# common base, collector, emitter

#### lokeycmos

Joined Apr 3, 2009
431
is there an easy way, or rule of thumb to identify whether a transistor circuit is common base, common emitter, or common collector? Im trying to research and am a little lost. thanks

#### crutschow

Joined Mar 14, 2008
33,369
It's whichever terminal is at AC common (ground).

#### Ron H

Joined Apr 14, 2005
7,014
Amplifiers have an input, an output, and a node that is common to both. Identify the input and the output. The common electrode is the remaining one.
A resonant circuit oscillator has feedback between two of the pins. The common electrode will be connected through a low impedance at the resonant frequency (capacitor or wire) to AC ground,

#### lokeycmos

Joined Apr 3, 2009
431
It's whichever terminal is at AC common (ground).

how do I identify AC Ground vs DC Ground?

#### t_n_k

Joined Mar 6, 2009
5,455
It's sometimes helpful to discuss specific examples and then move from the particular to the general as one's understanding grows.

If you posted some examples in which you are experiencing problems deciding which topology [CE, CB or CC] is represented, you then might move forward in gaining a clearer understanding.

#### crutschow

Joined Mar 14, 2008
33,369
how do I identify AC Ground vs DC Ground?
AC ground is a node that is connected to ground through a capacitor, thus it can have a DC value different from ground but will be at AC ground for any frequencies of interest. DC ground is the same as ground (i.e. it's ground for DC or AC).

#### WBahn

Joined Mar 31, 2012
29,528
AC ground is a node that is connected to ground through a capacitor, thus it can have a DC value different from ground but will be at AC ground for any frequencies of interest. DC ground is the same as ground (i.e. it's ground for DC or AC).
If you have a node connected to "ground" via an inductor, that "ground" will be DC ground but not AC ground.

It may just be a terminology thing.