Electrical engineers help with transistor?

Question

I have this design homework, where i have to design a biased circuit for a 2N3904 npn bipolar transistor with 5% tolerance resistors.
Here is what the circuit should look like. With a few changes, Vcc is 15V, Rc=0 ohm and the design is to give a nominal collector current of 1.2 mA. Unknown are R1, R2, RE.

http://i115.photobucket.com/albums/n288/johndoe_00/cir1.jpg

I need to design it based on worst case max and min to get a good design. Since i have to consider 4 factors ( beta,R1,R2 and Re). I also dont know what info i need of the datasheet. I need some help, i'm totally lost and confused on this problem.

Answer 1

Making a first assumption, the 1.2 mA in the collector is also the emitter current. This makes the voltage drop cross the emitter resistor = I*R=0.0012*2200=
Assuming a beta of at least 10 this will work. The 15 volt source less the voltage across the emitter resistor is the maximum peak to peak voltage at the output. Whatever that voltage comes out to be, calculate the collector resistor by using r=E/I

That leaves the biasing of the base to be calculated. In this case, the base voltage will be about 0.7 volts greater than the emitter voltage. The current through the bias resistors should be about 1/10 of the collector current. The voltage of the base being known, the current being known, calculate the resistor required.
Now since you need to use 5% resistors, they come in certain values. Select the next lower value resistor to a 5% value. Now recalculate the bias current knowing the bias voltage and resistor. The remaining bias resistor is found by dividing the source voltage minus the bias voltage, by the new bias current. Again find the nearest 5% value resistor.

I Hope this helps.

Answer 2

Your transistor data sheet should give both nominal values for beta and Re as well a guaranteed values. There should be a guaranteed minimum for beta and maximum for Re. The guaranteed values should be used for good design. The resistors, each will affect the collector current differently; pick the extremes of tolerance (+/- 5%) in the direction for each resistor that affects collector current in the same way. For example, select an extreme for max collector current expected by picking R1 at -5%, R2 at +5% and RE at -5%. Do the same for min, but with opposite percentage signs. This will give you the extremes of Ic. You will find that this give a very large range of Ic (+/-15%). In practice, it is very unlikely that all the resistors will be at the extreme ends of their tolerances, and all in the worst-case directions.

Answer 3

The info that you need is as follows....

Ic = beta * Ib

beta will vary between transistors and will also vary with actual current so you will need to look at 2 or three values, see the data sheet.

Ie = Ic + Ib

You can assume Veb to be about 0.7v although this can vary a little.

You now have enough info to calculate some values using ohms law. Give it a try.

Answer 4

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