# Finding current gain value for unknown NPN transistor

Discussion in 'Homework Help' started by Samir Jain, Apr 28, 2015.

1. ### Samir Jain Thread Starter New Member

Apr 28, 2015
6
0
Hi

I am trying to measure the β value (forward gain current) for a NPN transistor on a Breadboard circuit
I have the following circuit as a hint :

The Vcc is through a constant known voltage using a function generator and the resistor value is known. However, the transistor parameters is unknown (data sheet cannot be used) but it can be assumed to be in forward active mode with VBEon > VCEsat

I made a KVL loop Vcc - Ie*R-VBEon = 0.

How do i find the β value for the transistor without being allowed to measure the currents directly ?

Thanks!

2. ### #12 Expert

Nov 30, 2010
16,663
7,307
With no voltages, resistances, part numbers, or datasheets, you can only find a math formula for the answer.

3. ### Samir Jain Thread Starter New Member

Apr 28, 2015
6
0
We are allowed to design any circuit, with any amount of components such as resistors, capacitors, etc. The main goal is to create a circuit that would allow us to determine β for an unknown transistor. I can keep changing the component values and then measure the voltage across any terminals but not the current

This was the circuit that I came up with. Could u suggest what circuit I could design and use in order to find it ?

Thanks

4. ### crutschow Expert

Mar 14, 2008
13,472
3,359
Since β is current gain you need to measure base and emitter current, so is it allowed to measure the voltage across a known resistor value and calculate the current from that?

5. ### WBahn Moderator

Mar 31, 2012
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If you measure the voltage across the resistor in your circuit and use Ohm's Law to determine that current, that will tell you the sum of the base and collector currents, right? That doesn't help you much because you need to know them separately. But what if the current through a resistor WAS the collector current and the current through another resistor WAS the base current?

6. ### WBahn Moderator

Mar 31, 2012
18,079
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Who gave you that circuit as a hint?

7. ### Samir Jain Thread Starter New Member

Apr 28, 2015
6
0
Yes, that is allowed to do that.

8. ### Samir Jain Thread Starter New Member

Apr 28, 2015
6
0
the TA for my course gave me the circuit as a hint but we are pretty much allowed designed to any circuit. All that is not allowed is to measure the currents directly or know the transistor parameters (VbEON, VCEsat, β)

9. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
Once you are done with this assignment, ask the TA to show you how that circuit moves you in the direction of being able to determine the transistor beta. Use their response to help you gage whether you should be listening to that TA very much.

#12 likes this.
10. ### Samir Jain Thread Starter New Member

Apr 28, 2015
6
0
The TA only gave this as a basic circuit and said that we could add various resistors, etc to find the base and collector voltages.

Using the resistor placement above, would it be possible to solve for the transistor β ? If so, would u mind explaining how the KVL loop would look like ?

11. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
Yeah, I kinda would mind.

I think you will be better served by struggling with how to take this hint and figuring out how to construct a circuit that exploits the concepts. Post your best attempt as well as an explanation of how you think you might use the circuit to get the information you need to calculate beta.

Remember the key idea involved here: If a known resistance is in series with device pin then knowing the voltage across the resistor lets you determine the current into the pin.

12. ### #12 Expert

Nov 30, 2010
16,663
7,307
Now that we know the circuit you presented is not the circuit you will use, add a resistor between the voltage supply and the base, measure the voltage across the two resistors and do the math.

13. ### WBahn Moderator

Mar 31, 2012
18,079
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Well, so much for spending some time struggling with how to turn the goal and some concepts into a circuit.

14. ### #12 Expert

Nov 30, 2010
16,663
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Do you think I was wrong to say that?
I thought crutschow said that in post #4 and you said it in post #5

15. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
Not wrong, it just defeated the purpose of what I was trying to do. But there's nothing sacred about how one person is trying to interact with someone else on a forum. What crutschow said in Post #4 didn't take root, which led me to suspect (possibly incorrectly) that having the answer (or a method) spelled out probably would not result in the concepts being absorbed. Of couse that means that the same might be true of your suggestions. I think in general that it is a valid suspicion, but whether it is right in any specific instance is anyone's guess.

Also, note that crutschow did not say how to measure the currents or what kind of circuit to use to implement the suggestion he was making. That distinction is lost on you -- and I'm saying that in a positive regard -- unless you are looking for it because you read what he wrote and your mind immediately draws upon your experience and fills in the gaps and you know the circuit (or a circuit) that he is suggesting every bit as clearly as if he had literally drawn a picture. So, to your experienced mind, what he said and what you said are essentially indistinguishable -- again, unless you are looking for it. I very well would have viewed them that way, too, but for the fact that I was actively, in this case, trying to leverage that distinction and so I was in a frame of mind to be looking for it.

Last edited: Apr 29, 2015
16. ### Samir Jain Thread Starter New Member

Apr 28, 2015
6
0
Thanks for the help everyone!
Using the hints mentioned above, I was able to figure out how to solve this problem.

Apologies for being so vague in explaining the problem.

17. ### #12 Expert

Nov 30, 2010
16,663
7,307
Yes. I believe that is exactly what happened. I fail at trying to think on the level of a student, regularly!

18. ### WBahn Moderator

Mar 31, 2012
18,079
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So do I -- and given what I am now doing for a living....

19. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
Great!

Once you make your measurements, post your results. They'll be interesting to see. Feel free to wait until after the assignment is due if you want to.

20. ### #12 Expert

Nov 30, 2010
16,663
7,307
There are still interesting things to learn as you adjust the resistance values to get a useful measurement.