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  #1  
Old 08-29-2012, 07:12 PM
rougie rougie is offline
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Default 2N2222... hfe?

Hello,

I am getting into some basic transistor calculations.

I am using a 2n2222 transistor and I have done the following calculations as shown in the attachment. However, as shown in the 2N2222 spec sheet attachment it says that the HFE should be determined by the corresponding ic calculations.

If you look at my calculations, my ic is approximately 1.3 ma. According to the spec, shouldn't my HFE be somewhere around 60??

However, calculations with a HFE of 60 don't make sence with the actual physical readings? But HFE of 100 is much closer to the values I measure.

So, in my case how is one supposed to determine the HFE for a given transistor such as the 2N2222?

Thanks for all help!
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File Type: jpg T1_HFE.jpg (64.6 KB, 25 views)
File Type: jpg T1_HFE_SPEC.jpg (185.4 KB, 17 views)
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Old 08-29-2012, 07:17 PM
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Bill_Marsden Bill_Marsden is offline
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BJTs are wildly variable, even within the same lot. Each transistor has its own gain values, you design to allow for the variation. It has been like this pretty much from day on for BJT parts.
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Old 08-29-2012, 07:18 PM
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The HFE of any particular device isn't set in stone, it varies from one transistor to another of the same type. Looking at the datasheet here HFE can be anywhere between 35 and 300 for the 2N2222.
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Old 08-29-2012, 07:56 PM
rougie rougie is offline
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>>>The HFE of any particular device isn't set in stone, it varies from one transistor to another of the same type. Looking at the datasheet here HFE can be anywhere between 35 and 300 for the 2N2222.

Oh! I see what you are saying.... It is up to me to pick the HFE allowed by the part which would best suite the gain I need!!!

Thanks all for your help!
regards
r
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Old 08-29-2012, 08:06 PM
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crutschow crutschow is online now
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Quote:
Originally Posted by rougie View Post
>>>The HFE of any particular device isn't set in stone, it varies from one transistor to another of the same type. Looking at the datasheet here HFE can be anywhere between 35 and 300 for the 2N2222.

Oh! I see what you are saying.... It is up to me to pick the HFE allowed by the part which would best suite the gain I need!!!
More specifically, you design the circuit so that it will operate within the range of Hfe that the transistor may have. Typically this is done by using negative feedback, such as from an emitter resistor, so that the minimum gain the stage has is less than minimum Hfe.
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Old 08-29-2012, 09:26 PM
rougie rougie is offline
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now that i think about it ... how can this be?

if i select 200 as my hfe and use it in my ic calculations....
ic would equal to 2.6 ma???

At 2.6 ma my calculations would not match the measured values??

"you design the circuit so that it will operate within the range of Hfe that the transistor may have. Typically this is done by using negative feedback, such as from an emitter resistor, so that the minimum gain the stage has is less than minimum Hfe."

using my diagram, can you illustrate an example!

thanx
r
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Old 08-29-2012, 09:37 PM
ramancini8 ramancini8 is offline
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You design the circuit such that it will operate from the minimum specified Hfe, 35 in your case. If you do not have feedback high Hfe can cause saturation. You need feedback to stabalize the design.
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Old 08-29-2012, 10:04 PM
rougie rougie is offline
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What do you mean "35" in my case?? I am using "100" in my case!!!

ic = B * ib
ic = 100 * (2 - 0.7 / 100000) = 1.3 ma

if I do it with 35:

ic = 35 * (2 - 0.7 / 100000) = 455uA??

455uA is NOT what I measure on the bread board??

what I measure on the breadboard is approximately 1.3ma which is calculated with the 100 HFE!


>If you do not have feedback high Hfe can cause saturation. You need feedback to stabalize >the design.

I am sorry, in reference to my diagram, I am not familiar with the term "Feedback". I guess, "feedback", is used in more advanced transistor circuits than mine... Right now I would like to understand the simple "common emitter" transistor circuit. However, as my calculations are, at gate voltage of *2 volts*, I am not in saturation mode.

All I am asking is, based on my calculations in my initial post, why did I use 100 as HFE... why not 35, 70, 128, 200... which all would of given me wrong calculations when compared to the actual circuit measurements. I don't get it... discouraged.

r
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Old 08-29-2012, 10:12 PM
rougie rougie is offline
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I guess what you guys are trying to tell me is that if I would use 200 HFE in my calculations, and I would want a VCE of 3.7 V I would have to lower the value of RC thus increasing ic ... right?

So if I would of selected a HFE of 200. then:
ic = 200 * (2 - 0.7 / 100000) = 2.6 ma

And if I required a 3.7 v at VCE, then
RC1 = 1.3VRC/2.6ma = 500ohms

The only diff is that there would be a greater current of ic and my 1K would of reduced to 500 ohms...

r

Last edited by rougie; 08-29-2012 at 10:48 PM.
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Old 08-29-2012, 10:24 PM
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Markd77 Markd77 is offline
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Have a look here:
http://www.allaboutcircuits.com/vol_3/chpt_4/10.html
It explains some of the ways transistor are normally used.
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