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Common base amplifier realized with npn bjt is given in figure bellow.
Simulate npn bjt common base amplifier.


Common base amplifier, is class A amplifier. Current gain is close to unity (slightly less),


and voltage gain similar to one with common emitter amplifier with negative feedback (but with the same phase).


For large β, voltage gain is practically


Input impedance is small.

Purpose of capacitor C2 is to fix base potential. Since VBE is controlling voltage, and VB=const, VBE is smaller if VE (i.e. VIN) is higher. Then transistor is getting closer to be blocked, collector current is reducing, causing less voltage drop on RC, which means that for higher input voltage, output voltage increases, so output is in the phase with input, not like common emitter amplifier with negative feedback. This single stage amplifier can be used for current to voltage converter. In that case, output voltage is proportional to input current scaled with RC:


Since base separates input and output voltage, oscillations at high frequency are minimized, so this kind of amplifier can be used as high frequency amplifier.

Go to npn bjt common base amplifier.

External Links:

NPN BJT Common Emitter Amplifier on Transistoramp
NPN BJT Common Collector Amplifier on HyperPhysics
NPN BJT Common Collector Amplifier on AllAboutCircuits

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