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Taking a sample from TAOE Ch.2 Figure 2.9 for example. The book suggests that BJT constructs a constant current source for the LED and \$\beta\$ is subject to change from multiple parameters thus should be not used to calculate collector current in practical design.

Supposed \$\beta\$ is ~25, \$I_{b}\$ is (3.3-0.6)/10k = 0.27mA and hence \$I_{c}\$ is preliminarily "guessed" to be 6.75mA. Voltage drop across 330\$\Omega\$ resistor is "guessed" to be 330\$\Omega\$ * 6.75mA = 2.2275V, and if I assume voltage drop across LED is 1.65V, total voltage drop on collector side is 2.2275V + 1.65V = 3.8775V, which makes no sense since >3.3V suggests collector is pulled below ground. Now I know it's saturated. But to start the calculation, I cannot avoid using \$\beta\$ to estimate it. So I have the imagine \$\beta\$ is less than 25 and plug in each \$\beta\$ value until the voltage drop in the collector is 0.2V less than the supplied 3.3V, hence the beginning of my saturation current.

If we take a different approach, assuming LED drop voltage by 1.65V, then we calculate current across 330\$\Omega\$ resistor to be (3.3-1.65)/330 = 3.295mA (here I neglect \$V_{ce}\$ ~ 0.2). To drive a 3.295mA collector current, my base current should be at least \$\beta\$ * \$I_{b}\$. Again, I have to use \$\beta\$ which I am not supposed to.

  1. How is this a constant current source if either perspective requires me to use \$\beta\$ which is subject to much variation? Then I have to go about imagine \$\beta\$ to be a different value to find the "point" between saturation and forward active.
  2. If I change \$V_{cc}\$ from 3.3V to 10V, collector current increases more than 5%, which is not a constant current source.

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  • The collector looks similar to a current source if the BJT is not saturated but is instead in active mode. The collector looks similar to a voltage source when the BJT is saturated. I don't have the book handy, but figure 2.9 looks like a circuit intended to by operated in saturated mode. So not a current source behavior. – jonk Apr 14 '20 at 06:13
  • When BJT is in the saturation region the collector current is set by a resistor in the collector. https://electronics.stackexchange.com/questions/311243/bjt-base-current-calculation/311306#311306 Thus this circuit does not behave like a constant current source. – G36 Apr 14 '20 at 06:48
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    I looked into the book and this is what the book has to say about this circuit: "Then use an NPN transistor as a switch (Figure2.9), choosing the collector resistor to provide the chosen LED current, realizing that the voltage drop across the resistor is the supply voltage minus the LED forward drop at its operating current. Finally, **choose the base resistor to ensure saturation, assuming a conservatively low transistor beta (β ≥ 25 is pretty safe for a typical small-signal transistor like the popular 2N3904)**" I don't see any mention about the constant current source? – G36 Apr 14 '20 at 07:12
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    And next the authors added this: "Note that the transistor is acting as a saturated switch, with the collector resistor setting the operating current" And finally "_As we’ll see shortly, you can devise circuits that provide an accurate current output, largely independent of what the load does. Such a “current source” can also be used to drive LEDs. But our circuit is simple and effective._" Which means that you will learn about a constant current source in the future. But this circuit is simple and does not behave like a constant current source. – G36 Apr 14 '20 at 07:21

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