I found a datasheet to my 2N3904 transistor: Intelligent Power and Sensing Technologies | onsemi. I know that is has a maximum collector current of 200mA. But the h_FE value is a very large range of values. :~ I am familiar with Ohm's law: V = I*R, I = V/R, R = V/I. There is always 0.6V between emitter and base when the transistor is turned on, so that leaves 4.4V across the E and C. So I should do some formula like this: r_B = 4.4V / (0.2A / h_FE). Example: 4.4V / (0.2A / 100) = 4.4V / 0.002A = 2200 ohms. _ = subscript. A found a formula for h_FE: h_FE = i_C / i_B. But that isn't helpful, because I don't know the i_B [i_C / h_FE]; I need to know the h_FE first! Is there a formula for calculating the h_FE given the i_C and V_EC. etc.? Or better: formula for base current required.