BiPolar transistor

copper_pipe

Joined Sep 30, 2014
18

shteii01

Joined Feb 19, 2010
4,667
To me it means that you have to solve the problem twice. First time for resistors at -5%, 220k becomes 209k, 2k becomes 1.9k. Second time for resistors at +5%, 220k becomes 231k, 2k becomes 2.1k.

One more thing. I aint downloading 1.6 mb pic. You can waste your own time, I aint wasting mine.

Last edited:

MrCarlos

Joined Jan 2, 2010
400
Hello copper_pipe

Because you are not sure what to do with the tolerances on the resistors I will guide You a little.

What we know is:
The statement:
-For The circuit below, assume you That basis-emitter diode voltage drop is 0.7 V and Both resistors have tolerances of 5%. determine:
(a) The maximum basis and collector current.
(b) The maximum collector-emitter voltage.

And some values:
VBB = 5V.
VCC = 15V.
R1 = 220K.
R2 = 2K
Vb = 0.7V
Transistor DC Beta = 75.

Keep in mind that each part of the statement means something.
(a) The maximum base and collector current.
Ok.
(a) The maximum base and collector current will be when associated resistor (R1) have a value of 5%. Lower then its nominal value.
(R1 – 5%) = (220,000 x 0.005 =1,100) = (220,000 - 1,100 = 218,900) = (R1 – 5%).
Now. Ib = (VBB – Vb) / (R1 – 5%) = Ib) = (5 – 0.7) = 4.93
(4.94 / 218,900 = 2.2567e-5) = around 2.2567 uAmp. = Ib
So: 2.2567 uAmp. Is the maximum base current.
Now. IC = Ib x DC Beta. . . . . = maximum collector current.

Keep in mind that each part of the statement means something.
(b) The maximum collector-emitter voltage.
Curiously:
(b) The maximum collector-emitter voltage is when R1 has a value 5% higher than its nominal value.

WBahn

Joined Mar 31, 2012
24,576
What has been overlooked up to this point is that you can't just assume that just adjusting one resistor will yield the answer (it might, but it might not) or that both resistors should be off in the same direction (maybe they should and maybe the should be in opposite directions). You have to ask yourself what effect would a change in a given resistor value have on the desired measurement.

You can check yourself by simply performing the analyses for all four possibilities and looking at the results, but don't do this first thing. Try to see if you can identify the combinations that are appropriate.

For instance, for the second problem you might ask things like:

Q1) Given a fixed value of R2, will increasing collector current result Vce to increase or decrease?

Q2) What effect, if any, will changing R2 have on collector current?

Q3) If there's an effect in Q2, which way would R2 need to change to have the desired effect from the answer to Q1?

Q4) Given a fixed value of collector current, will increasing R2 cause Vce to increase or decrease?in more or less Vce.

Given the answer to these questionsL

Q5) To increase Vce. what do we want to do to R1? (a) increase, (b) decrease, (c) doesn't matter.

Q6) To increase Vce. what do we want to do to R2? (a) increase, (b) decrease, (c) doesn't matter.