Core idea
A transistor has three terminals: base, collector, emitter
Each configuration is named after the terminal that is common (shared reference) for both input and output
Input and output are defined with respect to that common node
Common Emitter (CE)
Common node: emitter
Input: base to emitter → small control current flows
Output: collector to emitter → large controlled current flows
What it does:
Small base current controls a much larger collector current
That current flows through a resistor
Resistor converts it into a large voltage change
Results:
Voltage gain: high (amplitude increases a lot)
Current gain: yes
Phase: inverted (180 degree shift)
Key behavior:
Base goes up → collector current increases → collector voltage drops
Input up → output down
Common Collector (CC) also called emitter follower
Common node: collector
Input: base to collector → small control current flows into base
Output: emitter to collector → large current supplied to load from emitter
What it does:
Emitter follows base voltage (slightly lower)
Transistor pulls extra current from supply to drive load
Results:
Voltage gain: about 1 (no amplitude increase)
Current gain: high
Phase: no inversion
Key behavior:
Input up → output up
Same voltage, but stronger (more current available)
Common Base (CB)
Common node: base
Input: emitter to base → large current injected into emitter
Output: collector to base → nearly same current flows to collector
What it does:
Current injected into emitter passes almost directly to collector
No real current amplification
Results:
Voltage gain: yes (amplitude increases)
Current gain: about 1
Phase: no inversion
Key behavior:
Current change flows through collector resistor
Resistor converts it into voltage gain
Final understanding
CE: increases voltage and current, inverts signal
CC: no voltage gain, high current gain, no inversion
CB: voltage gain, no current gain, no inversion
Simple memory rule
Output at collector → inversion (CE)
Output at emitter → follows input (CC)
Base fixed → no inversion (CB)
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