transistor amplifier design question (determining hie)..?

Question

the formula given forr determining hie (or Rpi) is (B VT) / IE , where VT = 0.026
according to the transistor 2N2222 datasheet , the minimum value for hie is 2K and the max value is 8K . Now suppose you have a very small value of IE (say about 10 micro ampere) , then according to the formula hie would be (assming B = 50) : 

 hie = 50 (0.026) / (10 x 10^-6) = 130 K ohm which is much greater than hie max (8K) , so how do 

we estimate the value of hie ? is that formula used for certain cases only ?

here is the link , plz refer to page 5 (tables 5 and 6)

http://www.aoc.nrao.edu/~pharden/hobby/HG_DS1.pdf

thanks

billrussell42 · Accepted Answer

2N2222 datasheet does NOT specify hie. The data in your link is what someone measured for a typical part, but is not part of the specification.

But anyway, hie varies with current. The 2k-8k number you have is typical at an unknown current, so that is pretty useless. The 130k you calculated is as good as any.

edit:
if all you want is the input impedance of a CE amplifier, use:

input Resistance Rb = 25(Hfe) / Ie (Ie in ma)
    This is paralleled by the bias resistors

For your example at 10µA, Rb = 25•50/0.01 = 125k ohms
But I'm unsure that HFE at 10µA is about 50, although that is possible. But it is an unspecified area of the transistor.

? · Answer

Why are all the answers so short these days?

? · Answer

it doesnot specify

transistor amplifier design question (determining hie)..?

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