Finding the Early voltage from the 2N5551 datasheet

gnurf

Is it possible to find the Early voltage (V_A) 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.

Jony130

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

Yes, but unfortunately I can't find this hoe in the datasheet.

Jony130

This is normal.
Why do you need to know Va voltage?

fbs7

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

In my book, the Early effect is modelled as a resistor, r_o, between the collector and emitter:

r_o = (V_A +V_CE)/I_C

I'm trying to find this value for a simple CE stage based on the 2N5551.

yungman

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

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

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.