Finding the Early voltage from the 2N5551 datasheet


by gnurf
Tags: datasheet, early, voltage
gnurf
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#1
Feb5-12, 10:28 AM
P: 330
Is it possible to find the Early voltage (VA) of the 2N5551 based on the information in the datasheet? As it happens, I see that the SPICE model in the datasheet lists this parameter at 100V, but that bypasses the purpose of the question. Thanks.
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Jony130
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#2
Feb5-12, 10:56 AM
P: 391
You need to find the datasheet that show hoe.
Knowing hoe we can without difficulty determine Early voltage.
See this datasheet as a example
http://www.datasheetcatalog.org/data...cs/mXyzzyw.pdf

Va = (Ic/hoe) - Vce = (1mA/5uS) - 10V = 200V - 10V = 190V

But as you can see we have a large manufacture spread. So in practice, if we don't know the exact value we assume a type value Va = 100V
gnurf
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#3
Feb5-12, 11:16 AM
P: 330
Yes, but unfortunately I can't find this hoe in the datasheet.

Jony130
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#4
Feb5-12, 11:28 AM
P: 391

Finding the Early voltage from the 2N5551 datasheet


Quote Quote by gnurf View Post
Yes, but unfortunately I can't find this hoe in the datasheet.
This is normal.
Why do you need to know Va voltage?
fbs7
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#5
Feb5-12, 11:35 AM
P: 127
Hmm... in the graphic of forward gain x collector current... the increase in forward gain in small signals is due to Early effect... should be able to get it from there, shouldn't it?

hf = hfo ( 1 + Vce/Va)

if one matches hf vs Ic, and Ic vs Vce... that should give Va...?
gnurf
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#6
Feb5-12, 12:51 PM
P: 330
Quote Quote by Jony130 View Post
Why do you need to know Va voltage?
In my book, the Early effect is modelled as a resistor, ro, between the collector and emitter:

ro = (VA +VCE)/IC

I'm trying to find this value for a simple CE stage based on the 2N5551.
yungman
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#7
Feb5-12, 12:54 PM
P: 3,844
Not with this data sheet. Look for another brand that has the Ic vs Vce curve. If you cut the graph and stick on a piece of paper with a lot of room on the left side, then extend the straight portion towards the left, they all should meet at Ic=0 at one point and that's the early voltage Va.

If it is a common transistor, there should be plenty of manufacturers. I notice different manufacturer give different information.

And other way if you have good meter, measure it yourself. Set up a bias condition with grounded emitter, then vary the collector voltage and read the current. This is hard as the change are very small. But it's doable.
Jony130
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#8
Feb5-12, 01:03 PM
P: 391
But do you know that
hoe = 1/ro
Also when load resistance (Rc||RL) of a CE stage is not such a larger we can ignore ro effect on the voltage gain.
yungman
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#9
Feb10-12, 02:43 AM
P: 3,844
Problem is data sheet of normal small signal BJT don't give you the h parameters, only β. Only the RF transistors that give you s-parameters, then you can do transformation.

Also the output impedance of a transistor is not ro alone as hoe is measure with ib=0. The collector curves are drawn with ib not equal to zero. It depend on the collector current.


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