The BJT can operate in 4 different modes/zones since there are two voltages on it:

  • Active mode
  • Inverse/Reverse active mode
  • Saturation mode
  • Cut-off mode

Because Emitter and Collector junctions can be represented as diodes (with amplification) there are 4 different operating modes of BJT.

But do all of these modes apply for each of BJT orientation: Common Base, Common Emitter, Common Collector?

Are there 12 (3x4) different BJT circuits regarding to polarity of junctions and orientation of BJT?

  • 1
    \$\begingroup\$ Conditions of each mode are as follows: Active (aka Forward Active): Vc > Vb > Ve, Reverse Active: Vc < Vb < Ve, Saturation: Vb > Ve & Vb > Vc, Cut-Off: Vb < Ve & Vb < Vc. You can easily check for these conditions on all sort of configurations by yourself (Sorry, I'm too lazy to do it for you). \$\endgroup\$ Jan 25, 2017 at 17:51
  • \$\begingroup\$ The CC, CE, and CB orientations are just examples of what can be built. Some circuits don't fall neatly into one of these categories. Some circuits that change categories during operation. For example, and oscillator can be in cut-off mode, linear mode, and saturation mode, all in each cycle of operation. The mode of operation is set by what the circuit does, not just the orientation. \$\endgroup\$ Jan 25, 2017 at 19:02

1 Answer 1


The terms common-emitter, common-base, common-collector usually refer to transistors operating in a linear circuit, where output signal is proportional to input signal. Saturation or cut-off or reverse-bias modes would not be considered, only active (linear amplifying) mode.
However, some circuits do traverse between modes in operation - an oscillator starts off as a linear, and signals grow until a signal peak reaches a current cut-off or voltage saturation. In this case, a designer very likely adjusts circuit biasing to ensure one limit is reached before another so that resonator Q is not adversely degraded. We might refer to a "common-base oscillator" only because its small-signal operation mimics a common-base amplifier.

A SPICE model of a transistor accommodates all biasing polarity possibilities, and has no knowledge of common-base, common-emitter, common-collector.


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