transistor and more please help

[llama]

firstly thankyou to whom ever runs this site it has some of the best information and the fact that i can ask a question is great so here is my question
i am very knew to electronics and i am still learning the basics and the one thing i cant seem to get clear is how to read hFE and does it matter how much current is applied to the base? thanks for your time if you take it.

 

[llama]

and...

before i forget i have a multi meter and it says 420 does that mean it has a gain of 420 %?

 

[Nigel Goodwin]

i am very knew to electronics and i am still learning the basics and the one thing i cant seem to get clear is how to read hFE and does it matter how much current is applied to the base? thanks for your time if you take it.

You simply inject a current into the base of the transistor, and measure the collector current. The hfe will vary according to the collector current, and in datasheets is usually specified at a specific current.

What you could do, is use a potentiometer to supply an adjustable base current, with a meter monitoring it, and a second meter monitoring the collector current. Slowly turn the base current up from zero, and keep writing down the collector current for each increment of base current. You can then plot this on a graph.

There have been many designs over the tears for automating this, either displaying the curve on a scope in real time, or feeding into a computer which controls the process and plots the graph for you.

 

[Nigel Goodwin]

Re: and...

before i forget i have a multi meter and it says 420 does that mean it has a gain of 420 %?

No, it means a gain of 420 times, which would be 42,000%.

BTW, a quick google search found this which you may find helpful.