Transistors

• Bipolar Junction Transistor (BJT): Two p-n junctions in NPN or PNP forms, using both electrons and holes.
• Field Effect Transistor (FET): Uses electric field control; includes JFET and MOSFET types.
• NPN Transistor: Electrons are the majority carriers; p-layer is between two n-layers.
• PNP Transistor: Holes are the majority carriers; n-layer is between two p-layers.

• Emitter (E): Emits charge carriers (electrons or holes).
• Base (B): Thin, lightly doped control terminal.
• Collector (C): Collects charge carriers; larger and moderately doped.

a. Common Base (CB)

• Input: Emitter–Base
• Output: Collector–Base
• Characteristics: Low input impedance, high output impedance, voltage gain > 1, current gain < 1
• Use: High-frequency amplifiers

b. Common Emitter (CE)

• Input: Base–Emitter
• Output: Collector–Emitter
• Characteristics: Moderate input/output impedance, high current and voltage gain, 180° phase shift
• Use: General-purpose amplifiers

c. Common Collector (CC) / Emitter Follower

• Input: Base–Collector
• Output: Emitter–Collector
• Characteristics: High input impedance, low output impedance, voltage gain ≈ 1, high current gain, no phase shift
• Use: Buffering, impedance matching

1. Active: Emitter-Base forward biased, Collector-Base reverse biased (amplification)
2. Saturation: Both junctions forward biased (acts as a closed switch)
3. Cut-off: Both junctions reverse biased (acts as an open switch)
4. Reverse Active: Uncommon; poor performance

Current Gain (β):

Voltage Gain (A_v):

Power Gain (A_p):

These parameters define the transistor's ability to amplify signals.

1. Amplifiers: Used in audio and RF circuits (CE mode).
2. Switches: Fundamental to digital logic circuits.
3. Oscillators: Create periodic waveforms for communication.
4. Voltage Regulators: Stabilize voltage in power supplies.
5. Microprocessors: Billions of transistors form integrated logic circuits.