the hfe in bjt transistor

[baby_1]

according my refrences the hfe is nott constant and its so varius with collector current,but is it so unkown for me what is varius?

 

[Audioguru]

The datasheet for every transistor part number gives a range for hFE. It also shows the hFE changing with collector current and temperature.
You should design a circuit with enough negative feedback so that it works when the hFE of the transistor is minimum or maximum and also works with the minimum and maximum collector current of your circuit and at the expected temperature range.

 

[baby_1]

thanks but i wanta know

how collector current has effected the hfe varius?

 

[Audioguru]

When the collector current is low then the minimum hFE is low but the typical hFE shown on a graph is the same as at higher currents.
When the collector current is high then the minimum, typical and maximum hFE is lower than at lower currents.

hfe is AC current gain. hFE is DC current gain.

 

[baby_1]

thanks but i want the reson?

 

[Audioguru]

I don't make transistors, I use them.
When a datasheet for a transistor says that its hFE is from 100 to 300 then I design a circuit for it that works when the hFE of a transistor is anywhere within that range.

 

[varuniyengar]

beta for BJT is 100

 

[Audioguru]

beta for BJT is 100

You cannot buy a transistor with a beta of 100. You can buy a few hundred transistors and measure them all then you might find one.
The beta is a range of numbers because they cannot make transistors accurately. Also the beta changes when the collector current is changed and the beta changes when the temperature changes.

Look at the datasheet for a 2N3904 transistor. At 25 degrees C the beta at a collector current of 10mA is 100 minimum to 300 maximum.
At a collector current of 0.1mA the beta has a minimum of 40 and at a collector current of 100mA the beta has a minimum of 30.
A graph shows that the typical beta is higher when the temperature is higher and the beta is lower when the temperature is lower. The graph also shows that the typical beta does not drop when the collector current is low but the beta drops a lot when the collector current is high.

 

[varuniyengar]

Audioguru

Yeah right i said in ideal condition beta is 100

 

[Audioguru]

"Ideal conditions" are used in dreams and in school.
You should learn how to bias a transistor aand use negative feedback so that the hFE, temperature, collector current and Vbe differences in transistors do not affect the performance of the circuit.

 

[varuniyengar]

Yeah rightly said Audioguru

 

[saikat36]

Simple relation Ic=hfe*Ib where Ib=(Vcc-0.7V)/Rb

Hope it will help.

 

[Audioguru]

Simple relation Ic=hfe*Ib where Ib=(Vcc-0.7V)/Rb

It is too simple:
1) A saturated transistor has a very low hFE of about 10 to 20.
2) Sometimes the hFE is low at low collector currents. The hFE low at high collector currents.
3) hFE is low at low temperatures and is high at high temperatures.
4) The hFE of each transistor is different even if they have the same part number so the hFE is a range of numbers unless you measure the hFE of your transistor.

 

[BJTMOSGAAS]

Hello baby_1,
the answer normally takes a few pages in textbooks (e.g. Ashburn).

A summary, so that you can look up the correct pages:

- At medium currents (what is medium depends on the transistor active area) hFE (DC current gain) is a roughly constant and given just by construction parameters like the widths and doping concentrations of emitter and base
- At high IC hFE decreases due to high injection (concentration of injected electrons in the base of the pnp comparable to the hole doping concentration in the base) and base widening (charge density of electrons contributing to IC comparable to collector doping): both effects decrease IC but do not affect IB, therefore hFE declines.
- At low currents, say IC in uA range, IB is even smaller, say some nA, and then kind of leakage current which is always present and generated in the BE junction is not overshadowed any more, but becomes relevant for the total IB. If you force IB in the hFE measurement you can imagine that first the leakage current needs to be compensated and then the IB needs to be provided for the IC according the transfer current, therefore hFE declines.

There is a correlation between AC current gain hfe and DC current gain hFE, so that IC dependence of hFE translates into same dependence of hfe.

Rgds

 

[azim.cy]

Sir. How to find the hfe in bjt transistor using Multimeter? Help me....

 

[goldsmith]

Dear baby_1
Hi
A short comment :
I think you are referring to the beta . of course it is not constant and it is variable according to the quiescent point of your amplifier . it depends on VEC and IB , when these parameters have changes then the beta has variations . this consideration needed in design steps , suppose that we want design a simple CE amplifier we know for class A , VEC= VCC/2 , then we have vce , and of course simply we have IC , and then according to the curves that derived from curve tracer or tables of datasheet , we can find IB and thus the beta will be known .
Best Wishes
Goldsmith

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Sir. How to find the hfe in bjt transistor using Multimeter? Help me....

Dear azim
Hi
Most of the multimeters havea connector for transistors , it hase some pins , you should put your transistor into it and then set the selector in HFE , mode , thus the LCD will show the HFE .
Best Wishes
Goldsmith

 

[azim.cy]

goldsmith sir. Thank you.......

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goldsmith sir. Thank you.......

 

[goldsmith]

Hi all , again !
A best friend of mine have noticed me to mention a thing : Beta of transistor in a little region is not constant and it depends on VCE ( when VCE increased we will need lower ib to have constant IC . ) . but in a transistor there is a region that beta is constant in that . on the other hands it depends on which quiescent point is your desired one ! at some of them you should effect this issue in your designs !
Let me give you an example , suppose that in an special region of operation we need 80 ma current as IC . and VCE is 5 volts , ( i.e it is an example ) and required ib is 10uA . and when we need 80 ma as IC and VCE is 2v we will need 20uA as ib .
But this issue will happen in some regions of operation ( it will happen in all of the regions of operation but sometimes this effect is insignificant and sometimes it is pretty important )

Best Wishes
Goldsmith

 

[Audioguru]

The beta of transistors having the same part number is not the same. Each transistor is different sometimes over a wide range (see the datasheet).
The beta of a transistor changes when the temperature changes.
As you say, the beta changes when Vce changes.

A transistor usually has negative feedback applied so that it is biased correctly. Negative feedback can be a biasing resistor from the collector to the base or a series emitter resistor.

 

[LvW]

A transistor usually has negative feedback applied so that it is biased correctly. Negative feedback can be a biasing resistor from the collector to the base or a series emitter resistor.

Does negative feedback ensure correct biasing? I don't think so.
For my opinion it should read instead:
Negative feedback can drastically reduce uncertainties in biasing caused by tolerances (hfe, resistors) as well as change of environmental conditions (temperature) .