Simple voltage regulator

Thread Starter


Joined Oct 31, 2010
hey, i am a bit stuck with this voltage regulator just a few things i need cleared up if any one could take a look it would be much appreciated.

first off sorry for the rubbish photo the only thing i have on me is my phone....
the question asks me to calculate vo which is the regulated ouput voltage over the resistor on the far side.
the Vce junction of the transistor
and the current in the 300 ohm resistor on the left.

okay so the transistor is the conrtrol element acting like a variable resistor over the CE junction depending on the value of VBE. The zener acts as my reference voltage of 15v (which i am told in the question). so i figure that Vo is = to Vz - Vbe = 15- 0.65 = 14.35 V

I then went on to guess that because the transistor is in parallel with the 300 ohm resistor its VC = 25v but because it acts like a variable resistor there is a voltage drop over it which i assume is 25 - 14.35 = 10.65V ???? but what about the Vbe ??

okay moving on there is 25v accross the zener and resistor. 15 volts over the zener so 10 over the resistor ??? or is the 0.65 taken out of this? so should it be 9.35 V over the resistor giving a current of 9.35/ 300 ?????

Thanks for any help, Lee.



Joined Feb 17, 2009
It's seems to me that you understanding this simply circuit quite well.
Vo = Vz - Vbe
And voltage across resistor is equal
Vr = Vin - Vz


Joined Mar 6, 2009
VCE= VC-VE = 25-Vo =25-14.35=10.65 as you have stated.

VBE is not part of this path. You have already accounted for VBE in finding Vo.