Sunday, 26 July 2015

transistors - Finding current through an LED in an amplifier using a BJT


Suppose you have the schematic:


enter image description here


My job is to find $I_{LED} $ knowing that the impedance of the capacitor in negligible against $V_s$. Here's my attempt:


I intend to do an AC analysis followed by a DC analysis, and get both components of the current that goes by the LED. enter image description here


Where $R_A=R_1||R_2$


For the AC analysis: I did the schematic in the figure and with the following set of equations I get to the final result: i1=i2+iBvsi1RBi2RA=0vsi1RBiERE=0iC=βiBiCiEiC=vsRB1β+RE(1R1+1R2+1RB)


For the DC analysis I use the following set, reaching the result: VccI1R1I2R2=0VccI1R1VBEIERE=0I1I2=IBIB=ICβICIEIC=VccR1VBE(1R1+1R2)1β+RE(1R1+1R2)


Finally I get to:


ILED=IC+ic=VccR1VBE(1R1+1R2)1β+RE(1R1+1R2)+vsRB1β+RE(1R1+1R2+1RB)



However the correct solution is: ILED=VccR1VBE(1RB+1R1+1R2)1β+RE(1RB+1R1+1R2)+vsRB1β+RE(1R1+1R2+1RB)


I know that this is a very long question and I don't ask for a solution to the problem, because I have one solution that leads to the correct result, however they don't do the DC and AC analysis separately and I was trying to do it because it should work, but I don't know where the mistake is, the answers are really similar and the AC analysis appears to be correct but the DC does not, and I can't find the mistake.


EDIT: I have found the mistake, In the DC analysis schematics I left out the resistance $R_B$, and if I leave it in the schematics connected to the ground then I can substitute, in the DC analysis only, all the $R_2$´s by the paralel resistance of $R_B$ and $R_2$ and the answers match. But a new question comes up, why do I have to leave $R_B$ connected to the ground? I thought that with a DC analisys capacitors would be replaced by an open circuit, therefore, no current should go through $R_B$, and that's why I ignored it. And this is just th case where I have a resistance $R_B$ there, if I didn't, but left that branch connected to the ground, then wouldn't it act as a short and we could ignore $R_2$?




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