All About Circuits

2N2222... hfe?

R

Thread Starter

rougie

Joined Dec 11, 2006
410
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!
 

Attachments

Wendy

Wendy

Joined Mar 24, 2008
23,421
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.
 
paulktreg

paulktreg

Joined Jun 2, 2008
835
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.
 
R

Thread Starter

rougie

Joined Dec 11, 2006
410
>>>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
 
crutschow

crutschow

Joined Mar 14, 2008
34,446
rougie said:
>>>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!!!
Click to expand...
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.
 
R

Thread Starter

rougie

Joined Dec 11, 2006
410
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
 
ramancini8

ramancini8

Joined Jul 18, 2012
473
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.
 
R

Thread Starter

rougie

Joined Dec 11, 2006
410
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
 
R

Thread Starter

rougie

Joined Dec 11, 2006
410
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:
Audioguru

Audioguru

Joined Dec 20, 2007
11,248
You cannot order a transistor with a certain amount of hFE. You get whatever amount they have available. It might be low, medium or high. Temperature also changes the hFE.

A transistor might have a low hFE or it might have a high hFE which is shown as a range of numbers on its datasheet. You do not "pick" the hFE.
Transistor amplifier circuits are designed using negative feedback so that the circuit works properly with any transistor whether its hFE is low or is high.

If the transistor is used as a simple on-off switch then its hFE is assumed to be only 10 because the saturated hFE is very low.

You mentioned "gain". hFE is current gain, not voltage gain.
 
R

Thread Starter

rougie

Joined Dec 11, 2006
410
<
A transistor might have a low hFE or it might have a high hFE which is shown as a range of numbers on its datasheet. You do not "pick" the hFE.
>

Don't mean to be stubborn here and I do apollogize if I am
but the 2n2222 that I used in my example comes with a range of 35 to 300 I
believe... And I somehow found a tutorial that exclusively picked
100????

Why 100?

If one needs to completely understand all the finest details
about transistors to understand why this is so, then I will
continue learning....

<
If the transistor is used as a simple on-off switch then its hFE is assumed to be only 10 because the saturated hFE is very low.
>

How 10?.... The spec says 35 to 300???

I know that transistors come with a predetermined HFE range.
But all I want to do is use a transistor as an on/off device.

I did my calculations with a HFE of 100 (and not 10) and using 6 volts as opposed to 2volts at gate I am still able to use it as an on/off switch.

The only problem is I need different voltages and currents!

still confused... But ready to put it to rest LOL
Regards
R
 
Papabravo

Papabravo

Joined Feb 24, 2006
21,225
rougie said:
<
A transistor might have a low hFE or it might have a high hFE which is shown as a range of numbers on its datasheet. You do not "pick" the hFE.
>

Don't mean to be stubborn here and I do apollogize if I am
but the 2n2222 that I used in my example comes with a range of 35 to 300 I
believe... And I somehow found a tutorial that exclusively picked
100????

Why 100?

If one needs to completely understand all the finest details
about transistors to understand why this is so, then I will
continue learning....

<
If the transistor is used as a simple on-off switch then its hFE is assumed to be only 10 because the saturated hFE is very low.
>

How 10?.... The spec says 35 to 300???

I know that transistors come with a predetermined HFE range.
But all I want to do is use a transistor as an on/off device.

I did my calculations with a HFE of 100 (and not 10) and using 6 volts as opposed to 2volts at gate I am still able to use it as an on/off switch.

The only problem is I need different voltages and currents!

still confused... But ready to put it to rest LOL
Regards
R
Click to expand...
The only reason a tutorial would pick a value is to walk you through the calculations you would make when and if you know the value. "Now your challenge, Jim should you decide to accept it, is to design a circuit that DOES NOT care what value of Hfe a transistor has and still meets the design requirements."

"This message will self-destruct in 5 seconds and the Secretary will disavow all knowledge of your actions"
--loosely adapted from a popular TV show "Mission Impossible"
 
R

Thread Starter

rougie

Joined Dec 11, 2006
410
i watched an interesting video at:

http://www.allaboutcircuits.com/videos/61.html

it seems that a transistor comes with the possible range of hfe values. so the manufacturer guarantees that the gain can vary somewhere between min hfe and max hfe based on construction, temperature and so forth.

i am new to this transistor stuff, but if i were to design a circuit that doesn't care about what the hfe is, i would have to read up further about transistor theory... but hey an insight would be very welcome :)

thanks for your reply.... its appreciated

r
 
Audioguru

Audioguru

Joined Dec 20, 2007
11,248
Negative feedback in a transistor circuit reduces the range of hFE, reduces the amount and range of voltage gain, reduces distortion and widens the bandwidth.
Frequently the negative feedback is produced by a resistor to ground in series with the emitter.
 

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R

Thread Starter

rougie

Joined Dec 11, 2006
410
<
Negative feedback in a transistor circuit reduces the range of hFE, reduces the amount and range of voltage gain, reduces distortion and widens the bandwidth.
Frequently the negative feedback is produced by a resistor to ground in series with the emitter.
>

Okay so it's an emitter bias type circuit!

Thanks for your help!
r
 
ramancini8

ramancini8

Joined Jul 18, 2012
473
The data sheet shows mostly minimum values. The beta range at 150mA is 100 to 300, and that shows the variability of beta. To measure beta, measure Vce and use this formula Ic = (Vcc-Vce)/Rc. What is your measured Vce? Your calculation for Ic should be Ic=Hfe(.013mA). Get Ic from the measured value of Vce and calculate Hfe. If you have a mA meter you can measure Ic and Ib.

With a guaranteed Hfe range of 3 to 1 you can see how a large value of Rc can limit the range of Vce causing saturation, and that is why feedback is used. If you want to persue this subject more contact me offline.
 
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