Task about BJT Led

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Circuit: https://www.circuitlab.com/editor/5kx6tz8jk36j/

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To set up the LED driver. The requirements are:

The current through the LED should be 20mA ± 1mA

The IC/IB should be around 25

I find: RB=43000 ohm, and Rc=180ohm , that meet the requirements, so, the LED driver works properly.

I have a couple questions if someone can give some help!

_Which area it uses and which area can be used to vary the brightness if needed.

_ What are the values VCE, VBE

_What kind of amplifier this is and how the transistor β affects RB. (VOLUNTARY)

  • 2
    \$\begingroup\$ Your questions sound too much like homework questions to me. Are they? \$\endgroup\$ Commented May 25, 2020 at 11:00
  • \$\begingroup\$ @Maikel - Hi (a) You say "BJT" in the title, but your first schematic shows a MOSFET. Please clarify/fix. (b) Your question includes the word "area" (twice), in a context which doesn't make sense. I'm quite sure that you mean something else. Please clarify/fix. \$\endgroup\$
    – SamGibson
    Commented May 25, 2020 at 19:24

1 Answer 1


1.By changing RC u can vary brightness. 2.Vbe,Vce are BJT parameters,mostly Vbe is around 0.7,at different collector currents Vce will be mentioned in datasheet of BJT. 3.current amplifier and you should select Rb based on minimum current gain(beeta).

  • \$\begingroup\$ No, \$V_{BE}\$ and \$V_{CE}\$ are not BJT parameters found in the datasheet. Select Rb based on minimum RB...what does that mean? \$\endgroup\$ Commented May 25, 2020 at 13:24
  • \$\begingroup\$ sorry that was minimum current gain(beeta) \$\endgroup\$
    – yugi
    Commented May 26, 2020 at 3:57
  • \$\begingroup\$ well i have used BJT and i have seen Vbe and Vce in its datasheet. \$\endgroup\$
    – yugi
    Commented May 27, 2020 at 3:36
  • \$\begingroup\$ You will see Vbe and Vce in datasheets but not as parameters of the transistor. These voltages will often have a specified value during the measurement of some other parameter. \$\endgroup\$ Commented May 27, 2020 at 16:55

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