I am going through this tutorial, they state that "Ic is the current through the collector and function of the transistor" however I haven't really been able to look at the data sheet and figure out where that information is. I am guessing you need to calculate it somehow but I am not sure what information to use.

I see the section where they give some typical values of beta with different ranges of current but I don't get how that is a function of the transistor.



They just mean that different transistors have different values of beta.

Since it's a beginners tutorial they're glossing over the fact that even for a given transistor, beta will vary depending on temperature, bias point, etc.

I haven't really been able to look at the data sheet and figure out where that information is.

Some data sheets will use the symbol hfe instead of \$\beta\$.

Usually the temperature dependence will be given only as a typical curve, not a minimum/maximum specification. Even the nominal beta spec range will usually be quite broad, pften with ratios in the neighborhood of 5:1 between maximum and minimum.

What this means is you need to design your circuit (usually by including feedback) so that the overall behavior of the circuit is insensitive to the beta of the individual transistors.

  • \$\begingroup\$ So, why did they choose .001 amps in this case. For that particular transistor what in the datasheet made them make that decision? \$\endgroup\$ – Kenn Mar 25 '14 at 17:50
  • \$\begingroup\$ Probably because they're using a 9 V battery for power, and they don't want to run it down too quickly. The 2N4401 datasheet they linked gives hfe values for collector currents at 0.1, 1.0, 10, and 150 mA; You could design your circuit to operate at any of those bias points depending on what you're trying to do. You can see that as Ic goes up, the gain goes up. But you would also run down your battery faster. \$\endgroup\$ – The Photon Mar 25 '14 at 17:55
  • \$\begingroup\$ Right on, intuitively I would have thought OK - this is just telling me that if I put .001 amps of current on the collector at X volts this is what to expect. They didn't gets some information from the data sheet and do a math problem, they just said, I would like to figure out the resistance I will need to make sure that the current on the collector it .001 amps. If that statement is wrong please let me know. \$\endgroup\$ – Kenn Mar 25 '14 at 18:55
  • \$\begingroup\$ @Kenn, Actually, they have to decide how to bias the base before they will be sure to get 1 mA into the collector. They pick the resistor value knowing they want to have 1 mA Ic, and an output bias point of about halfway between the supplies (4.5 V). \$\endgroup\$ – The Photon Mar 25 '14 at 19:07

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