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I don't know what is the forward beta of the bipolar junction transistor KN2222A, I'm an amateur and i don't found the value in the data sheet or i just don't understand where. Please let me know

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    \$\begingroup\$ Have a look in the datasheet: buchangelec.com/data/transistor/general/2222a.pdf on page 2 there's a section: "DC Current Gain" which is \$h_{FE}\$ which is also \$\beta\$. There are mostly minimum values there. For hobby projects you could just assume \$\beta\$ = 50 and that would fit the needs of 99% of generally used circuits. If your circuit relies on the BJT having a specific value of \$\beta\$ then you should use a better (less \$\beta\$ dependent) circuit. \$\endgroup\$ Jun 12 '19 at 6:48
  • \$\begingroup\$ @Bimpelrekkie Thank you! \$\endgroup\$ Jun 12 '19 at 6:53
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    \$\begingroup\$ Each manufacturer who applies such a part number can decide how to answer that question. Sometimes a specification covers multiple manufacturers (2N2222A is such a specification), but probably not KN2222A. \$\endgroup\$
    – Whit3rd
    Jun 12 '19 at 6:56
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    \$\begingroup\$ That datasheet is poorly specified. It relies upon \$V_\text{CE}=10\:\text{V}\$ and it doesn't show the values for different temperatures (which is more important to know.) \$\endgroup\$
    – jonk
    Jun 12 '19 at 6:57
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And for what it's worth there are multiple \$\beta\$ factors to consider when designing BJT circuits:

  • \$\beta\$, \$\beta_{DC}\$, \$h_{FE}\$ - These refer to the BJT's DC forward current gain when the BJT is operating in forward-active mode (small signal amplification). Used when performing the DC design/analysis.
    (n.b. The 'FE' refers to Forward current gain, common-Emitter configuration.)
    (n.b. Parameters written with UPPERCASE subscripts typically refer to DC parameters.)

  • \$\beta_{ac}\$, \$h_{fe}\$ - These refer to the BJT's AC forward current gain when the BJT is operating in forward-active mode (small signal amplification). Used when performing the AC design/analysis. Typically, \$\beta_{ac}\ll\beta_{DC}\$.
    (n.b. Parameters written with lowercase subscripts typically refer to AC parameters.)

  • \$\beta_{sat}\$ - Refers to the BJT's forward current gain when the BJT is operating in "hard" saturation mode (when the BJT is turned ON fully). Typical values are \$5 \le \beta_{sat} \le 30\$ with \$\beta_{sat}=I_{C(sat)}/I_{B(sat)}=10\$ being a fairly common value for low power and medium power transistors.
    (n.b. Data sheets often use lowercase text for parenthetical text—e.g., \$I_{C(sat)}\$, \$V_{BE(sat)}\$, etc. This does not indicate or imply that these are AC parameters.)
    (n.b. \$\beta_{sat}\$ is not an AC parameter because the transistor is not performing small-signal AC amplification when it is operating in saturation mode.)

And as a general rule do not use \$\beta\$, \$\beta_{DC}\$, \$h_{FE}\$ to perform saturation calculations. If you do, the BJT will likely operate in "soft" saturation—i.e., in the transition region between the BJT's forward-active mode and its "hard" saturation (fully ON) mode.

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