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Determining transistor HFE

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axro

New Member
How do you determine what the gain will be for a transistor. I have a PN2222A transistor and the datasheet give lots of different numbers. How can you determine what HFE number to use when designing a circuit?
 

Papabravo

Well-Known Member
You can construct an experimental setup that controls or measures three things.
  1. Base Current, Ib
  2. Collector Current, Ic
  3. Colector-Emitter Voltage, Vce
Now you set about plotting the points on a family of curves. This family of curves will tell you what hfe is for a particular transistor.

Some additional observations.
  1. Think of hfe as being a normally distributed random variable with a mean and a variance.
  2. Don't EVER design a circuit that depends on a specific value, or even a range of values, for hfe
 

audioguru

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Most Helpful Member
A transistor has a wide range of hFE. So its circuit is designed with enough DC negative feedback so that any hFE value will work acceptably.

An emitter resistor with the base biased from a stable supply through a voltage divider, or a resistor from the collector to the base to bias it.
 

axro

New Member
A transistor has a wide range of hFE. So its circuit is designed with enough DC negative feedback so that any hFE value will work acceptably.

An emitter resistor with the base biased from a stable supply through a voltage divider, or a resistor from the collector to the base to bias it.
Would you like to elaborate on that a little? Maybe with some circuits or examples?
 

audioguru

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Most Helpful Member
Sorry, I am not a teacher. Please find basic transistor circuits and a tutorial yourself.
 

bountyhunter

Well-Known Member
How do you determine what the gain will be for a transistor. I have a PN2222A transistor and the datasheet give lots of different numbers. How can you determine what HFE number to use when designing a circuit?
You can use the typical curves if you like to live dangerously, or the guaranteed specs if you are smart. You need to know the collector current and VCE to figure out the gain.
 

bountyhunter

Well-Known Member
So sorry to ask a question about electronics in a forum about learning electronics.
Then ask a question with a finite answer like "how do you design XXX" or "I am trying to build a circuit that does XXX". You will get buried in answers when people know what you are asking.
 
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axro

New Member
You can use the typical curves if you like to live dangerously, or the guaranteed specs if you are smart. You need to know the collector current and VCE to figure out the gain.
Where on the datasheet where they show guaranteed gain?

Also to know what VCE and Collector Current(without measuring them in the circuit) are going to be, don't you need to know HFE and Base Current?
 

audioguru

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Most Helpful Member
Where on the datasheet where they show guaranteed gain?
Here is the guaranteed gain on the Fairchild datasheet of the 2N3904 transistor.

Also to know what VCE and Collector Current(without measuring them in the circuit) are going to be, don't you need to know HFE and Base Current?
Your circuit design sets the Vce and collector current.
If you use negative feedback then the transistor can have any value of hFE and will work properly at many temperatures.
 

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axro

New Member
Here is the guaranteed gain on the Fairchild datasheet of the 2N3904 transistor.


Your circuit design sets the Vce and collector current.
If you use negative feedback then the transistor can have any value of hFE and will work properly at many temperatures.
Thanks. Sounds like I need to read up some more on negative feedback, to get a good grip on this.
 

axro

New Member
Is negative feedback just a resistor on the emitter?

I'm looking on the net for good examples on what negative feedback is and how it applies, but I can't seem to find anything good.
 

audioguru

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A resistor in series with the emitter adds negative feedback and reduces the difference of the Vbe of different transistors.

A bias resistor from the collector to the base also adds negative feedback but then a resistor from the input to the base is needed.
 

MrAl

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How do you determine what the gain will be for a transistor. I have a PN2222A transistor and the datasheet give lots of different numbers. How can you determine what HFE number to use when designing a circuit?

Hi,

I am surprised that you did not find the minimum gain spec for that
transistor on the data sheet. This is the gain they say will be the
minimum found in any of their transistors.

The thing is though, is that a circuit can not usually be designed that
depends highly on the gain of the transistor anyway, as you have to
assume that it changes over temperature too. This means you should
test your circuit and make sure it still works as planned.
For example, a circuit that requires a gain of 50 using a transistor
with a min gain of 50 wont work right, so you design a circuit so
that it only requires a gain less than the min gain spec and set up
the design to force a gain of say 25 to 40. This will allow the
transistor to work at a gain that it should have even with a change
in temperature.
With some transistors they show the change in beta with temperature,
but with most they dont. It starts out low with lower temps, then
goes up to some peak, then dips low again for very high temps.
 

axro

New Member
A resistor in series with the emitter adds negative feedback and reduces the difference of the Vbe of different transistors.

A bias resistor from the collector to the base also adds negative feedback but then a resistor from the input to the base is needed.
If you put a resistor in series with the emitter, you would then have a emitter follower correct? And then you would have no gain at all?
 

audioguru

Well-Known Member
Most Helpful Member
An emitter-follower does not have a collector resistor and its output is from its emitter. Its voltage gain is 1.
A common-emitter transistor circuit has a collector resistor and its output is from its collector.

The voltage gain of a common-emitter transistor is RC/RE. If RC is 10k and Re is 1k then the voltage gain is 10.
 

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