On Wed, Nov 13, 2024 at 02:53 PM, john23 wrote:
Hello Andy, I biased my NPN to design the limiting resistor.
What "limiting resistor" are you talking about?
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As I explained already, resistor R1 does not limit anything in your circuit.? The current into the Base exactly equals the current from I1.? Adding that resistor R1 changes nothing.? (Well, it changes the voltage at the top of I1, but it does not change anything at the NPN BJT.? The current source I1 drives its specified current exactly, regardless of the voltage.)
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by your previus advice my maximal current should be 1000*(5/200)=25mA which is exactly the saturation current i got in the plot below.
I'm sorry, but I do not folllow.? Where is this "maximal current"?? Through what?? Where did you get that formula?
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If "200" represents the NPN transistor's Beta, note that that number is very approximate.? In fact, the 2N2222 in the simulation had a Beta greater than 200 when I simulated it.
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In real circuits with real transistors, the Beta can vary over a very large range.? The datasheet (referred to earlier in this topic) shows a range of 30 to more than 325, and the range at a single operating point?can be at least as much as 325/75 which is a ratio of 4.3:1.? In other words, one 2N2222 might have a Beta that is 4.3 times another 2N2222, under the same conditions.? Note that some of the numbers on the datasheet are omitted.? For example, there is no maximum Beta at Ic=10 mA and Vce=10 V.? It might be as high as 1000 or more.? It probably won't be, but where do you put the upper limit?? There is none.
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... 25mA which is exactly the saturation current i got in the plot below.
That is not what you showed in your plot.? It does not reach 25 mA.
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I have made a simulation shown below by your advice where I lowered the current going into the base.?
Three questions:
1.In the plot I saw the when the collector? current is saturated? Vce is very low .
Why the Vce is very low in this state?
Because that is what it means to be "saturated".
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Get a book about bipolar transistors, and read about them.? You need to learn how transistors work, before you can use them.
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When the base current increases, the collector current TENDS to increase also, while the transistor turns ON more.? But when Ic increases, the voltage drop across R3 makes the collector voltage fall.? Eventually it reaches the point where Vce is nearly zero.? When that happens, the transistor can not turn ON any more.? It can not pull the collector voltage any lower.? If it could, it would make Vce negative, which is impossible.
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When Vce becomes very low and it can't go any lower, that means the transistor is saturated and can not make its Ic larger than it is.
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2. suppose I have a 3V voltage source instead of? the current source.
I want to stay in linear state where Ib=45uA Ic=9mA as shown in the plot below.
How do i calculate the needed limiting resistor if I replace the curent source with 3V voltage source?
What is the "limiting resistor"?? Do you mean R1?? R2?? R3?
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Is there something you are trying to accomplish with this circuit, or are you just fooling around to learn about circuits with transistors?? It feels like it is pointless trying to answer a question like that, when there is no particular reason to hand-hold someone through all the calculations that they are not willing to do, or incapable of doing.
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Andy
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