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NPN transistor and 555 timer

  1. Aug 26, 2012 #1
    I'm trying to create a circuit to drive a pretty big speaker using a 555 and an NPN transistor. The transistor part # is PN2222A. The output from the 555 is roughly 66% duty cycle, Vmax = 6.6V. When I connect the output from the 555 to the base of the BJT, and put a current limiting resistor (~750Ω) between Vcc and the collector of the BJT, the emitter has Vmax = 5.6V and Vmin = 2V.
    555 fout = 37kHz

    From what I observed, if I lower the frequency, the emitter Vmin = 0V, but when the frequency is quite high, Vmin = 2V. What exactly is preventing the voltage of the emitter to drop to 0V at high frequencies? Where would I find out the limit on the data sheet, if there exists such a limit?

    EDIT: Fixed
    Last edited: Aug 26, 2012
  2. jcsd
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