Newb question about transistor

Discussion in 'General Electronics Chat' started by canith, Dec 29, 2013.

  1. canith

    Thread Starter New Member

    Dec 29, 2013
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    In a basic transistor how does the base resistor feed current to the base?
    Isn't the role of the resistor there to limit current flow? I got confused about the current feeding role of resistors. I would appreciate for explanation on this point.
     
  2. k7elp60

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    Nov 4, 2008
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    The current in a bipolar transistor is split two ways. The emitter current is the total current. The base current + the collector current = the emitter current. The base current determines the collector current, controlled by the β,(beta) or hfe. For silicon transistors the differential voltage between base and emitter is close to 0.6V. For germanium (old) transistors the differential is about 0.2V.
    The base resistor does limit the current.
     
  3. ActivePower

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    Mar 15, 2012
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    While designing a BJT circuit, you'd need to know the exact amount of base current that needs to be fed into the Base terminal as that determines the controlled current i.e. through the Collector to the Emitter. Keep in mind that the base-emitter junction acts almost like a diode with constant 0.7 V barrier to the supply.

    The purpose of the resistor is just to limit the base current to the above value (specified by your design).

    For example, to achieve a current of 100 mA across a transistor with a Beta ratio of 100, you'd need 100/100 = 1 mA of base current. To get this from a battery of say, 5 V you'd need a resistance of (5 - 0.7)V/1 mA = 4.3k.

    HTH.
     
  4. bertus

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  5. canith

    Thread Starter New Member

    Dec 29, 2013
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    Thank you all for the useful points. but in all three responses you did not mention about the current feeding role of resistor to the base.
     
  6. ActivePower

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    A resistor "feeding" some amount of current is just a turn of phrase, not to be taken literally. A resistor, as you know, is a passive element and does not actually feed energy into the transistor.

    As mentioned in earlier answers, its sole purpose is to regulate/decide the amount of current flowing into the base terminal. Changing the R value changes the amount of charge going into the base (assuming a constant voltage source).

    You might also find this a useful reference about BJTs.
     
    canith likes this.
  7. crutschow

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    Mar 14, 2008
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    The sole function of a resistor is to "resist" the flow of current through it. This current is directly proportional to the voltage across the resistor and inversely proportional to the resistance value of the resistor.

    As per Ohm's law the equation for this is I = V / R where I is the current, V is the voltage across the resistor, and R is the resistance value.

    Thus if you had a 10kΩ base resistor and 5V at the resistor input, then the base current would be (5V-0.7V) / 10kΩ = 0.43mA (assuming the transistor has a base-emitter voltage drop of 0.7V to ground).
     
  8. canith

    Thread Starter New Member

    Dec 29, 2013
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    Thank all:

    That helped. It is clear now.
     
  9. LvW

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    Jun 13, 2013
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    QUOTE k7elp60: "The base current determines the collector current, controlled by the β,(beta) or hfe."

    Quote ActivePower: "While designing a BJT circuit, you'd need to know the exact amount of base current that needs to be fed into the Base terminal as that determines the controlled current i.e. through the Collector to the Emitter."

    I am aware that some forum members don`t like to discuss the question how the transistor really works.
    However, I must confess I always feel provoked to read that the base current would "control" or "determine" the collector current.
    And I cannot resist to correct these obviously false statemets. Sorry.
    I agree that during design of a transistor amplifier we can make use of the simple and nice-looking formula Ic=beta*Ib.
    However, this formula does not tell the PHYSICAL TRUTH regarding the control mechanism inside the transistor.
    And, for my opinion, every good engineer who is going to design a transistor stage should not only be able to handle formulas but, rather, should know what he is doing and WHY he is allowed to do this.
    That means: He must be informed about the physical background of the effects he is exploiting.
    And therefore I repeat: The BJT is a device with a collector current Ic which can be externally controlled by the base-emitter voltage Vbe. The current Ib is nothing than an unwanted by-product that cannot be avoided!
    There are many simple and easy-to-understand justifications for this fact - and I am always surprised that some people still believe that it should be possible that a small current (Ib) should be able to control a large current (Ic). This is impossible!
     
    Last edited: Dec 30, 2013
  10. crutschow

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    It's nice to know the "PHYSICAL TRUTH" about BJT's as voltage-controlled devices but that's of little use when designing the bias circuits for BJT linear amp stages or when using them as a switch. For those purposes viewing them as a current controlled device with the base-emitter junction appearing as diode with a fairly low input impedance is the easiest way to calculate the bias values needed. That's why the beta or Hfe value is used for these calculations, not the transconductance.
     
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  11. MikeML

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    deleted post
     
    Last edited: Dec 30, 2013
  12. MikeML

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    Sort of like a very pedantic fellow who likes to talk about how many angels can dance on the head of a pin we have encountered before...

    To Canith, K7elp, and ActivePower: Listen to crutschow; he designed circuits for a living. He knows what he is talking about.
     
  13. LvW

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    Jun 13, 2013
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    Why so ironically? Is this really appropriate?
    Everybody - including myself - has it`s own experience (in designing and teaching).
    Do you really think it is "pedantic" to correct technical statements that are simply false?
    Didn`t I mention that, of course, the base current has to be taken into account during bias calculation? I never did argue against it.
    Are you afraid that engineers (or students as future engineers) are overstrained by discriminating beween theory (yes: physical truth) and practical circuit design (design using the current gain) ?

    Final - design oriented (!) - question to all you "non-pedantic" fellows:

    What is your answer if somebody wants an explanation
    *how feedback using an emitter resistor Re works?
    *how the classical current mirror works?

    Are you able to give an explanation based on current control ?
    Perhaps I can learn something from your answers.
    Thank you.

    EDIT:
    @MikeML: If you read the OP`s question again (post#1 and #5) you will notice that he didn`t ask how to design a circuit. Rather, he wants a more theoretical EXPLANATION. (Quote: I got confused about the current feeding role of resistors. I would appreciate for explanation on this point.).
    By the way: Do YOU have any opinion regarding current vs. voltage control of the BJT?
     
    Last edited: Dec 30, 2013
  14. crutschow

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    If you noticed, I stated specific design situations where a current-mode view of BJTs is appropriate. If you are considering AC voltage gain or circuits such as current mirrors than obviously the input voltage characteristics of the BJT come into play. I never said you only consider the current mode of operation in BJT design, just where it's appropriate, the same as the voltage-mode.

    But, as you noted, the OP was asking about the role of a resistor in feeding current to the base of a BJT. To me, mentioning that a transistor is really a voltage operated device and the base current is an "unwanted byproduct" just muddies that discussion. I don't see how that knowledge is applicable to his question, since he is obviously interested in the current into the device. Insisting that he understand the "PHYSICAL TRUTH" does seems rather pedantic, considering he apparently barely understands Ohm's law. :rolleyes:
     
  15. t_n_k

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    Hi LvW,

    With respect to the two posts on which you made particular note:
    - my concern would focus more on the notion that one designs for an "exact" base current. As far as I understand the matter, BJT bias design is concerned with avoiding that very matter of exactness. The variability in beta and temperature dependence of device parameters requires the designer to take steps to overcome these limitations and stabilize the bias conditions as best as can possibly be achieved - notwithstanding those limitations.
     
    Last edited: Dec 30, 2013
  16. LvW

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    Hi Crutschow, I am fully with you.
    You may have noticed that in my post#13 I did not argue against your contribution (post#10). Like you, I also have designed a lot of transistor amplifer circuits - using the standard design formulas. And, of course, it was not necessary continuously to recall the voltage-control feature of BJT`s.
    However, I`ve got the feeling that the OP did have some problems to UNDERSTAND some BJT operation. That was the background of my contribution. Nothing else.

    (On the other hand, I think everybody who has designed a BJT amplifying stage has used resp. exploited the BJT`s voltage-control property - perhaps without knowing or realizing this fact. This is true, in particular, for calculation of stabilizing feedback elements. But a good designer should know what he is doing and why!).)

    PS: I am a bit disappointed that a technical discussion in a technical forum - from time to time - contains some sort of personal attacks. Why? (Perhaps the term "attacks" is to strong - excuse my limited linguistic knowledge, but you will know what I mean).
     
  17. t_n_k

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  18. LvW

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    @t_n_k:
    Quote: The variability in beta and temperature dependence of device parameters requires the designer to take steps to overcome these limitations and stabilize the bias conditions

    Yes - that`s an important point.
    In this context, there is an interesting effect:
    If we have relatively strong current-controlled voltage feedback (emitter resistor Re), we all know that the voltage divider at the base node for proper bias can be calaculated without knowing the "exact" value for Ib (due to the beta tolerances).
    More than that, we even can completely forget the base current Ib - and the error in the resulting Ic is tolerable in many cases. Such a design example helps to understand the role of the base current Ib.
     
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