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#1




Circuit with BJT and JFET transistors
Hi,
For the following circuit determine the voltages VG,VC and VD VG For determing Vc i need to know the value of the current Ic of the BJT. For determing Ic do i need to determine Id first? For determing Ib (not sure if this equation is correct) I need to know what the value of the current Id and Vgs in order to determine the value of VD right? Can i determine VD from this equation: Suposing that the BJT works in the ative region therefore IE is equal to Id How can i determine VD and VDS from what i already know.. Thanks 
#2




Firstly, check your incorrect equation for VG from which you (luckily) obtain the right value ....
Next you need to set up some equations to solve for the FET drain current. The base equation is You already know VG and VS is found from So You solve equations 1 & 2 to find FET drain current. The rest will follow from this. 
#3




Quote:
So i have to do a system of equations with: I have found and Now suposing that the BJT is in the ative region.Id=Ie ,and Ie is aprox equal to Ic Therefore For determing Vd The problem is that i don't know neither vd or vds.Do i need to find ib first for determing Vd? Thanks Last edited by AD633; 07162013 at 03:05 PM. 
#4




The problem is that i don't know neither vd or vds.Do i need to find ib first for determing Vd?

#5




OK, but how can Id current be larger than Idss and and also Vp and Vgs?
So you need solve this equation again. 
#6




@ AD633
.Remember that quadratic equations usually have two roots  not one. You chose the wrong root. The value must make sense physically as well as mathematically. 
#7




Quote:
I have found and Now suposing that the BJT is in the ative region.Id=Ie ,and Ie is aprox equal to Ic Therefore For determing Vd The problem is that i don't know neither vd or vds.Do i need to find ib first for determing Vd? Thanks 
#8




Well the correct answer is
Id = 4.23621mA and Vgs = 0.958448V And yes, you need to know Ib first to find Vd = Ve voltage. 
#9




Quote:
Or assuming that the BJT is in the ative region and that Ie is approx equal to Ic Thanks 
#10




Quote:
Then you need to check if your assumption about BJT active region was right. What?? Ic = ?? 
Tags 
bjt, circuit, jfet, transistors 
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