# Are the equations used in transistor problems valid for all the 3 configurations?

#### Devika B S

Joined Mar 8, 2017
144
The various equations such as alpha = Ic/ Ie and Beta = Ic/Ib, Vce = Vcc - IcRc valid for all the three configurations - such as common base, common emitter and common collector configuration? All the problems that I worked out only have the common emitter configuration. So I am confused whether these equations are valid just for that one particular mode of operation. Do the other 2 modes have some other equations and gain parameters?

#### Jony130

Joined Feb 17, 2009
5,093
These equations are always valid if BJT is working in active region. In saturation Ic = β*Ib is not valid. And to be more precise Vce = Vcc - IcRc this equation is only true if Re resistor is 0Ω. But Vc = Vcc - Ic*Rc is always valid.

• Devika B S

#### DickCappels

Joined Aug 21, 2008
6,015
The various equations such as alpha = Ic/ Ie and Beta = Ic/Ib, Vce = Vcc - IcRc valid for all the three configurations - such as common base, common emitter and common collector configuration? All the problems that I worked out only have the common emitter configuration. So I am confused whether these equations are valid just for that one particular mode of operation. Do the other 2 modes have some other equations and gain parameters?
Those describe the inner workings of the transistor and are not related to how it is connected to the rest of the circuit.

#### WBahn

Joined Mar 31, 2012
24,987
The various equations such as alpha = Ic/ Ie and Beta = Ic/Ib, Vce = Vcc - IcRc valid for all the three configurations - such as common base, common emitter and common collector configuration? All the problems that I worked out only have the common emitter configuration. So I am confused whether these equations are valid just for that one particular mode of operation. Do the other 2 modes have some other equations and gain parameters?
By definition, α = Ic/Ie and β = Ic/Ib. However, there is nothing that says that α and β are constant. In the active region they are often treated as approximately constant, but in saturation they vary substantially. In cutoff and reverse operation they become particularly troublesome.

Any equation involving a component external to the transistor, such as Rc, is going to depend on the configuration.

Don't spend a bunch of time memorizing equations and when you can and can't use them. Instead, learn and understand the fundamentals of circuit analysis so that you can analyze circuits that you have never seen before and, perhaps, the world has never seen before.

• Sinus23