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In all the configurations common collector, common base and common emitter, is the base always the input?

enter image description here

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The base node is - obviously - ONE single node only.

However, a voltage is defined as the difference in potential BETWEEN TWO nodes. As a consequence - and according to the working principle of the BJT - the voltage between the base node and the emitter is the input for ALL three configurations.

However, one of these nodes is grounded (ac wise) and , thus, defines the three basic configurations.

EDIT (Comment): (Regarding the comment from Spehro Pefhany)

Also in common-base configuration, it is the base-emitter voltage that controls the collector current Ic.That means: When the base is grounded, it is the emitter voltage referenced to ground.

(After changing the PC machine a new account was automatically created and I am not yet allowed to write a "comment".)

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Obviously (?) not for common base. Because it's common.

Input is to the emitter, which give you the potential of voltage gain, not current gain. Image from Wikipedia.

enter image description here

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  • \$\begingroup\$ I dont know about common base yet, but for the image i just added. Does the graph show a common base configuration? \$\endgroup\$ – Aura Divitiae Jan 6 at 6:15
  • \$\begingroup\$ @AuraDivitiae What terminal does the base connect to in the circuit above? Ground. This terminal is common to both input and output circuits. That's why it's called common base. \$\endgroup\$ – Neil_UK Jan 6 at 6:38

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