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


Common collector amplifier, also called emitter follower is class A amplifier, similarly as common emitter amplifier. But there is big difference. Voltage gain of this amplifier is close to unity, but smaller then one! It means that common collector amplifier is voltage buffer and current amplifier. Since voltage gain is slightly smaller then one, emitter follower is applied in situations when input voltage is large enough, so further voltage amplification is not needed, and current should be amplified. How come that voltage gain is several hundreds times less with common collector amplifier comparing to common emitter amplifier? Bipolar transistor needs to operate in active regime in order to amplify the signal. It means that VBE is in 0.5 to 0.7V range. VBE is not the limitation factor if output voltage is taken from transistor collector, since VCE can vary from VCES that is typically 0.2V to VCC. If, however output voltage is taken directly from the emitter, then output voltage VOUT is lower then input voltage for VBE amount of volts (VOUT = VIN VBE). Equivalent circuit for small signals is given in figure bellow.


From the equivalent model of common collector amplifier for small signals, it can be calculated that voltage gain is:


As it can be seen from the equation, output voltage is with the same phase as the input voltage. Current gain is significant and it is 1+β. Operating class of signal amplifiers depends from both amplifier topology and the amplitude and polarity of the input signal. By shifting quiescent point to VOUT = VCC when input voltage is zero (this can be adjusted by removing voltage divider R1R2), common collector amplifier will operate as an amplifier in class B. Actually, this way modified common collector amplifier is upper half of push – pull amplifier.

Go to npn bjt common collector amplifier.

External Links:

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

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