Current through a lightbulb after flowing through transistor

[4Phreal]

Homework Statement

Consider a 2N3904 NPN transistor in a circuit. Treat the lightbulb as a 100 ohm resistor. For different values of R, what is the current through the lightbulb? Enter two digits rounded to the nearest 0.1 amps.

If R=500 ohms, the current through the lightbulb is:

If R=5 kohms, the current through the lightbulb is:

If R=50 kohms, the current through the lightbulb is:

http://i.imgur.com/mbk0cKK.jpg

Homework Equations

Ic = βIb

The Attempt at a Solution

For the 500 ohm resistor:
Ib = (10V - 0.6V) / 500 ohms=0.0188A
Ic=β*0.0188A=100*0.0188A=1.9A

0.6V is the common value used for silicon transistors
It appears beta for the 2N3904 NPN is 100. In other words, Ic/Ib = 100
For the 5k and 50k ohm resistors, in order for the circuit to have any current, the potential difference between base and emitter has to be large enough to activate the transistor. For the 2N3904 NPN, there must be a 0.2 mA current for it to be amplified.

4. Other relevant information
The answer to this question using a 2k ohm resistor is 0.1 A. No idea how they got that answer

 

[Hesch]

You must find some characteristics for the transistor ( I_collector vs. I_base characteristics ).

It's very hard to find ( I've tried ) because the transistor is typical used as a switching transistor, where of the mentioned characteristics are not of interest.

 

[4Phreal]

What kind of characteristics?

For the 2N3904 NPN transistor, when the base is more positive than the emitter by 0.6 volts (for the silicon devices) current will flow from the base to the emitter. When this base current IB is sufficiently large, the transistor is “turned on”. A current of 1 mA or larger will turn on the 2N3904 NPN transistor. As mentioned in my original post, the beta value of amplification here between IB and IC is 100. What other characteristics am I needing?

 

[Hesch]

You must find some characteristics for the transistor ( I_collector vs. I_base characteristics ).

Something like this:

Plot two points ( A and B ) and draw a load line. Calculate I_b and you can read the Q-point, determining I_c.

( The figure is not for a 2N3904 ).

 

[4Phreal]

Something like this?

I kind of doubt that I have not been given sufficient information to answer the question without finding some obscure characteristics. But I also clearly have no idea what I'm doing, so my doubts don't hold much weight

 

[Hesch]

I cannot read the base currents in the figure, but yes, that's it.

The characteristics are not "obscure"

 

[4Phreal]

Oh sorry, the graph is for IB = 10 µA, 20 µA, 30 µA, ... , 80 µA.

 

[Hesch]

The highest resistor value in #1 is 50 kΩ → I_b = 188 μA.

That's a problem.

 

[4Phreal]

It's a problem because we don't have that data point on the graph? I'm almost certain the answer for the 50 kΩ is 0.0

 

[Hesch]

Are the curves in #5 for a 2N3904 ?

( Curves differ for different types of transistors ).

 

[Andrew Priest]

0.1, 0.1, 0.0. I assume you're in Physics 140 at BYU? All you do for this problem is see if the transistor is on or not. To do this, the current going into the transistor from the R side has to be greater than 1 mA. If it is, then the current through the light bulb=9.8/100, or 0.1 in his class.