Finding the Early voltage from the 2N5551 datasheet

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Discussion Overview

The discussion revolves around determining the Early voltage (VA) for the 2N5551 transistor using information from its datasheet. Participants explore various methods and considerations related to this parameter, including the relevance of the SPICE model, the need for specific parameters like hoe, and the implications of the Early effect in transistor operation.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that the SPICE model lists VA as 100V but questions the usefulness of this value for their inquiry.
  • Another participant suggests that knowing the parameter hoe is essential for calculating VA and provides a formula to derive it, though they acknowledge variability in manufacturer specifications.
  • Several participants express difficulty in finding the hoe parameter in the datasheet, indicating that it is not commonly provided.
  • One participant proposes that the Early effect can be inferred from the forward gain versus collector current graph, suggesting a method to extract VA from the relationship between hf, Ic, and Vce.
  • Another participant describes the Early effect as being modeled by a resistor ro between the collector and emitter and seeks to find this value for a common emitter stage using the 2N5551.
  • A suggestion is made to look for alternative datasheets from different manufacturers that might provide the necessary Ic versus Vce curves to determine VA visually.
  • One participant mentions that the output impedance of a transistor is influenced by both ro and hoe, and that typical small signal BJTs do not provide h parameters, complicating the analysis.

Areas of Agreement / Disagreement

Participants express a lack of consensus on how to determine the Early voltage for the 2N5551, with multiple competing views on the necessity of specific parameters and the methods to derive VA. The discussion remains unresolved regarding the best approach to find VA.

Contextual Notes

Limitations include the absence of the hoe parameter in the datasheet, variability in manufacturer specifications, and the challenge of measuring small changes in collector current. The discussion highlights the dependence on specific definitions and parameters that are not universally available.

gnurf
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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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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/datasheet/SGSThomsonMicroelectronics/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
 
Yes, but unfortunately I can't find this hoe in the datasheet.
 
gnurf said:
Yes, but unfortunately I can't find this hoe in the datasheet.
This is normal.
Why do you need to know Va voltage?
 
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...?
 
Jony130 said:
Why do you need to know Va voltage?
In my book, the Early effect is modeled 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.
 
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.
 
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.
 
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.
 
Last edited:

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