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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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