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Frequency range for 1N4148

Discussion in 'Electronic Design' started by tester, Sep 28, 2005.

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  1. tester

    tester Guest

    I am testing the switching function of 1N4148 that I thought would work
    under VHF or at least HF. I input a 200KHz square wave of 100mv-800mv
    to a 1N4148, which has a 100k ohm between the output end and the
    ground. There is also a 50 ohm resistor between the input end and the
    ground. I am expecting to see also a square wave with 0v-300mv at the
    output end. But what I got has a DC component in it. Where does this DC
    come from? Does it mean the diode didn't cut off even when the input
    voltage is below its threshold? Or What's the frequency range for
    1N4148 to work well? Thanks a lot.
  2. Diode switches are either AC coupled or use two diodes against each
    other. I'd suggest two diodes anyway for compensation.
    As to the frequency range, I'd expect a few MHz.

  3. Remember that the diode has a junction capacitance (about 4 pf for low
    bias voltages). Think of the time constant of that capacitance with a
    100k resistor. 100k*4pf=0.4us

    At any frequencies higher than 1/(2*pi*.4us)= 400kHz, the diode will
    look mostly like a capacitor, charged to some offset by the average
    diode current. At 200kHz, the capacitance should be showing a
    measurable, but not totally dominant effect.

    In other words, the frequency capability has a lot to do with the rest
    of the circuit it takes part in.
  4. DaveM

    DaveM Guest

    Such as the capacitance of the probe being used to see the waveform. This
    can have a significant effect on the displayed waveform in a high impedance

    Dave M
    MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
    the address)

    Never take a laxative and a sleeping pill at the same time!!
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