I can't seem to get over this but this seems to be the basics in electronics. I have an NPN darlington transistor TIP122 and have built to simple test circuit around it with Rb = 150 ohm, Rc = 80 ohm and Re = 6.8 ohm. My base supply Vbb = 3.3V and collector supply Vcc = 20V.
I have used the equation from this site as mentioned for emitter biasing.
Ie = (Vbb - Vbe)/(Rb/beta + Re)
However I am getting different values theoretically and practically.
With Re = 6.8 I get theoretical Ie = 273mA and practical Ie = 222mA then With Re = 6.8||6.8 I get theoretical Ie = 535mA and practical Ie = 228ma.
I don't understand why could this be happening, how does the emitter resistor work in the transistor circuit... can you help me out?