If * Find . * If changes to , find . * For what value of will voltages V(B),V(E),V(C) get back to previous values. ================================================== Here's what I got. If R(B) changes to 270k: Here comes the 'problem', finding Beta... I calculated using the previous (first) V(B) value. So If V(C) gets back to the previous value, I(C) should be the same. And . But I realized I can find Beta with another method, using K-laws. using: and the value of I(C) found first, I get: Can anyone tell me if any of the solutions is right or wrong and why. Thanks in advance.
Check your original VE. It's inconsistent with your emitter current. The emitter voltage should be ~0.7 V above the base voltage.
For Hfe = 30 and Rb = 27K Ib = (9V - 0.7V)/ ( 27K + 31*2.7K) = 74.977μA Ic = 30 *Ib = 2.249mA Ie = 31 *Ib = 2.324mA Ve = 9V - Ie*Re = 2.725V Vc = Ic*Rc - 9V = -2.927V Now for Rb = 270K and β = 30 Ib = (9V - 0.7V)/ ( 270K + 31*2.7K) = 23.466μA Ic = 30 * Ib = 703.986μA Ie = 31 * Ib = 727.452μA Ve = 9 - Ie*Re = 7.035V Vc = Ic*Rc - 9V = 7.099V And I don't see any problem ?
@mlog - Yeah you're right. Ve should be 2.724V. @Jony130 - Thanks for the confirmations, but I'm having problems with the third part of the problem. so with Rb=270k, I have to find the value of Beta so the terminal voltages get their previous values. I think Beta should be 300, because Rb changed from 27k to 270k. So if Beta was 30 first, it should be 300 when Rb=270k and the terminal voltages get back to their previous values. I tried another method (you can see in the first post) and I get Beta=309. What am I missing ? Is Beta 300 or 309 or none?
I think that both of your answer are correct. Simply it is impassible to get exact voltage for R = 270K. The the reason for this is that emitter current is equal Ie = Ic + Ib but for R=270K the Ib is ten times smeller than previous values. And this cause the "error".