Unit 2: Bipolar Junction Transistors

1. Working of n-p-n and p-n-p Transistors

A Bipolar Junction Transistor (BJT) consists of three doped regions: Emitter (E), Base (B), and Collector (C).

• n-p-n Transistor: A thin layer of p-type material is sandwiched between two n-type layers. Conduction is primarily due to electrons.
• p-n-p Transistor: A thin layer of n-type material is sandwiched between two p-type layers. Conduction is primarily due to holes.

2. Configurations and Operating Regions

Transistors are typically used in three main configurations: Common Base (CB), Common Emitter (CE), and Common Collector (CC).

Operating Regions:

• Active Region: E-B junction is forward biased, and C-B junction is reverse biased. Used for linear amplification.
• Cut-off Region: Both junctions are reverse biased. The transistor acts as an open switch.
• Saturation Region: Both junctions are forward biased. The transistor acts as a closed switch.

3. Current Gains (α and β)

These parameters define the amplification capability of the transistor.

α (Alpha): The ratio of collector current to emitter current in CB configuration. (α = Ic / Ie)
β (Beta): The ratio of collector current to base current in CE configuration. (β = Ic / Ib)

Relation: β = α / (1 - α) or α = β / (1 + β).

4. DC Load Line and Q-point

The DC Load Line is a graph of all possible values of collector current (Ic) and collector-emitter voltage (Vce) for a given circuit.

The Q-point (Operating Point) is the specific point on the load line representing the DC values of Vce and Ic when no signal is applied. For faithful amplification, the Q-point should remain stable in the center of the active region.

5. Classification of Amplifiers

Amplifiers are classified based on the position of the Q-point and the duration of current flow during the input cycle.

6. Transistor Biasing and Stabilization

Biasing is the process of providing proper DC voltages and currents to ensure the transistor stays in the active region. Stabilization is required to keep the Q-point fixed despite variations in temperature or transistor parameters.

Common Biasing Circuits:

• Fixed Bias: Simplest but provides poor stability.
• Voltage Divider Bias: Most widely used as it provides excellent stability of the Q-point.

The Stability Factor (S) indicates how much Ic changes with temperature-related variations.

7. Feedback in Amplifiers

Feedback involves returning a portion of the output signal to the input.

• Positive Feedback: Feedback signal is in phase with input. Used in Oscillators.
• Negative Feedback: Feedback signal is out of phase with input. Used in Amplifiers.

Effects of Negative Feedback:

• Reduces Gain but increases Stability.
• Increases Bandwidth (BW).
• Reduces Distortion and Noise.
• Modifies Input and Output Impedance.