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bjt operation

Adham157

Why the bipolar junction transistor works in the active mode.

I mean if i were to imagine that a bjt is like 2 diodes the base-emitter junction is forward biased while the collector-base junction is reversed biased that implies that there should not be an current passing from the collector to the emitter in case of NPN transistor(because a reverse biased diode does not pass alot of current) but it does pass current. not only that but also the collector current is proportional to the base current. how that works.

Also why we use the bjt in active mode for an amplifier.

by looking at the Vce(collector to emiter voltage)/Ic(collector current) graph it looks like the bjt works best as an amplifier in the saturation region (because the curve between the Vce and Ic is steep while in the active region its less steep).

i know that these 2 questions could be silly for many of you but forgive me for that because i am just new here and i am starting to learn about transistors.image

  • 0


    Hi,

    BJTs (actually, even a diode) have extremely complex physics behaviour. When you're using rules like a diode passing current or not passing a current, that's a model, and you can get more complex models the deeper you want to go. 

    You can tell that a BJT-as-two-diodes model is a very limited model because if you buy two diodes, you'll never be able to connect them to behave like (say) a transistor amplifier. That suggests that a BJT cannot be formed from two diodes (there are physical differences between two diodes with metal caps and wires between each diode that is connected, versus what's inside a BJT), and that the understanding will be limited or inaccurate if all one does is try to understand a transistor using a two diode model.

    There are many books if you wish to dig into deeper models, but the chart you have shown is trying to represent a model of a BJT when in a certain configuration.
    However, you're not applying the chart correctly.. look how evenly spaced the near-horizontal lines are to each other, with an evenly spaced change of base current. You can use that to the advantage of producing a linear amplifier, which is what that red line is trying to help with (whatever book/article you have obtained that image from, will explain how to do that with the red line).

    If you want book recommendations, there is a list here (or your school/college may have a recommended book): /members-area/personalblogs/b/john-wiltrout-s-blog/posts/i-want-to-do-electronics-where-do-i-start

  • 0

    Questions are seldom silly.

    The physics of how BJTs work is not simple and it is quite hard to accurately explain in simple terms.

    Understanding the band gap voltage of a pn junction and how electrons overcome this bandgap via "attraction" to holes on the other side is usually taught as a leap-of-faith fact or observational fact rather than from a true understanding.

    Here is one  attempt at a "simple" explanation...

    https://www.youtube.com/watch?v=fIvZen2tq_w&ab_channel=CircuitBread

    https://www.youtube.com/watch?v=VuxAz0fvMnA&ab_channel=CircuitBread