Please help me with the following assignment question. A certain 2N3904 dc basis circuit with the following values is in saturation. Ib = 500 uA Vcc = 10V and Rc = 180 ohm and hfe = 150. If you increase Vcc to 15V, does the transistor come out of saturation?

My attempt at a solution:

Ic (sat) = (Vcc - Vce (sat))/Rc but Vce (sat) then work out whethere Ib is capable of producing Ic (sat) but Vce (sat) isn't given. It's only a two mark question. Please help.

 

If Vce(sat) isn't given, then what do you think a reasonable value for it would be?

 

If Vce(sat) isn't given, then what do you think a reasonable value for it would be?

0.2v but, there's a question that follows this one which asks me to find Vce.

 

Welcome to AAC! And thank you for showing your solution.

How is the base current generated? What causes the transistor to become saturated?

When discussing circuits, schematics are appreciated.

 

Welcome to AAC! And thank you for showing your solution.

How is the base current generated? What causes the transistor to become saturated?

When discussing circuits, schematics are appreciated.

There's no schematic on the question paper, just the same statement I quoted. I think it's safe to assume that the Base has its own Vbb dc supply. According to my knowledge, if BETA*Ib > Ic (max) then the transistor is in saturation(basically when the transistor is used as a switch with Vce being at its lowest value).

 

According to my knowledge, if BETA*Ib > Ic (max) then the transistor is in saturation(basically when the transistor is used as a switch with Vce being at its lowest value).

Under what conditions is beta 150?

From the datasheet:

 

Under what conditions is beta 150?

From the datasheet:
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When Ic = 10mA and Vce = 1V, according to this then, the transistor should not be in saturation which is contrary to what the question says.

 

When Ic = 10mA and Vce = 1V, according to this then, the transistor should not be in saturation which is contrary to what the question says.

I'm not sure what you were taught, but documented beta values don't apply for saturation (unless they're stated for saturation).

What happens to beta when Ic changes? Does beta remain constant? If you've studied IV curves for transistors, what happens to beta when Vce decreases?

 

Beta varies with Ic and Vce, that I do know but like you said, when the transistor operates in saturation, beta values aren't very useful. I'm still a lil confused as to how I can prove that an increase in Vcc will push to you the active region or keep it on saturation.

 

I'm still a lil confused as to how I can prove that an increase in Vcc will push to you the active region or keep it on saturation.

What have you been taught? Any guidelines regarding Ib? Do you have device graphs to consult?

 

When Ic = 10mA and Vce = 1V, according to this then, the transistor should not be in saturation which is contrary to what the question says.

You aren't reading the data sheet properly. Note that the test conditions for the Ic = 10 mA and Vce = 1 V says nothing about the base current, only that the DC current gain is somewhere between 100 and 300, making the base current somewhere between and 33 μA and 100 μA. At 500 μA the gain is only about 20, which puts it effectively into saturation; there is no hard demarcation between saturation and non-saturation -- for most data sheets and designs a criterion of a gain of 10 or less is often used, but for some transistors (power transistors, most commonly) the current gain in normal operation may not be too much higher than ten and a lower saturation gain is then frequently used.

 

Beta varies with Ic and Vce, that I do know but like you said, when the transistor operates in saturation, beta values aren't very useful. I'm still a lil confused as to how I can prove that an increase in Vcc will push to you the active region or keep it on saturation.

Assume it is in saturation at a reasonable saturation voltage. Find what the gain is. Is that gain significantly above the gain that it is showing under the conditions in which it is stated that it is in saturation, particularly in comparison to what they would be at the given non-saturated gain?

 

If the transistor is in saturation, what would be the maximum Ic at Vcc = 10V?
What would be the maximum Ic at Vcc = 15V?

 

Please help me with the following assignment question.

I did some calculations and think I know what you're expected to do. Can you post the exact wording of the problem?

 

I did some calculations and think I know what you're expected to do. Can you post the exact wording of the problem?

A certain 2N3904 dc basis circuit with the following values is in saturation. Ib = 500 uA Vcc = 10V and Rc = 180 ohm and hfe = 150.

1.If you increase Vcc to 15V, does the transistor come out of saturation?

2. What is Vce at this Vcc?

 

A certain 2N3904 dc basis circuit with the following values is in saturation. Ib = 500 uA Vcc = 10V and Rc = 180 ohm and hfe = 150.

1.If you increase Vcc to 15V, does the transistor come out of saturation?

2. What is Vce at this Vcc?

We can't help you with hypothetical scenarios without knowing all of the assumptions being made. I looked at a few datasheets and they both assumed a beta of 10 for saturation mode for 2N3904 and PN2222. Two from Onsemi and one from Fairchild.

Your problem appears to be assuming a beta of 150 when the transistor is saturated, but no 2N3904 will behave that way. When Vce is small, beta drops off severely.

For my information, what is the name of your school?

 

We can't help you with hypothetical scenarios without knowing all of the assumptions being made. I looked at a few datasheets and they both assumed a beta of 10 for saturation mode for 2N3904 and PN2222. Two from Onsemi and one from Fairchild.

For my information, what is the name of your school?

Unisa (University of South Africa)

 

1.If you increase Vcc to 15V, does the transistor come out of saturation?

Yes, it will come out of a saturation.

What is Vce at this Vcc?

Do the calculation, simply by using your given beta value and KVL.
https://www.physicsforums.com/threa...-gain-in-saturation-mode.865392/#post-5432159
http://electronics.stackexchange.co...an-npn-bjt-in-saturation-region/276266#276266

 

A certain 2N3904 dc basis circuit with the following values is in saturation. Ib = 500 uA Vcc = 10V and Rc = 180 ohm and hfe = 150.

1.If you increase Vcc to 15V, does the transistor come out of saturation?

2. What is Vce at this Vcc?

I have to wonder if they are expecting you to guess at a circuit topology in which these values make sense. The obvious circuit that comes to mind is a transistor with the emitter tied to ground, a current source providing the base current, and then just the transistor and the collector resistor. But is this consistent with the problem?

Work it from two directions. First, assume that "saturation" means that beta is ten. What does that mean the collector current is? What does that mean that the collector voltage is? Is that in any way consistent with a transistor in saturation? Now assume that "saturation" means that the Vce is very low and compute what the collector current is. What does that mean that beta is? Is that in any way consistent with a transistor in saturation? If neither is (and if one is then the other better be, too) then either the problem is completely messed up, or this is the wrong circuit topology.

So what other topology might make sense? Well, this is now in the realm of crystal ball work. At the risk of perhaps giving too much away, the first thing that comes to mind would be to add an emitter resistor and figure out what it has to be in order to be consistent with the stated problem conditions. But that is only a guess.

 

Yes, it will come out of a saturation.

What circuit topology are you assuming?