# transistor gain formula

#### fran1942

Joined Jul 26, 2010
58
Hello, I understand the gain formula is RC/re+RE.

My notes say:
"The term 're' being a transistor parameter which represents the internal resistance of the transistor. The formula for 're' is 26/IC mA. The 26 value is a internal resistance constant used for silicon transistors if you are running at 1mA. If you were running at 2mA you would use 13. In our case we are using １mA, so we use 26.

My question is:
In my case, I am running at 2.5mA, so what value would I use for 're' ?

Thanks kindly for any help.

#### t_n_k

Joined Mar 6, 2009
5,455
re=26/2.5=10.4Ω

#### steveb

Joined Jul 3, 2008
2,436
The formula for 're' is 26/IC mA. The 26 value is a internal resistance constant used for silicon transistors if you are running at 1mA. If you were running at 2mA you would use 13. In our case we are using １mA, so we use 26.
Note that the value of 26 mV is only for room temperature operation. The more general formula is Vt=KT/q, where K is Boltzmann's constant, T is temperature in Kelvin and q is the magnitude of the electron charge. At room temperature T is about 300 K (just at 273 to the temperature in degrees Celsius), so

$$V_t={{K\; T}\over{q}}={{(1.38)10^{-23}(300)}\over{(1.6)10^{-19}}}=25.6 {\rm mV}$$

Sometimes people multiply another factor in to Vt and use nVt, where n is 1.2 for typical silicon transistors. This brings the value up to 31 mV at room temperature.