Darlington base-emitter resistors

Discussion in 'General Electronics Chat' started by Norfindel, Apr 11, 2008.

  1. Norfindel

    Thread Starter Active Member

    Mar 6, 2008
    235
    9
    Hello,

    I readed that base-emitter resistors are included in darlington pairs to better the switching-off times, but not found too much info about that. Why are they slow to switch off? Because of base-emitter capacitancy? How do you know what value of resistor to use? If someone has info on this, it would be much appreciated.

    Thank you.
     
  2. rwmoekoe

    Active Member

    Mar 1, 2007
    172
    0
    I think you are right. It's for the be and bc capacitance. Going from that point, the value can be determined for given Cbe // Cbc and maximum rise time wanted for the transistor's swing.
     
  3. Norfindel

    Thread Starter Active Member

    Mar 6, 2008
    235
    9
    Thank you for your response.
    I was building a circuit with a darlington pair to boost the output of an opamp (it was the LM358 single supply opamp), and to my surprise, the circuit would not go to 0v. The opamp output was 0v, but the base of the *second* transistor was at a higher value. I just used a low value resistor, because frequency response was not critical.
    Now i am seeing the possibility of using a power mosfet or igbt to avoid loading the LM358, but never used them before, so it would take a bit of research to see if i use them or the darlington.
    Oh! i am planing to build a bench power supply, with variable voltage and current limit.
     
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