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BJT Basics questions

Discussion in 'General Electronics Discussion' started by max_torch, Aug 6, 2015.

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  1. max_torch

    max_torch

    111
    1
    Feb 9, 2014
    I'm reviewing my electronic basic fundamentals and now I'm trying to tackle questions that I never bothered to ask while I was in the classroom. (You might have noticed some of my other threads) I actually know some things, but there are huge gaps in my knowledge.
    It is said that there are three BJT configurations namely, common collector, common base, common emitter.
    - Sometimes the drawings of these configurations are shown with two sources, yet in actual practice most of the time BJTs in circuit have only one supply so I find this confusing. Why is it that sometimes it is shown with two sources and why sometimes with only one?
    - Sometimes drawings of these configurations focus only on which terminal is being grounded: if it's grounded emitter then it's common emitter, etc. And highlighted is the input side and output side, but where are the voltage sources connected? There are actually two voltage sources for it to start to operate?
    - We also have different kinds of biasing modes such as voltage feedback, voltage divider, etc, but what is the difference between the modes of bias and the configurations? Can you have a fixed bias, emitter stabilized, etc for common base, then have a fixed bias, emitter stabilized, etc for a common collector, then have a
    a fixed bias, emitter stabilized, etc for a common emitter? So confusing.
    - I know how to tell how diodes are biased, but not how a transistor is biased. I am having difficulty in telling whether the BE junction is forward or reversed and whether the BC junction is forward or reversed, especially in large circuits such as the internal circuitry of Integrated Circuits that are composed of so many transistors.. And this problem is carrying on even to my analysis of thyristors, UJTs, etc which are just extensions of the BJT concept, for instance with the SCR it is just approximate to an NPN and a PNP, but I am having trouble understanding basics of BJT concept in the first place so I don't know what to make of the SCR's BJT equivalent unless someone explains it to me.
    - Also I am aware about the conditions for active, saturation, and cut-off, operating regions and what those regions imply, I just need to master how to determine the biasing so that I can conclude what regions the transistors are in. The books focus on how the transistor is biased in order to determine the region of operation yet here: https://www.electronicspoint.com/resources/how-a-bjt-transistor-works-base-current-version.37/ there is no mention of how the transistor is biased. (He did say that his guide is under construction).

    I want to master the concept of BJT, so that I can extend that mastery to the analysis of other thyristors and devices based on the BJT, and to the analysis of the internal circuitry inside ICs. What I would like to know is how to merely look at the BJT equivalent of an IC or of a thyristor, and without having been told anything about the IC or thyristor but only on the BJT equivalent alone, be able to deduce its function and limitations.
    And if you have general or specific advice/explanations regarding any of these matters it would be appreciated. I am trying to bridge the gap from being a carefree student to being a serious, responsible engineer.
     
    Last edited: Aug 6, 2015
  2. Ratch

    Ratch

    1,089
    331
    Mar 10, 2013
    You need to start from ground zero. Specifically, take a course on BJT's by attending a class, self study, or online course if available. We can help you with specific questions about a particular aspect, but we are not equipped to give you a complete course on BJT's. I learned most of what I know about BJT's by self study.

    Ratch

    Ratch
     
  3. max_torch

    max_torch

    111
    1
    Feb 9, 2014
    Okay then can you help me with a specific question on a particular aspect? In an SCR equivalent circuit if I set it up like this: how do I know which junctions are forward or reverse biased?
    scr.png
    I'm talking about:
    Q1 BE --> forward or reversed?
    Q1 BC --> forward or reversed?

    Q2 BE --> forward or reversed?
    Q2 BC --> forward or reversed?

    This specific small concept could help me a lot.
     
    Last edited: Aug 6, 2015
  4. Ratch

    Ratch

    1,089
    331
    Mar 10, 2013
    For a PN junction, if you put a more positive voltage on the P slab, or equivalently a more negative voltage on the N slab, then you are forward biasing the junction. For the schematic shown, both transistors have their BE junctions forward biased and their BC junctions reversed biased. If you have trouble seeing that, ask for help, and specify which transistor and junction you are having trouble.

    Ratch
     
  5. AnalogKid

    AnalogKid

    2,425
    690
    Jun 10, 2015
    In the latched (on) state, both junctions are forward biased. In the off state, neither junction is biased at all. When Vb = Ve, the transistor is off but the base-emitter junction is not reverse-biased. Also, Q1 should have a resistor from base to emitter for reliable operation.

    ak
     
  6. Ratch

    Ratch

    1,089
    331
    Mar 10, 2013
    There are four junctions in the submitted circuit. Which two junctions are you referring? What transistor are you referring for Vb=Ve? The circuit is a junction model of a SCR. It will not really work as a SCR anymore than two diodes will work as a BJT. Therefore, it is spurious to suggest additional components for this circuit.

    Ratch
     
    max_torch likes this.
  7. LvW

    LvW

    604
    146
    Apr 12, 2014
    Max - just a short notice: The mentioned link is completed by a long discussion about some specific transistor features. Don`t overlook this sequence of questions and answers (understanding comes through questions and discussion).
     
  8. max_torch

    max_torch

    111
    1
    Feb 9, 2014
    I made an analysis with regards to the posted SCR BJT equivalent, Also Ratch I will try to base my conclusion on what is and is not biased based on what you said:
    scr2.png
    I substituted the transistors with their diode approximations. D1 is BE of Q1, D2 is BC of Q1, D3 is BC of Q2, and D4 is BE of Q2.
    If the switch to the 50V supply is shorted, and the gate switch is open, I can understand why D2 and D3 would be reverse biased, what I don't understand is how the diodes D1 and D4 could be forward biased at this point, because one of the requirements for proper biasing is for the junction potential that is typically 0.7V to be overcome, but with D2 and D3 in reverse bias mode they are open so approximately no current is flowing through either D1 and D4 as well and if there is no current flowing then nothing is breaking the junction threshold so they are also not forward biased. I just know that until such time that the reverse breakdown voltage of D2 and D3, plus the 0.7 thresholds of D1 and D4 are overcome, then the SCR will trigger and supposedly will remain on after being triggered even if the voltage goes down because a "regenerative loop" of current will occur between D2 and D3, not until the switch is opened and all current is stopped, and this explains the characteristic curve. And that adding a gate voltage reduces the reverse breakdown of D3 and D2 that would have to be overcome before the SCR gets triggered. I sort of know the explanation, but I feel like I'm missing something in the way I analyze. Is it correct to say that only when the reverse breakdown of D2 and D3 is reached will D1 and D4 be forward biased? And is the reverse voltage to overcome be the combined reverse breakdown of D2 and D3 or only of one of them?
    Is my logic correct?

    By I just want to clarify that in this profession I know that I am not supposed to expect to be completely spoonfed but I am just asking for some nudges or guidance along the way.
     
  9. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    For a beginner, you are trying to go into far too much detail.
    Here is a simple circuit that shows 2 transistors acting as an SCR:

    SCRwithTrs.gif
     
  10. AnalogKid

    AnalogKid

    2,425
    690
    Jun 10, 2015
    With the addition of two resistors to limit base currents to safe values it will work exactly like an SCR. Really.

    ak
     
  11. AnalogKid

    AnalogKid

    2,425
    690
    Jun 10, 2015
    1. In the transistor diode model of an NPN transistor, both anodes are connected to the base. Therefore, in normal operation with collector current flowing, the base-emitter junction is forward boased and the base-collector junction is reverse biased. This is why when a transistor is talked about being reverse biased, the assumption is that it is the base-emitter junction that is being discussed.

    2. Both transistors.

    ak
     
  12. Ratch

    Ratch

    1,089
    331
    Mar 10, 2013
    D1 and D4 are not biased to anything because D2 and D3 are nonconductive at this point. A voltage has to be applied to both ends of a diode for it to become biased. Once D2 and D3 become conductive, then D1 and D4 will be biased.
    0.7 volts for a silicon diode or 0.4 volts for a germanium diode is not a voltage to overcome. Current will still be present at any voltage above zero, but it becomes significant at those voltages.
    You basically have the theory correct, but read my remarks at the end. You are using technical slang and looking for a detailed explanation. Current flow means "charge flow flow", which is redundant and ridiculous. Current means charge flow. So say "charge flow" or current exists or is present.

    Now consider this, The BC junction of a BJT is reversed biased in the active mode, and yet, charge flows through this junction readily. Why is that? The reason is that a BJT does not operate like two diodes connected back to back. The same principle applies to a 4-layer semiconductor like a SCR. You cannot model the operation of a SCR with two transistors, nor especially with 4 diodes. Those models only show the electrical connections of the P and N slabs, but not the true operation of the device. You might want to read post #75 of this thread. https://www.electronicspoint.com/th...stor-works-base-current-version.269901/page-4 Keep asking questions.

    Ratch
     
    LvW likes this.
  13. Ratch

    Ratch

    1,089
    331
    Mar 10, 2013
    There are many circuits that will latch like a SCR does, but that does not mean they will have the same electrical characteristics and specification that a real SCR does. Making a latching circuit from two transistors does not demonstrate how a SCR works. It is a different and alternative method of achieving the same functionality.

    Ratch
     
    Last edited: Aug 6, 2015
  14. Ratch

    Ratch

    1,089
    331
    Mar 10, 2013
    First time I heard that. When someone tells me that a multi-junction device is biased a certain way, I want to know what junction they are talking about.

    Ratch
     
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