# Transistor on resistance

Discussion in 'Homework Help' started by srikar sana, Jul 23, 2016.

1. ### srikar sana Thread Starter New Member

Feb 3, 2016
18
0
hi
i want to know about the way to find the on resistance of a transistor at a particular collector current
let the transistor be 2N2222 and pls explain me how to find the on resistance at a collector current of 10mA

[ i ve heard that in on state transistor is equivalent to .a resistor in series with a voltage source why is it that way and how to find the resistance and voltage source values ]

note i am a slow learner some what detailed description helps

2. ### hp1729 Well-Known Member

Nov 23, 2015
2,098
232
Measuring collector resistance.
Once you have done the exercise with R1 = 470 ohm change R1 to 390 ohm (or something else) and see if collector resistance changes.
R col = V c-e / Ic

Last edited: Jul 23, 2016
3. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
The concept of on resistance is not really applicable to a bipolar junction transistor; it arises in connection with the behavior of a field effect transistor. If you want to look at a set of characteristic curves for a 2N2222 you can pick any point in the operating range, divide Vce by Ic and get a resistance for that point and that point only. You have in effect an infinite set of on resistances controlled by the base current Ib.

4. ### BobTPH Active Member

Jun 5, 2013
806
121
More interesting is the saturation voltage. This will tell you how much power is dissipated when used as a switch.

Bob

5. ### srikar sana Thread Starter New Member

Feb 3, 2016
18
0
i have done the same procedure but my sir said it is a wrong approach and he told me that in one stage transistor is equivalent to a voltage source and a resistor those drop together gives saturation voltage and he did proved to me by doing some circuitary and it is with me but the thing is i dont understand why he did like that and what made him think like that ...i too first told him Rcol=Vce/Ic

6. ### srikar sana Thread Starter New Member

Feb 3, 2016
18
0
bob pls tell me how to find the resistance bob not the power dissipation ? i think you know the answer spit it out to help me bob

7. ### DickCappels Moderator

Aug 21, 2008
2,773
669
Similar to but a little more accurate than "Rcol=Vce/Ic"

Or to see what the resistance of the collector looks like when "looking" into the collector, but a little more accurately than "Rcol=Vce/Ic", you can use this graph in the datasheet for your particular brand of transitor, find the slope of the VCE(sat) curve as it crosses 10 ma and use that along with Ohm's law. R = (change in voltage)/change in current). This also includes change in emitter resistance.

8. ### hp1729 Well-Known Member

Nov 23, 2015
2,098
232
The point is that there is no specific resistance. With a different load you get a different V c-e at that 10 mA, so resistance will change. Are you looking for a minimum resistance at 10 mA? Find a load that hits saturation at10 mA.

9. ### DickCappels Moderator

Aug 21, 2008
2,773
669
I should clarify, when not in the region of saturation, the collector of a resistor looks like a current source with some shunt resistance in parallel -that is what hp1729's method in post #2 finds. It does not look like voltage source in series with a resistance as somebody told you (the voltage source is the battery, external to the transitor.

So now you know how to determine two "kinds" of collector resistance, which one you use depends on what you are trying to accomplish.

10. ### srikar sana Thread Starter New Member

Feb 3, 2016
18
0
sir i am a new bie but here are the details what my sir told me about this you people decide and pls expalin me so that i can understand my final aim is to understand this topic no matter what i request you all people to achieve my aim thank you in advance

sir i have attached a pdf sir which my sir made for explaining me see that once sir

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11. ### hp1729 Well-Known Member

Nov 23, 2015
2,098
232
Ah, this helps. The exercise is looking for AC resistance, impedance.
What is calculated and what is measured are not the same. Is this your question?