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  #1  
Old 07-13-2013, 09:36 PM
AD633 AD633 is offline
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Default 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
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  #2  
Old 07-14-2013, 01:44 AM
t_n_k t_n_k is offline
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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.
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  #3  
Old 07-14-2013, 05:22 PM
AD633 AD633 is offline
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Quote:
Originally Posted by t_n_k View Post
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.

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; 07-16-2013 at 03:05 PM.
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  #4  
Old 07-16-2013, 03:06 PM
AD633 AD633 is offline
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Quote:
Originally Posted by AD633 View Post
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


Thanks
The problem is that i don't know neither vd or vds.Do i need to find ib first for determing Vd?
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  #5  
Old 07-16-2013, 04:05 PM
Jony130 Jony130 is online now
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OK, but how can Id current be larger than Idss and and also Vp and Vgs?
So you need solve this equation again.
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  #6  
Old 07-16-2013, 11:36 PM
t_n_k t_n_k is offline
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@ 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.
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Old 07-17-2013, 01:48 PM
AD633 AD633 is offline
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Quote:
Originally Posted by t_n_k View Post
@ 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.





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
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  #8  
Old 07-17-2013, 02:18 PM
Jony130 Jony130 is online now
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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.
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  #9  
Old 07-17-2013, 02:44 PM
AD633 AD633 is offline
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Quote:
Originally Posted by Jony130 View Post
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.
For finding Ib do i need to use a system of equations?Something like:





Or assuming that the BJT is in the ative region and that Ie is approx equal to Ic



Thanks
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  #10  
Old 07-17-2013, 03:07 PM
Jony130 Jony130 is online now
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Quote:
Originally Posted by AD633 View Post
Or assuming that the BJT is in the ative region and that Ie is approx equal to Ic
Yes, first assume that BJT is in active region and find Ib current.
Then you need to check if your assumption about BJT active region was right.


Quote:
Originally Posted by AD633 View Post


Thanks
What?? Ic = ??
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