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Hello ,I have build NPN which base is being? biased with a current source ,the? problem that My collector current is very low ?no matter the current i put in the base as shown in the photo and simulation.
Why i cant see the Ic/Ib=beta? ratio?
Thanks.

/g/electronics101/files/john233/npn_circuit.zip

On Mon, Nov 11, 2024 at 01:38 PM, john23 wrote:

Why i cant see the Ic/Ib=beta? ratio?

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Because your NPN transistor is in saturation.? Saturation must be avoided if you want current gain.

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Plot Vce.? It is less than 30 mV throughout your sweep.? It is saturated.? In fact, the collector voltage is lower than the base voltage and the collector-base junction has become forward-biased.? Under those conditions, your transistor is not an amplifier.? Current goes backwards out of the collector.

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Now repeat the simulation using this sweep command:

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.dc dec I1 1n 100u 10

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Make sure to delete the plot window, then re-run the simulation, to make sure that the X-axis in the plot is logarithmic.

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When the base current from U1 is smaller than about 10 uA, the transistor is not in saturation, and Ic/Ib is reasonable.? Even when I1 is as high as 20 uA, Ic/Ib is almost 200, even though Vce falls to 0.65 V by that point so it is very nearly (not quite) in saturation.? But above 20 uA, the transistor saturates and Ic/Ib takes a nosedive.

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You can plot Ic/Ib directly.? Just edit the label at the top of the plot pane so that it is "Ic(Q1)/Ib(Q1)".

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Andy

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I wrote:

When the base current from U1 is smaller than about 10 uA, ...

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Sorry, that should be I1, not U1.

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Andy

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With 1K in the emitter, the most Ic you will see with Vcc of 5 volts is 5ma. Take the transistor out and put in shorts and see what currents will be in each path. Do some basic current flow.

Hello,the purpose of biasing NPN threw current source is to calculate the limiting resistor needed for supplying the base of the NPN.
Why in saturation Ic/Ib is ver very small?
Thanks.

I think you meant Collector-Emitter saturation voltage is around 0.2V, correct?? The Base-Emitter voltage is typically 0.6V.

On Tue, Nov 12, 2024 at 04:04 PM, john23 wrote:

Why in saturation Ic/Ib is ver very small?

Because you can't get any more current than that.? When it is in saturation, it can't turn on any harder and that is all the collector current that is possible.? It is basically limited by the rest of the circuit around the transistor.

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In your case, you subjected the NPN to a base current from 1 mA to 20 mA.? If the transistor's Beta was 200, then the collector current would have to be between 200 mA and 4 Amps.? It is physically impossible.? The absolute maximum collector current in your circuit would be 5/(1K+100) = 4.5 mA, which is MUCH smaller than either 200 mA or 4000 mA.

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In order to get Ic = 4000 mA, the collector voltage would have to be pulled down to -395 volts!? But there is no negative supply voltage in your circuit, so that is just plain impossible.? Transistors can not generate voltages from nothing.

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Andy

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By the way, the 1K resistor R1 does nothing useful.? Because you drove the Base with a current source, the resistor R1 does not change that in any way.

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Andy

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On Wed, Nov 13, 2024 at 10:08 AM, wn4isx wrote:

On Tue, Nov 12, 2024 at 08:45 PM, Andy wrote:

In order to get Ic = 4000 mA, the collector voltage would have to be pulled down to -395 volts!

Color me confused,

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Look at the circuit external to the NPN.

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In order to have 4 Amps of current into the NPN's collector pin, its collector voltage would need to be at +5volts - (4amps * 100ohms) = -395volts.? That is the only way to get 4000 mA through the resistor and into the collector.? That only works if the other stuff, connected to the NPN's base and emitter pins, have some different voltages on them.

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If you managed to do that, and if nothing goes up in smoke, then yes, you could have Ic = 4000 mA.? And if the 2N2222's Beta is still around 200 at that level - which it probably is not - and if the 2N2222 is even capable of sinking 4000 mA - which its datasheet says it is not - then you would see Ic/Ib where you thought it might be.? But there are rather significant IF's in that situation and it simply would not work even under those conditions.? Indeed the hfe drops significantly by the time you reach only 500 mA.? By 4000 mA it has probably disappeared.

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Putting it another way, the emitter and base voltages would need to be somewhere around -400 V or more negative.? And then you still get V(c) > V(e) and V(c) > V(b), just like normal NPN operation.

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All normal NPN circuits I've seen have the collector tied to V+ either through a resistor, inductor or transistor, or directly for an emitter follower.

Then I think you have not seen that many NPN circuits.? (No, I'm only joking.? I am sure you have seen many others, which have voltages all over the place.? Everything is relative.)

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Andy

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Hello Andy, I biased my NPN to design the limiting resistor.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 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?

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?
Thanks.

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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.

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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On Wed, Nov 13, 2024 at 02:53 PM, john23 wrote:

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?

Here is one approach to answering that question:

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(*) But there are two more things to consider, and they can affect the calculations:

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Andy

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Hello Andy,When I changed current to 3V voltage source?

given Ib=11.723618?A Ic=2.4mA in linear state , I got by the calculation below that Rb=8.5k

Why do you say is not a limiting current resistor?I current limiting resistor is a common name.

If i increase the Rb Ic will be smaller.

3-R*11.723618?A=0+1k*2.4mA+0.7
3-R*11.723618*10^-6=0+2.4+0.7

/g/electronics101/photo/296358/3856221?p=Created%2C%2C%2C20%2C2%2C0%2C0

On Sun, Nov 17, 2024 at 03:05 PM, john23 wrote:

Why do you say is not a limiting current resistor?

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If i increase the Rb Ic will be smaller.

That happens after you replace current source I1 by a voltage source.

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Andy

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