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Transistor dc gain ?

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tron87

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On a transistor data sheet how do you know which min hFe value to use for example for the 2n2222a.

DC Current Gain
(IC = 0.1 mAdc, VCE = 10 Vdc)
(IC = 1.0 mAdc, VCE = 10 Vdc)
(IC = 10 mAdc, VCE = 10 Vdc)
(IC = 10 mAdc, VCE = 10 Vdc, TA = –55°C)
(IC = 150 mAdc, VCE = 10 Vdc)(1)
(IC = 150 mAdc, VCE = 1.0 Vdc)(1)
(IC = 500 mAdc, VCE = 10 Vdc)(1)

hFE
35
50
75
35
100
50
40


If my Ic was going to be 50ma then which hFe value would i use ?

If the voltage supplied to the transistor is less than 10 vdc say 5 vdc insteed that obviously means that vce can not be 10v so how do you choose the hFe from data sheet in this case if the characteristics are based on vce of 10 vdc ?
 
hi,
It depends upon the application, a good starting point are the graphs for the transistor.
 

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Normally you design a circuit to be somewhat insensitive to the actual Hfe since it typically has a variation of 3:1 or more from unit to unit. This is often done with an emitter resistor. This stabilizes both the bias point and the AC gain (if the emitter resistor is not bypassed) of the stage.
 
For the saturation collector region how do I find the expected vce for ib .5 ma and ic 50 ma if they only have 1 ma,10 ma,150 ma,500 ma IC values ?

Also how do i find the hFe value on the dc current gain graph for ic of 50ma and if vce is not 1v or 10v ?
 
For the saturation collector region how do I find the expected vce for ib .5 ma and ic 50 ma if they only have 1 ma,10 ma,150 ma,500 ma IC values ?

Also how do i find the hFe value on the dc current gain graph for ic of 50ma and if vce is not 1v or 10v ?

You have to interpolate from the graph,, at 25C for 50mA the gain is close to 250 for 1Vce and 10Vce.

If you have Ib=.5mA and Ic=50mA then the MINIMUM gain would have to be 100, so 250 will be more that adequate to saturate the transistor
 
Datasheets normally have a different set of specifications for the saturation region. Vce(sat) is normally specified for Ic/Ib=10, or Ic/Ib=20, depending on the part number. The beta specs for non-saturated Vce values are basically irrelevant for saturation.
 
Don't use the graph because it is only for a "typical" transistor that you cannot buy and might not have.
If you design a circuit for a "typical" transistor then it won't work when the hFE of the transistor is lower than "typical".
But if you design the circuit properly for a low hFE transistor then any passing transistor will work in the circuit.
 
When working with dc only on transistors most of time is hFe mainly acquired just to calculate required base current to ensure saturation and not operation in the active region for dc?
 
When working with dc only on transistors most of time is hFe mainly acquired just to calculate required base current to ensure saturation and not operation in the active region for dc?

Use the hFE when using transistors as switch in order to saturate the transistor.

With ref hfe , small signal gain for say amplifier use, I would still recommend that you use the typical gain values.
When you get more experience you can be more selective on whats the best choice for the circuits you design
 
Would you say that dc only transistor configurations mainly are used to operate as a switch(saturated) and not in the linear active region ?

Can a transistor using only dc and fully saturated be classified in some ways as a dc amplifier because of the difference between input and output current or is the term amplifier only applied when its in the linear active region?
 
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Would you say that dc only transistor configurations mainly are used to operate as a switch(saturated) and not in the linear active region ?

Can a transistor using only dc and fully saturated be classified in some ways as a dc amplifier because of the difference between input and output current or is the term amplifier only applied when its in the linear active region?

Hi,
Look thru some of these links, lots of info.

**broken link removed**
 
Tron and Eric are both wrong.
HFE is used for a linear amplifying transistor that is not saturated because it has plenty of Vce voltage.
The datasheet for most transistors shows that the base current must be 1/10th the collector current for the transistor to saturate with the max Vce saturation voltage that is spec'd. A few BCxxx high gain European transistors (example is BC549) have their max Vce saturation voltage spec'd when the base current is 1/20th the collector current even when their minimum hFE (hFE is spec'd when the Vce is 5V) is 420.
 
Tron and Eric are both wrong.
HFE is used for a linear amplifying transistor that is not saturated because it has plenty of Vce voltage.
The datasheet for most transistors shows that the base current must be 1/10th the collector current for the transistor to saturate with the max Vce saturation voltage that is spec'd. A few BCxxx high gain European transistors (example is BC549) have their max Vce saturation voltage spec'd when the base current is 1/20th the collector current even when their minimum hFE (hFE is spec'd when the Vce is 5V) is 420.

I dont agree, hFE as I quoted is used for high current applications, so why do say that my post is wrong.???

Use the hFE when using transistors as switch in order to saturate the transistor.

With ref hfe , small signal gain for say amplifier use, I would still recommend that you use the typical gain values.
When you get more experience you can be more selective on whats the best choice for the circuits you design
 

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I dont agree, hFE as I quoted is used for high current applications, so why do say that my post is wrong.???
Also look at post #6 that agrees with me. hFE is not used for a transistor that is a saturated switch.

In the datasheet you posted showing the hFE, the transistor has a Vce of 10V so it is not saturated, instead it is a linear amplifier.
 
Also look at post #6 that agrees with me. hFE is not used for a transistor that is a saturated switch.

In the datasheet you posted showing the hFE, the transistor has a Vce of 10V so it is not saturated, instead it is a linear amplifier.

The transistor datasheets dont support your argument.!
 
The transistor datasheets dont support your argument.!
But they do. hFE is used for bias purposes, which is why it is given at a Vce of 10V or 1V. If it were for turning on a transistor as a switch then it would be given at a low Vce voltage of perhaps 0.1V.

As Uncle $crooge noted, you normally use an hFE of 10 or 20 to insure saturation of a switched transistor.
 
But they do. hFE is used for bias purposes, which is why it is given at a Vce of 10V or 1V. If it were for turning on a transistor as a switch then it would be given at a low Vce voltage of perhaps 0.1V.

As Uncle $crooge noted, you normally use an hFE of 10 or 20 to insure saturation of a switched transistor.

hi Carl,
I am not talking about ensuring the saturation point, I am fully conversant with required 10% bias current overdrive for adequate VCsat.

The point I am making is in line with datasheet. ie: hfe is for small signal gain and hFE is for high signal [ current ] gain.

Are you saying that is wrong.????

The problem with 'agu's post is the generalisation that my post was wrong, which if he took the trouble to read it properly he would understood what I was saying.

Regards
 
hi Carl,
I am not talking about ensuring the saturation point, I am fully conversant with required 10% bias current overdrive for adequate VCsat.

The point I am making is in line with datasheet. ie: hfe is for small signal gain and hFE is for high signal [ current ] gain.

Are you saying that is wrong.????
No, that is correct.

But your post #9 in apparent contradiction says: "Use the hFE when using transistors as switch in order to saturate the transistor." which was the point of my comment.
 
No, that is correct.

But your post #9 in apparent contradiction says: "Use the hFE when using transistors as switch in order to saturate the transistor." which was the point of my comment.

hi Carl
I think we are talking at cross purposes here.
Can you define what you mean by hFE.??
 
Hi Eric,

You appeared to refer to hFE for use in both the transistor linear large signal and saturated conditions, which are not the same. But there seems to be confusion by many in this forum on that point.

The definition of hFE (or sometimes Hfe) I use is that it is the high current (large signal) DC current gain when used in applications such as linear voltage regulators or power amplifiers. It is not directly used for calculation of the base current required to saturate the transistor in switching applications.
 
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