# Finding current gain value for unknown NPN transistor

#### Samir Jain

Joined Apr 28, 2015
6
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!

#### #12

Joined Nov 30, 2010
18,216
With no voltages, resistances, part numbers, or datasheets, you can only find a math formula for the answer.

#### Samir Jain

Joined Apr 28, 2015
6
With no voltages, resistances, part numbers, or datasheets, you can only find a math formula for the answer.
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

#### crutschow

Joined Mar 14, 2008
26,058
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?

#### WBahn

Joined Mar 31, 2012
26,398
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?

#### WBahn

Joined Mar 31, 2012
26,398
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 :
Who gave you that circuit as a hint?

#### Samir Jain

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

#### Samir Jain

Joined Apr 28, 2015
6
Who gave you that circuit as a hint?
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)

#### WBahn

Joined Mar 31, 2012
26,398
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

#### Samir Jain

Joined Apr 28, 2015
6
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.
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.

But what if the current through a resistor WAS the collector current and the current through another resistor WAS the base current?
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 ?

#### WBahn

Joined Mar 31, 2012
26,398
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 ?

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

Joined Nov 30, 2010
18,216
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.

#### WBahn

Joined Mar 31, 2012
26,398
Well, so much for spending some time struggling with how to turn the goal and some concepts into a circuit.

#### #12

Joined Nov 30, 2010
18,216
Do you think I was wrong to say that?
I thought crutschow said that in post #4 and you said it in post #5

#### WBahn

Joined Mar 31, 2012
26,398
Do you think I was wrong to say that?
I thought crutschow said that in post #4
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.

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#### Samir Jain

Joined Apr 28, 2015
6
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.

#### #12

Joined Nov 30, 2010
18,216
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.
Yes. I believe that is exactly what happened. I fail at trying to think on the level of a student, regularly!

#### WBahn

Joined Mar 31, 2012
26,398
Yes. I believe that is exactly what happened. I fail at trying to think on the level of a student, regularly!
So do I -- and given what I am now doing for a living....

#### WBahn

Joined Mar 31, 2012
26,398
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.
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.

#### #12

Joined Nov 30, 2010
18,216
There are still interesting things to learn as you adjust the resistance values to get a useful measurement.