Transistor hFE equation question

In summary, the equation relating \Delta I_E and \Delta I_B can be substituted into to get: \Delta I_B = \frac{1}{h_{FE} + 1}\frac{\Delta V_B}{R}
  • #1
TFM
1,026
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Homework Statement



Showing how to get:

[tex] Z_{out} = \frac{Z_{in}}{h_F_E + 1} [/tex]

from

[tex] \Delta I_E = \Delta V_B/R [/tex]

Homework Equations



[tex] \Delta I_E = \Delta V_B/R [/tex]

[tex] Z_{out} = \frac{Z_{in}}{h_F_E + 1} [/tex]

[tex] \Delta V_E = \Delta V_B [/tex]

The Attempt at a Solution



I am trying to prove the above, but the book makes quite a large jump (again, "The Art of Electronics")

It goes from:

[tex] \Delta I_E = \Delta V_B/R [/tex] - (1)

straight to

[tex] \Delta I_B = \frac{1}{h_{FE} + 1}\Delta I_E = \frac{\Delta V_B}{R(h_{FE} + 1)} [/tex] -(2)

Can anyone help show how they've gone from (1) to (2)?

Thanks,

TFM
 
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  • #2
TFM said:

Homework Statement



Showing how to get:

[tex] Z_{out} = \frac{Z_{in}}{h_F_E + 1} [/tex]

from

[tex] \Delta I_E = \Delta V_B/R [/tex]

Homework Equations



[tex] \Delta I_E = \Delta V_B/R [/tex]

[tex] Z_{out} = \frac{Z_{in}}{h_F_E + 1} [/tex]

[tex] \Delta V_E = \Delta V_B [/tex]

The Attempt at a Solution



I am trying to prove the above, but the book makes quite a large jump (again, "The Art of Electronics")

It goes from:

[tex] \Delta I_E = \Delta V_B/R [/tex] - (1)

straight to

[tex] \Delta I_B = \frac{1}{h_{FE} + 1}\Delta I_E = \frac{\Delta V_B}{R(h_{FE} + 1)} [/tex] -(2)

Can anyone help show how they've gone from (1) to (2)?

Thanks,

TFM

They don't. They substitute (1) into the equation relating [tex]\Delta I_E[/tex] and [tex]\Delta I_B[/tex] (the first half of (2)). I believe AoE uses [tex]h_{FE}[/tex] in place of [tex]\beta[/tex]
 
  • #3
Okay so:

[tex] \Delta I_E = \Delta V_B/R [/tex]

substitute into:

[tex] \Delta I_B = \frac{1}{h_{FE} + 1}\Delta I_E [/tex]

gives:

[tex] \Delta I_B = \frac{1}{h_{FE} + 1}\frac{\Delta V_B}{R} [/tex]

[tex] \Delta I_B = \frac{\Delta V_B}{(h_{FE} + 1)R} [/tex]

R is the load, so I am assuming that this is the R output. Multiply it out:

[tex] R\Delta I_B = \frac{\Delta V_B}{(h_{FE} + 1)} [/tex]

divide by I_B

[tex] R = \frac{\Delta V_B}{(h_{FE} + 1)\Delta I_B} [/tex]

V = IR
R = V/I

thus:

[tex] R_{output} = \frac{R_{input}}{(h_{FE} + 1)} [/tex]


[tex] Z_{out} = \frac{Z_{in}}{h_F_E + 1} [/tex]

is also the same as:

[tex] R_{out} = \frac{R_{in}}{h_F_E + 1} [/tex]

Z is just a complex version of R

Does this look correct?

TFM
 
  • #4
I think that's okay. Though you might want to make some justifications as to what the input and output (and input and output impedances) are of the BJT (unless this was part of the setup for the question). And you should probably start with complex impedances (Z=V/I) instead of changing from R to Z mid-way through.

Just my 2c.
 
  • #5
Okay, Thanks for all your assistance :smile:

Thanks,

TFM
 

FAQ: Transistor hFE equation question

What is the hFE equation for a transistor?

The hFE equation for a transistor is hFE = IC/IB, where IC is the collector current and IB is the base current. This equation represents the ratio of the output current to the input current in a transistor.

Is hFE a constant value for a transistor?

No, hFE is not a constant value for a transistor. It varies depending on the operating conditions of the transistor, such as temperature, collector voltage, and collector current.

How does hFE affect the performance of a transistor?

hFE plays a crucial role in determining the gain and amplification of a transistor. A higher hFE value means a higher gain and better amplification, while a lower hFE value results in lower gain and poorer amplification.

Can hFE be measured or calculated?

Yes, hFE can be measured using a multimeter or calculated using the hFE equation mentioned above. However, the measured or calculated value may not always be accurate due to other factors that can affect hFE.

What factors can affect the hFE value of a transistor?

Some factors that can affect the hFE value of a transistor include temperature, collector voltage, collector current, and aging of the transistor. Additionally, different types and models of transistors may have different hFE values.

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