hfe

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Electronman

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Hi,

I have a question regarding to The current gain of a bipolar transistor,
Current gain or hfe is the ratio of collector current to base current.
Now I want to see when I design a circuit by a transistor like BC108 then is hfe constant so that I could determine the Ic by multiplying the Ib by hfe or hfe is determined by the ratio of Ic and Ib?
Hope my question makes sense to you guys
P.s: my examination by simulator proteus showed that hfe changes by changes of Ib so??
 
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But please wait, If hfe does not remain constant then we would not be able to have Amplifiers made by transistors???!
So It seems to me that hfe has to be constant for different Ib's to have Ic's Proportionate to Ib's? ???!
Sorry but I am getting confused??!!!
besides I am talking about common emitter configuration.
 
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hfe is seldom constant. You need to look at the characteristic curve. Generally there will be a linear section & a non linear section & depending on the bias you can adjust the part of the curve it operates on. This is GENERALLY speaking.
 
you can say hfe is a constant in a range of voltages and temperature. component values on a particular configuration and hfe all together will reflect in the gain of the transistor stage.

when the transistor is operated in the linear region you always will get Ic/Ib nearly equal to hfe.
 
When I design a transistor amplifier I use the minimum hfe for the quiescent current of the circuit. That way any of the same transistor will always work.
 
The parameter "hfe" is hard to control. It varies with value of collector current, with temperature, and with speciman. Even if bjt devices are selected to tight tolerances, the temperature variation can be substantial. If hfe = 160 at room temp, 25 C, then it can drop to 100 value at low temp, -40 C, and increase to 300 at high temp of 100 C.

Generally for operation in the active region (linear amplification), emitter resistance is used to provide degenerating feedback. This nulls out variations in stage gain due to beta variation (beta is "hfe").

The same is true with gm, r_pi, etc. By using a good bias network, the amp stage performance varies very little with speciman, supply rails, and temperature. Have I helped?
 
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Look at the datasheet for a BC108 (it is extremely old) transistor. Some have an hFE of only 110, most have an hFE of 350 and some have an hFE of 800.

Transistor circuits are designed so that they work with any transistor, not just a transistor selected for a certain hFE.

An emitter resistor helps because it provides negative feedback.
A voltage divider with two resistors to feed the base also is much better than a single resistor from the positive supply to the base.
 

Ok, so I'll show you my problem with an example,
Here's it is:
I want to drive a load at collector of a BC108 (the load happens to be a resistor or a led).
We know that the said LED needs 10mA to safely be lighten; it has an operating voltage of 2V in our example. So we know that Ic has to be 10mA and we have to have 10V drop on emitter-collector of BC108.
As you mentioned every transistor has a different hfe even if they are the same.
Now please tell me how I determine the Ib while The hfe is not constant for a specific transistor?
Makes sense?
 

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Electronman said:
Ok, so I'll show you my problem ....
Click to expand...
You want a switching transistor to be saturated.
hFE is not used when a transistor is saturated, hFE is used for an linear transistor amplifier that has plenty of collector voltage.
The datasheets for all transistors show that they saturate pretty well when their base current is 1/10th the collector current.

I changed your schematic so that the base current is 1mA and the collector and LED current is limited to 10mA by a 1k resistor.

You could also change the transistor circuit into a more complicated 10mA current source.
 

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Thanks a bunch genius audioguru!
Now it makes some what sense,
But please tell me why you could put an exact 1k Resistor on collector while there is a led in series with it and the led has a resistance which adds with 1K resistor so we do not follow 1/10th law??

Please tell me if I could use a Pot instead of R1 and change its value so that I get 10mA at collector for the led too?

And please tell me if I could use any resistor lower than 12k for base yet remain at Saturation situation?

Thanks
 
Electronman said:
But please tell me why you could put an exact 1k Resistor on collector while there is a led in series with it and the led has a resistance which adds with 1K resistor so we do not follow 1/10th law??
Click to expand...
Are you a Noobie who knows nothoing about electronics?

The power supply is 12V. The LED is 2V. The transistor is saturated (turned on hard) with a voltage drop of only 0.05V.The current for the LED is 10mA.
Duh! Ohms law says that the current-limiting resistor for the LED must be (12V - 2V)/10mA= 1000 Ohms.

Please tell me if I could use a Pot instead of R1 and change its value so that I get 10mA at collector for the led too?
Click to expand...
The hFE of every transistor is different and it changes when the temperature changes. Are you going to match a pot to a transistor then change the pot each time the temperature changes?

Why not use a known resistor to limit the current instead of an unknown amount of current from a partially turned-on transistor?

And please tell me if I could use any resistor lower than 12k for base yet remain at Saturation situation?
Click to expand...
Yes, of course. But it would simply waste power. Use 10k.
A little transistor is completely saturated when its base current is 1/10th or more of the collector current.
 
Thanks for your comments,

Well I want to know what if I replace the led with a small 3V motor (those CORED not coreless which are used in mobile phones as vibrators) I just want one speed (full throttle) so do I have put the transistor in saturation to do so too???!
 
when you put the transistor in saturation it is acting like a switch, up to the point of saturation it acts in accordance to its characteristic curve depending on its bias
 
yes you can use, make sure you chose a propper tanssitor with rating. also dont use any resistor in between. as used for LED.
 
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mbarazeen said:
yes you can use, make sure you chose a propper tanssitor with rating. also dont use any resistor in between. as used for LED.
Click to expand...

Thanks,
So do I need to take any care to the Inductance of my motor (it is a very small motor)??? Or I can consider it like a real resistance (measure its resistance by an ohm meter and then consider just its resistance parameters on BJT?)
 
Your transistor might be a typical one as shown in the datasheet.
But yours has a base current that is only 1/88th of the collector current with a saturation voltage of 390mV, and the one in the datasheet has a base current that is 1/20th of the collector current and a saturation voltage of only 70mV which is much better.

Your circuit probably will not work with many BC108 transistors that have less than typical gain.
 
If you are running a motor I believe a fly-back diode should be used so that any EMF is happily run back to the positive instead of possibly messing with what is controlling your transistor.
 

I am wondering why my transistor shows just 0.30V at CE while the base resistor is 100K ie collector resistor is 1/100 base resistor?
 
Birdman Adam said:
If you are running a motor I believe a fly-back diode should be used so that any EMF is happily run back to the positive instead of possibly messing with what is controlling your transistor.
Click to expand...

I am wondering if I want to use a motor how do I consider it? A real resistor or...?

Besides is flyback diode equal to free wheeling diode?
 
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