Common collector (CC) configuration: Collector terminal is common between the input and the output side.
Input: Applied between base and collector terminal.
Output: Measured between emitter and collector terminal.
AC Signal Perspective: Collector terminal grounded.
DC Biasing Perspective: Similar to common emitter configuration with resistor on emitter side.
Circuit Analysis
DC Biasing: Base-emitter junction is forward biased, Collector-base junction is reverse biased.
AC Analysis: DC voltage sources considered as zero; collector terminal grounded.
Current Directions:
Base current (Ib): Input side
Emitter current (Ie): Output side
Voltages:
Input side voltage = Vcb
Output side voltage = Vce
Characteristics
Output Characteristics
Defines: Relationship between emitter current (Ie) and voltage (Vce).
Similarity to Common Emitter Config: Ie ≈ αIc (α close to unity; Ie ≈ Ic).
Input Characteristics
Defines: Relationship between base current (Ib) and voltage (Vcb).
Voltage Relations:
Vce = Vcb + Vbe
As Vcb increases → Base current (Ib) reduces.
Fixed Vcb → Increase Vce → Increase Ib.
Current Gain (γ)
Definition: Ratio of emitter current (Ie) to base current (Ib).
Symbol: γ = Ie / Ib
Relation to α and β:
γ = β + 1
β = α / (1 - α)
γ = 1 / (1 - α)
α * γ = β
Properties
Input Impedance: Very high
Output Impedance: Very low
Current Gain: Very high (γ = β + 1)
Voltage Gain: Low (close to unity)
Power Gain: Low
Applications
Frequently used for impedance matching in amplifier circuits.
Summary
Common collector configuration is essential for impedance matching due to its high input impedance and low output impedance, along with a high current gain.