Transistor questions

Anonymous

Hello,

I am a TI chip user and I am trying to control a stepper motor with a simple transistor-based switch. Although these transistors does not come from TI, I still with to consult with the experts here on some basic questions:

The transistor I bought from en electronic market has labeling as "S8050 D 331". I used a multimeter to measure its hfe value and it shows 186 (varying between 186 and 188).

However, I googled "S8050" and found a document from http://www.unisonic.com.tw/datasheet/S8050.pdf which appears to me somewhat confusing:

There are h_fe1, h_fe2, h_fe3 for this S8050 transistor and for each there are associated MIN, TYP and MAX value. for h_fe1and h_fe3 there are only a MIN value, but for h_fe2 the values MIN, TYP and MAX are all exist. Why is the difference?
What does h_fe1, h_fe2and h_fe3 mean? If they all exist, do I still have a unique value? Which one of them correspond to the 186 value which I got from the multimeter?
Why for each of them there are a range of values (MIN, TYP, MAX)? If the hfe has, for example like h_fe2, MIN value as low as 100 and MAX as high as 400, then how can I know exactly which hfe value the transistor will operate on, and consequently, how should I design the circuit (resistance values, saturation mode, etc.)?

I could hardly see any clue in this data sheet. Help will be greatly appreciated.

Zheng

over 13 years ago

0 Bruce Trump over 13 years ago

Guru 21975 points

Zheng,

The forward current gain of a BJT varies depending on the collector current. This manufacturer has provided information on Hfe at three different operating currents that they call 1, 2, and 3.

Manufacturers generally only provide min and max characteristics for parameters that they actually test on each device that they ship. Other information is provided so that the user can get a sense of how the device may behave in various circuits.

The meter you are using probably tests beta or Hfe at a relatively low collector current, perhaps around 1mA. The instructions for the meter may provide this information.

Regards, Bruce.

0 Anonymous over 13 years ago in reply to Bruce Trump

Guru 17045 points

Dear Bruce,

Bruce Trump said:

"The meter you are using probably tests beta or Hfe at a relatively low collector current".

I am a bit confused: Isn't beta and hfe the same thing?

http://en.wikipedia.org/wiki/Bipolar_junction_transistor said:

"The common-emitter current gain is represented by β_F or h_fe; it is approximately the ratio of the DC collector current to the DC base current in forward-active region."

From the syntax of the above sentence it seems they are the same thing, but why do you still use an "or" between them, which might suggest beta and hfe are two different quantities?

Another question is that in textbook examples, it is usually assumed that beta is known, and the steps of calculation would be:

Because V_BE's change in negligible with respect to I_B change, V_BE is therefore usually taken as 0.7V, and I_B is determined by I_B=(V_BB-0.7)/R_B.
Using β, I_C = β × I_B.
V_CE=V_CC - I_C × R_C.

In this treatment, β (=hfe?) is basically regarded as a constant and hence the calculation is straightforward. But if β (=hfe?) changes with I_C, then how can I_C be determined when I_B is known? The requirement is that

I_C = β × I_B, hence I_C depends on β.

Bruce Trump said:
The forward current gain of a BJT varies depending on the collector current.

β depends on I_C

Therefore I_C and β are mutually dependent, which constitute a chicken-egg dilemma in their determination. How can one then solve this and determine the correct value for both I_C and β? The most unfortunately way I could thought of is to use "iteration", which I happened to see somewhere in semiconductor textbooks, merely for this basic task. Is it true? Or did I make any mistake here?

Zheng

0 chandrakant kulkarni over 9 years ago in reply to Anonymous

Prodigy 30 points

its depends on beta...Ic/Ib..Dc current gain

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