For the three basic common BJT amplifiers, Common Emitter, Common Base, Common Collector.

How do you differentiate them? I was told that the term "common" because the particular leg is the common node or grounded between the BJT. However, I do see where there is an resistor at the Emitter (so it not connected directly to ground) and that BJT is still consider Common Emitter. What give?


Building on what Janka said in a comment of Essaim's answer, here is a graphical way of thinking about it:


simulate this circuit – Schematic created using CircuitLab

Bear in mind any or all of these terminals can have series resistances and it won't change the formulation of the amplifier, it is defined by the common terminal between input and output.


Often there will still be a resistor placed between the emitter and ground in a CE amplifier.

It's referred to as a degenerative resistor, and helps to provide bias stability to the amplifier. It's a form of negative feedback that helps to limit dependence on the specific beta values (which vary dramatically from device to device)

Often, you may notice that this resistor is significantly smaller, and the output for a CE amplifier will always be above the collector.

Another common disconnect from ground that you may see is a capacitor. This is often used to counteract the negative effects of biasing the device by providing something close to a short to ground at high frequencies.

One way to think about the type of amplifier is to look at which terminal the signal doesn't pass through. In a CE amplifier, it goes into the base and out the collector. In a CB amplifier, it goes into the emitter and out the collector. In a CC amplifier, it goes into the base and out the emitter.

  • 3
    \$\begingroup\$ +1. It's called the common node because it is shared between the input and the output of the two-port network. \$\endgroup\$ – Janka Nov 15 '17 at 3:55

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