Basic Electronics : Basics, Working & Circuit configuration of BJT

Junction Transistor

• Both the electrons and holes take part in the conduction process for bipolar devices.
• BJT consists of two p-n junctions manufactured in a special way and connected in series, back to back.
• The transistor is generally a 3-terminal device with emitter, base and collector terminals.
• From the physical structure, BJTs can be divided into 2 groups: NPN and PNP transistors.

• Modes of operation:

      The transistor consists of two p-n junctions, the emitter-base junction (EBJ) and the collector-base junction (CBJ). Depending on the bias condition (forward or reverse) of each of these junctions, different modes of operation of BJT are obtained, as shown in below table.

• Active Mode:

       When the emitter-base junction of the transistor is forward biased and the collector-base junction is reverse biased, the transistor operates in active region. In this mode transistor is used as amplifier. This bias configuration is shown in below figure for n-p-n and p-n-p transistors.

• Saturation Region

           When both the emitter-base junction and collector-base junction are forward biased, the transistor operates in saturation region. Transistor has a large current in saturation mode. In the saturation mode , the transistor is used as closed switch.

• Cut-off region

         When both the emitter-base and collector-base junctions are reverse biased, transistor operates in cut-off region. In cut-off mode the current through the transistor is zero(ideally). In this case the transistor is operated  as an open switch. Both n-p-n and p-n-p transistor are biased in cut-off mode as shown in below figure.

• Reverse Active Region or Inverse Region

          When the emitter-base junction of the transistor is reverse biased and the collector-base junction is forward biased, the transistor is said to be in reverse active mode. This mode of operation is not often used. In below figure, transistors are biased in reverse active mode.

4. Current Relationship in BJT

Transistor Configuration

The transistors can be connected in the following three different configurations, Depending upon the terminals which are used as a common terminal to the input and output terminals.

• Common Base Configuration
• Common Emitter Configuration
• Common Collector Configuration

    a) Common Base Configuration

• In this configuration base terminal is connected to a common terminal.
• The input signal is applied between the emitter and base terminals.
• The output signal is taken between the collector and base terminals.

     Properties of CB configuration

• Lowest input resistance (R_i < 100 Ω)
• Highest output resistance (R₀ > 1 Ω)
• Lowest current gain (α < 1)
• Highest voltage gain
• Medium power gain (Typical value 68)
• Output and input voltages are in phase i.e. phase shift is 0°.
• In CB amplifier current gain is less and therefore bandwidth is large and hence CB amplifier is widely used as high frequency amplifier.

    Applications

• As a non-inverting voltage amplifier
• As a high frequency amplifier
• As an impedance matching device between low resistance to high resistances.

     b) Common Emitter Configuration

• In this configuration, emitter terminal is connected to a common terminal
• The input signal is applied between the emitter and base terminals.
• The output signal is taken between the collector and base terminals

    Properties

• Moderate input resistance (around 1 kΩ).
• Moderate output resistance (50 kΩ to 500 kΩ).
• Moderate current gain (Typical value 49).
• Moderate voltage gain.
• Highest power gain (Typical value 4226).
• Output and input voltages are out of phase i.e. phase shift = 180°.

    Application

     It is the most common and frequently used amplifier circuit.

     c) Common Collector Configuration

• In this configuration, collector terminal is connected to a common terminal.
• The input is applied between the base and collector terminals.
• The output is taken between the emitter and collector terminals.

    Properties

• Highest input resistance (50 kΩ to 500 kΩ).
• Lowest output resistance (< 100 Ω).
• Highest current gain.
• Lowest voltage gain.
• Voltage gain is less than 1 or very close to 1.
• Lowest power gain (Typical value 48).
• Output and input voltages are in phase i.e. phase shift is 0°.
• Common collector configuration is also known as emitter follower.
• Emitter follower is basically a Current Controlled Voltage Source (CCVS).

     Applications

• As an audio frequency power amplifier.
• As a butter (impedance matching device between high resistance to low resistance).
• In designing of voltage sweep circuits.
• As a high input resistance device.
• As a “Boot strap emitter follower”.

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