Solve for IB, IC, IE, VB, VC: Figure 7, Bdc=40

[Loke]

Homework Statement

Find I_B, I_C, I_E, V_B and V_C in Figure 7, where B_dc is 40.

The Attempt at a Solution

V_CC-I_CR_C-V_CE=0
I_C=B_dcI_B
V_BB-I_BR_B-V_BE=0
I_E=I_B+I_C
V_BE=V_B
V_CE=V_C

 

[stevenb]

Why don't you try to apply your equations? For example, why can't you solve for Ib using your third equation? Let's start there. Make an attempt, and if you can't figure that out, let's find out why.

Once you know Ib you can progress further in the problem.

 

[Loke]

i tried ady...but i still can't find it...3rd equation got 2 unknown Ib and Vbe...i just don't know how to solve it without the type of transistor given...can you help me to figure it out?

 

[stevenb]

i tried ady...but i still can't find it...3rd equation got 2 unknown Ib and Vbe...i just don't know how to solve it without the type of transistor given...can you help me to figure it out?

Technically you are correct that you don't know Vbe, but there is an approximation you can make in this type of circuit. Assume Vbe is 0.6 V and you will get a decent answer.

The alternative is to use the Schokley diode law, but then you need more information about temperature and transistor characteristics. Given those uncertainties, assuming Vbe=0.6 should be good enough.

 

[Loke]

ermmm...can you solve it with kirchhoffs law because i haven study schokley diode law... i cannot apply it on my work yet.

 

[stevenb]

ermmm...can you solve it with kirchhoffs law because i haven study schokley diode law... i cannot apply it on my work yet.

ermmm ... you missed my main point. Assume Vbe is 0.6V and you can solve.

 

[Loke]

Teacher only taught me silicon transistor,Vbe=0.7 and germanium transistor,Vbe=0.3 ...but i don't know can assume one ^^? ...and in this question both type of transistor is not given that's why i don't know how to proceed..:(

 

[stevenb]

Teacher only taught me silicon transistor,Vbe=0.7 and germanium transistor,Vbe=0.3 ...but i don't know can assume one ^^? ...and in this question both type of transistor is not given that's why i don't know how to proceed..:(

Well, you are making good points that technically the problem does not specify the transistor. But, this approach is not going to let you solve the problam.

Traditionally, if the transistor is not specified, one can assume it is a silicon device, and 0.6 or 0.7 V can be used for Vbe.

Try it. You need to think like an engineer on this one.

 

[Loke]

ohh... thanks...i ask u 1 more question...if let say you don't assume Vbe=0.7...can you actually find an answer? I'm wondering.

 

[stevenb]

ohh... thanks...i ask u 1 more question...if let say you don't assume Vbe=0.7...can you actually find an answer? I'm wondering.

Yes, you can find an approximate answer based on assumptions, but it amounts to the same thing. Any reasonable assumptions will put Vbe in the 0.6 V to 0.7 V range and the answer is not very sensitive to changes to Vbe when Vbb is 3 V or greater.

There is going to be sensitivity to changes in temperature and transistor characteristics and part of the challenge to designing transistor circuits is making circuits that are less sensitive. You will be learning about this soon.

 

[Loke]

As long as the question does not provide the type of transistor,then we just assume it to be Vbe=0.7 ...am i right? i meant other than this question also?...

 

[stevenb]

As long as the question does not provide the type of transistor,then we just assume it to be Vbe=0.7 ...am i right? i meant other than this question also?...

Typically, yes, but there are always exceptions to any rule. The basic idea in most circuit design is to make robust circuits that are not sensitive to temperature and transistor characteristics. The end result of this is that usually changes in Vbe are not important and the above approximation gives reasonable answers.

One example of an exception is the well known current mirror circuit.

 

[Redbelly98]

As long as the question does not provide the type of transistor,then we just assume it to be Vbe=0.7 ...am i right? i meant other than this question also?...

stevenb gave a good answer. Also, you can think of the base-to-emitter path as similar to a diode. As long as there is something providing voltage between the base and emitter, it will (usually) be about 0.7V.

 

[Loke]

Determine I_B, I_C, I_E, V_CE and V_CB in Figure 8 for the following values:
R_B = 5 k ohm, R_E = 500 ohm , V_BB = 3.0 V, V_CC = 20 V and B_dc = 80

problems:
-If i assume the V_be in this question also = 0.7...i still can't solve it right away...can u help me figure it out?
-can i assume I_C=I_E...so I_E=B_dcI_B ...any idea?


attempt at solution:
V_BB-I_BR_B-V_BE-I_ER_E=0
V_CC-V_CE-I_ER_E=0
V_BE=0.7
I_C=B_dcI_B

 

[stevenb]

Determine I_B, I_C, I_E, V_CE and V_CB in Figure 8 for the following values:
R_B = 5 k ohm, R_E = 500 ohm , V_BB = 3.0 V, V_CC = 20 V and B_dc = 80

problems:
-If i assume the V_be in this question also = 0.7...i still can't solve it right away...can u help me figure it out?
-can i assume I_C=I_E...so I_E=B_dcI_B ...any idea?


attempt at solution:
V_BB-I_BR_B-V_BE-I_ER_E=0
V_CC-V_CE-I_ER_E=0
V_BE=0.7
I_C=B_dcI_B

Remember that Ie=(beta+1)*Ib

 

[Loke]

oh...should use Ie=(beta+1)*Ib instead of Ie=beta*Ib right?... THANKS a lot ^^ !