Bipolar Junction Transistors: Operating Region, Characteristics, and Application

Bipolar Junction Transistors

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.

Operating Region of BJT

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).

Characteristics of BJT

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. It is called a dc load line because its slope depends upon the load resistance Rc which is a dc load.

• 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.

Biasing of BJT

• 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.

BJT as an Amplifier

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.
• The difference between the output and input waveform is called Distortion. Distortion can be reduced by reducing the peak to peak value of base voltage.
• 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.
• When the load draws current more than transistor amplifier capacity this event is called Loading. Loading Effect is caused by impedance matching.

Reference:

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