Most Important Points | Bipolar Junction Transistors | CompetitiveExamination




Most Important Points | Bipolar Junction Transistors | Competitive Examination


Suitable for the post of Junior Engineer in BSNL, DMRC, DRDO, PTCUL, RRB, UPCL, etc examination.

  1. Bipolar Junction Transistors (BJT) is a three terminal, two junction, non-linear semi-conductor device, that can amplify electronic signals.

    • Emitter is highly doped, base is lightly doped and collector is moderately doped.

  2.  BJT can operate in four regions:

    1. Active Region: Emitter-Base Junction is Forward Biased (FB) and Collector-Base Junction is Reverse Biased (RB).

    2. Saturation Region: Emitter-Base Junction is Forward Biased (FB) and Collector-Base Junction is Forward Biased (FB).

    3. Cut-Off Region: Emitter-Base Junction is Reverse Biased (RB) and Collector-Base Junction is Reverse Biased (RB).

    4. Inverted Region: Emitter-Base Junction is Reverse Biased (RB) and Collector-Base Junction is Forward Biased (FB).





  3. When two points (saturation point and cut off point) on the output characteristics of CE transistor are joined by a straight line, we get a dc load line.

  4. Quiescent Point, Q-Point OR Operating Point is a point of the transistor (I CQ, V CEQ) at which it is biased.

    • The concept of Q-Point is used when transistor is used as an Amplifier i.e. operate in active region of output characteristics.

    • Q-Point is generally taken to be the Intersection Points of dc load line with the output characteristics of transistor.

    • For Base Bias: Q-Point is dependent on β which depends on Transistor, Ic and Temperature.

      • Base Bias is a prototype used in the design of Switching Circuits.

    • For Emitter Bias: Q-Point remains stable as Ic is independent of β.

      • Emitter Bias is a prototype used in the design of Amplifying Circuits.

    • Voltage Divider Bias: It is a most widely used biasing circuit.

      • Base contains a voltage divider so it is called a voltage divider bias.

      • Ic and Vce  are independent of β, Hence Q-Point is independent of β.





  5. Amplifier is a device which increases the current voltage OR power of an input signal with the help of transistor by producing additional power from a separate source of power supply.

    • Amplifier must have high input impedance (ideally infinite) and low output impedance (ideally zero).

    • The process of raising the strength of a weak signal without any change in its shape is called Amplification.

  6. The difference between the output and input waveform is called Distortion.

  7. Two Transistors Model (Ebers Mall Model) for a transistor used to simulates how it behaves when an ac signal is present.

    • Emitter diode of transistor acts like an ac resistance r_e'.

    • Collector diode acts like a current i_c.

  8. When the load draws current more than transistor amplifier capacity this event is called Loading.

    • Loading Effect is caused by impedance matching.





  9. Transistor can be used in three configuration as given in following table:



    Author: Mr. Amarjeet Singh Jamwal, Assistant Loco Pilot, Ambala Division, INDIAN RAILWAY.




    Reference: Albert Malvino and David J Bates, “Electronic Principles”, 7th Edition, TATA McGRAW HILL.


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