# BJT - Common Emitter - exact RE calculation

#### Guenter

Joined May 24, 2021
4
Hi there,

in a common emitter circuit, I calculate the RE ( Emitter Resistor) and RC with

Ib = Ic / BF
R3 = (Vcc - Uce - Ure) / Ic
RE = Ure / (Ic + Ib)

But these Formulas do not consider the internal resistors rb and rc.

A comparison with Qucs and LTSpice confirms this. ,The BJT Model in this case is

.model myNPN NPN
+ IS=2e-16
+ BF=140
.ENDS

If RE is attached to the model (0.4 for example), the calculates RE is wrong.

Does anyone know the exact formulas?

THX
Guenter

#### Veracohr

Joined Jan 3, 2011
765
Usually the circuits are designed in a way that the effect of the internal resistances is negligible.

#### Guenter

Joined May 24, 2021
4
Yes, you are right.

But this is for my understanding how the mathmodel of the BJT and the RE calculation depends of each other.

#### BobaMosfet

Joined Jul 1, 2009
2,002
Hi there,

in a common emitter circuit, I calculate the RE ( Emitter Resistor) and RC with

Ib = Ic / BF
R3 = (Vcc - Uce - Ure) / Ic
RE = Ure / (Ic + Ib)

But these Formulas do not consider the internal resistors rb and rc.

A comparison with Qucs and LTSpice confirms this. ,The BJT Model in this case is

.model myNPN NPN
+ IS=2e-16
+ BF=140
.ENDS

If RE is attached to the model (0.4 for example), the calculates RE is wrong.

Does anyone know the exact formulas?

THX
Guenter
Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

#### neonstrobe

Joined May 15, 2009
181
Some basic points - if you mean RE is an external resistor to bias the emitter, it is set by the emitter voltage (to whatever rail) divided by IB+IC=IE.
If you mean RE is the physical internal resistance it is generally very low (.1ohm for a small signal transistor or less for a higher power transistor) and usually only really matters at high currents (it has to be measured at high currents too usually or with RF kit).
There is also another "RE" which is the effective impedance of the emitter looking into the emitter, which applies to a "small signal" analysis but is usually derived from the IV characteristics - it is an apparent resistance but not a fixed value as it depends on the current.