative to the current supplied to the base .
    (this is all crudely said , but it's how the transistor works)

    https://www.circuitlab.com/circuit/64a2gu/transistor-voltage-divider/
     
    Last edited: May 26, 2013
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  11. Shagas

    Active Member

    May 13, 2013
    802
    74
    V++ is the power supply voltage (your positive terminal of the battery or whatever)
    The Vin is the small signal that you supply to the base of the transistor which is 'amplified' , increased in magnitude (within the limits of the power supply voltage !)
     
  12. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    If you go to the section "The Common Emitter Configuration" which is halfway down the page, you will get an explanation for the figure you posted.
     
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