vindicate,



I mean the later, the external resistors Re and Rc.



You need Ie for Re and Ic for Rc. How you get Ie and Ic is up to you. Ie and Ic are usually about the same. If you know Hfe is high enough, then you can estimate Ie from the feedback principle. "Higher than Vcc" for saturation to occur, yes?



What?

Ratch

I guess this is what I'm trying to say. If you wanted to design a circuit. And you wanted a saturated transistor. To determine how much base current to give the transistor for it to be saturated you would need to know HFE would you not? How else would you know how much current is going to be running through the Collector and Emitter?

You cannot figure the saturation voltage loss because each transistor is different. Simply feed it the amount of base current that is shown on the datasheet (1/10th of the collector current for most little transistors) and a good one will have a typical voltage loss and a poor but passing one will have the spec'd max voltage loss.

Audioguru, where do you get that 1/10th current spec on the datasheet?

 

Audioguru, where do you get that 1/10th current spec on the datasheet?

All datasheets for transistors have a spec for Maximum Collector-Emitter Saturation Voltage. It is shown for most small transistors with the base current 1/10th or 1/20th of the collector current. HFE is used only when the transistor has plenty of collector to emitter voltage (10V on most datasheets) so it is not saturated.

 

All datasheets for transistors have a spec for Maximum Collector-Emitter Saturation Voltage. It is shown for most small transistors with the base current 1/10th or 1/20th of the collector current. HFE is used only when the transistor has plenty of collector to emitter voltage (10V on most datasheets) so it is not saturated.

Where do you see that on this datasheet?
http://www.fairchildsemi.com/ds/PN/PN2222A.pdf

The reason I refer to the PN2222A is because that is the transistor I have.

 

vindicate,

I guess this is what I'm trying to say. If you wanted to design a circuit. And you wanted a saturated transistor. To determine how much base current to give the transistor for it to be saturated you would need to know HFE would you not? How else would you know how much current is going to be running through the Collector and Emitter?

Yes, if you wanted to saturate the transistor using the least amount of Ib, you could calculate it using the minimum Hfe expected from the spec sheets, once you know the Ic needed. But no matter what the Hfe, the Vsat voltage itself is not dependent on that parameter. Hfe only affects the minimum Ib nessessary to enter saturation region. Using 1/10 of Ic for Ib usually gives a generous oversupply, thereby insuring saturation.

Ratch

 

vindicate,



Yes, if you wanted to saturate the transistor using the least amount of Ib, you could calculate it using the minimum Hfe expected from the spec sheets, once you know the Ic needed. But no matter what the Hfe, the Vsat voltage itself is not dependent on that parameter. Hfe only affects the minimum Ib nessessary to enter saturation region. Using 1/10 of Ic for Ib usually gives a generous oversupply, thereby insuring saturation.

Ratch

Alright, thats kinda what i was thinking, and It makes sense to me now. Thanks .

Audioguru was saything that that 1/10th was listed in the datasheet, but I don't see it anywhere in there. Where do I find that?

 

vindicate,

Audioguru was saything that that 1/10th was listed in the datasheet, but I don't see it anywhere in there. Where do I find that?

I will let the guru answer that.

Ratch

 

This is a correct version of Fairchild's PN2222A datasheet, the same as all other manufacturers' datasheets.
The 2009 datasheet with the error must have been made by little school kid NOOBs.

Notice that the Vce is not listed for the saturation voltage spec's.
Notice that the base current is 1/10th the collector current in the saturation voltage spec's.

 

I tried saturating the 2N3904 transistor. Below, you'll find an analysis of my design.

As you can see, I'm not sure about Ie but I went ahead and made it the same current as Ic. Therefore, Re = 230Ω. I am just guessing for this but I thought it might make the transistor more stable; or could I have it the same current as the base?

So this is supposed to act like a switch, right? Then where is the ternimals for this, just like the actual ternimals in a regular switch?

Thanks!

 

The 2N3906 is a PNP transistor. you show an NPN transistor.

When a transistor is used like a switch then it does not have an emitter resistor. The emitter is connected to ground so that the collector can saturate to near ground when the transistor is turned on.

Nearly all datasheets show a saturated transistor with the base current 1/10th the collector current. hFE is not used in the calculation of current for a saturated transistor. hFE is used when the transistor is linear and is not saturated because it has plenty of collector to emitter voltage.

 

Some time we uses this equation:

\(Rb\ \le \ \frac{Vin-Vbe}{Vcc-Vcsat}*\beta*Rc\)
where:
Vin - minimum input voltage
Vcc - maximum supply voltage
Rc - minimum load resistance
β - minimum value of a current gain (Hfe)

For safety we pick Rb from 3 to 5 times less than the calculated value.
​

 

The 2N3906 is a PNP transistor. you show an NPN transistor.

When a transistor is used like a switch then it does not have an emitter resistor. The emitter is connected to ground so that the collector can saturate to near ground when the transistor is turned on.

Nearly all datasheets show a saturated transistor with the base current 1/10th the collector current. hFE is not used in the calculation of current for a saturated transistor. hFE is used when the transistor is linear and is not saturated because it has plenty of collector to emitter voltage.

I fixed it, it is a 2N3904. Thanks!