Unit 2: Bipolar Junction Transistors
Table of Contents
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.
| Class | Operating Point Position | Conduction Angle | Efficiency |
|---|---|---|---|
| Class A | Center of Load Line | 360° | Low (Max 25-50%) |
| Class B | At Cut-off | 180° | Medium (Max 78.5%) |
| Class C | Below Cut-off | < 180° | High (> 80%) |
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.
Exam Focus Corner
Frequently Asked Questions
- Why is the CE configuration most commonly used? Because it provides both high voltage gain and high current gain.
- Define Thermal Runaway. It is the self-destruction of a transistor due to an uncontrolled increase in collector current caused by rising temperature. Biasing circuits prevent this.
Common Mistakes
- Current Gains: Thinking α can be greater than 1. (α is always slightly less than 1).
- Feedback: Confusing the effects of negative feedback—remember, it *decreases* gain to *increase* everything else (stability, bandwidth).
Mnemonics
Operating Regions: "ACS" -> Active (Amplifier), Cut-off (Off), Saturation (Short/On).