Transistor Equivalent

Discussion in 'General Electronics Chat' started by sudar_dhoni, Jan 16, 2010.

  1. sudar_dhoni

    Thread Starter Member

    Nov 9, 2009
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    Can someone give me a detailed on the circuit given below.
    i.e I want to know the purpose of each and every component in the circuit.
    Also in the diagram the direction of electron flow in the speaker portion is not draw. Could someone draw it ?
    Also what is that component which looks like a galvanometer near the diode.
    In the net its given that it is a current source. But i cant understand it as there is already a source Vcc. Why another one?
    Could someone explain in detail about that transistor equivalent model?
     
  2. radiohead

    Active Member

    May 28, 2009
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    What does the source you got the circuit from say its supposed to do? The V Input looks like an audio signal (maybe a microphone). It doesn't look like an amplifier.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    The thing labeled Q1 is a transistor. It is shown as a diode (for the BE PN junction) and a current source (for the CE junction). The external resistors set an operating bias on the base to the transistor and limit current from the AC signal.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    A transistor is never biased like that. It should haver a bypassed emitter resistor for DC stability.
    A speaker should never have DC flowing in it.
     
  5. VoodooMojo

    Active Member

    Nov 28, 2009
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    sudar
    could this be your equivalent circuit?

    the thing that looks like the galvonometer is an amp meter representation?
     
    Last edited: Jan 17, 2010
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The thing that looks like a galvanometer is a current-dependent current source (Ic=β*Ib).
     
  7. sudar_dhoni

    Thread Starter Member

    Nov 9, 2009
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    What is that ?
    COuld you give a detailed explanation on it.
     
  8. Ron H

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  9. sudar_dhoni

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    Nov 9, 2009
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    Initially i was against conventional current and wanted only electron flow movement. But since all my books deal only with conventional current, i have given up and have decided to follow the conventional current.

    Please help me
    How can conventional current provide bias to the base of the transistor so as to allow collector current to flow ??

    How can conventional theory account for the temperature dependance current which is produced in collector which i had earlier called it as leakage current??

    For understanding the load line analysis, i require these doubts to be clarified .
     
  10. Jony130

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    Feb 17, 2009
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    Maybe this helps
    [​IMG]

    Note that arrow on a BJT (on a emitter) symbol show how current will be flow through BJT. Of course from "+" to "-".

    [​IMG]

    And when BJT is cut-off there is a small current that is flow (of course from "+" to "-") form Vcc through collector-emitter to gnd.
    And its called collector-emitter cut-off current (Iceo) (leakage current).
    But the current is so small so in practice we forget about it.
    And Iceo doubles when temperature increase by 7.5K
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    Heh, this site deals uses electron flow in it's text book. With something like transistors it doesn't matter much, though there are applications it does.

    You have to understand the 1st picture you pointed to was a model, trying to describe how a transistor worked. It is not the transistor itself.

    There are several models for transistors out there, all have their strengths and weaknesses. The formula

    Ic = ß Ib

    is a current model, and will let you understand how mathematically understand how other biasing schemes work.

    While it is not in the formula, it is understood there is a diode drop between Base and Emitter, 0.6V to 0.7V. No such drop exists if a transistor is fully turned on between the Collector and Emitter, they can get within 0.1V of each other.

    Beta (ß) is an unstable parameter, it can change according to temperature. Also, even in the same batch of transistors made on the same die no two transistors have quite the same ß. In addition to changing the actual ß value, temperature also acts like a slight base current. For this reason BJT transistors can run away thermally, the hotter they get the more current they allow to flow, which means they get hotter. This cycles continues until they can burn up.

    More advanced bias schemes (which is what AudioGuru was talking about) can prevent this from happening. Basically you add more resistors in strategic locations.

    Finally, to help you understand the illustrations you were seeing, you have to understand that these two schematics are basically the same.

    [​IMG]

    This was discussed in depth in a much older thread.

    This thread belongs to you, use this one to ask questions.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    It doesn't matter which direction the current flows. Base current in a transistor causes it to conduct from collector to emitter. Base current flows when the base-emitter junction is forward-biased.

    You just need to remember that the emitter of an NPN transistor is usually the most negative voltage and the emitter of a PNP transistor is usually the most positive voltage.

    Collector current increases when the transistor is hot because when it is hot its hFE is higher and its base-emitter voltage is lower.
     
  13. t06afre

    AAC Fanatic!

    May 11, 2009
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    Also as many have pointed out. Your circuit is a typical "text book" circuit used for explanation. You can do calculations on it, but you should never use it in any real world audio project
     
  14. sudar_dhoni

    Thread Starter Member

    Nov 9, 2009
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    EVen i know the water analogy for the BJT.
    Initially i was against conventional current and wanted only electron flow movement. But since all my books deal only with conventional current, i have given up and have decided to follow the conventional current.

    So to accept the conventional theory i took the help of water analogy.
    It did help me with the bias.
    But it could not account for the flow of minority current and the leakage current i.e the temperature dependance current.
    Here onwards i got stuck and could'nt proceed.
    So i asked


    1. How can conventional current provide bias to the base of the transistor so as to allow collector current to flow ??

    2. How can conventional theory account for the temperature dependance current which is produced in collector which i had earlier called it as leakage current??


    IF possible could you give a water analogy for the second point.
    Or could you deal with pure positively charged conventional current to clarify both the doubts.(dont use electrons there)
    The first one, you cleared it with water analogy.
    But the second one is not convincing.
    So i would prefer you to explain both these in terms of water analogy.
    That would be very helpful for me.
     
  15. sudar_dhoni

    Thread Starter Member

    Nov 9, 2009
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    Usually the output is first connected to coupling capacitor to allow only ac to loudspeaker. How does the capacitor separate ac from dc.
    Also if possible could you show how the a.c current flows through in the output circuit.
     
  16. Audioguru

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    Dec 20, 2007
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    A capacitor blocks DC but passes AC.
    Look at Capacitor Theory in Google.
     
  17. sudar_dhoni

    Thread Starter Member

    Nov 9, 2009
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    THats ok but the ouput current in alternating only in magnitude but not in direction. ITs like a sine wave above the X axis i.e not below the X axis.
    In that case how can the capacitor segragate.
    Also could you also help me with my previous post just before this which i have mentioned in bold.
     
  18. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Current in the capacitor is proportional to the rate of change in the voltages.
    I=dU/dt
    So voltage change, current is flowing, no change in voltage, current does not flow.
    This is a general principle how the capacitor work.
     
  19. Audioguru

    New Member

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    A capacitor passes an entire sine-wave, not just half of it.

    Don't think about conventional or electron current direction. Just think that a forward-biased base-emitter junction conducts and causes the transistor to conduct a much higher current through its collector-emitter.

    The temperature dependance of a transistor is caused by its leakage current increasing, its current gain increasing and its base-emitter forward voltage decreasing when the temperature increases. Conventional current direction theory has nothing to do with it.
     
  20. sudar_dhoni

    Thread Starter Member

    Nov 9, 2009
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    What is a current dependant current source?
    What's inside it?
    How does it operate in a transistor equivalent model.
    Please explain how the transistor equivalent model works with a diode and current source.
     
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