I'm looking at my friend's notes on choosing resistor values for a simple infrared generator circuit. I don't quite understand it and was wondering if you guys could help me.

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I understand how they got V(RE2) and V(comp) but I'm not sure what Isink is. Could someone please tell me where on the diagram Isink would be and how they got a value of 6mA for it? I'm not sure if they calculated that value or knew it from a datasheet.

Thanks. :)

  • \$\begingroup\$ What does your friend say when you ask them this question? \$\endgroup\$
    – gbulmer
    May 13, 2016 at 0:35
  • \$\begingroup\$ They haven't been replying on facebook unfortunately and I need to have this figured out before my test this evening. \$\endgroup\$ May 13, 2016 at 0:44

1 Answer 1


Isink = Vcc/R1 seems to be the source of this value.

Its the current that would flow if the input was grounded.

The voltage drop across R1 is (Vcc - 9.6) = 2.4V giving a value of 1.09mA through the voltage divider (R1 and R2).However this seems unnecessarily high.

If the first transistor needs 2mA through it and ( and another 1 mA to supply the second transistor) then assuming a minimum gain of 100 the base current would be 30uA so the current required through R1 and R2 would be about 10x that (to prevent load the mid point voltage too much) or 0.3mA so the 1mA current is acceptable but sightly larger than it needs to be.


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