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Re: Parasitics?

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On 3/16/2017 9:21 PM, Sylvia Else wrote:
> On 17/03/2017 10:44 AM, rickman wrote:
>> I'm running a simulation where I'm pumping charge into a capacitor to
>> generate a voltage.  A transistor emitter/source is connected to the
>> positive rail and the collector/drain is connected to a 1 nF cap through
>> a 47k resistor.  When the base/gate is driven high to shut off the
>> current I see the current go up initially then taking almost 2 us to
>> shut off.  I've tried 2N3906 and BSS84 and a couple others selected
>> randomly.  The switching period is around 8 uS.
>>
>> Is this from the parasitic capacitance in the transistor?  The BSS84
>> only has 1 nC gate charge and still the effect is very pronounced.
>>
>> Is there a way to mitigate this effect other than using a larger cap and
>> smaller resistors?
>>
> When you try to drive the gate high, the gate-drain capacitance has to
> be charged, and the only way than can happen is for current to flow
> through R3. A back of the envelope calculation suggests that will take
> about the time you're seeing.
>
> To get it to switch more quickly, you need to reduce R3, or provide some
> other path for the current.
>
> A diode in series with R3, and a 1K resistor from the drain to ground
> improves things substantially.

That actually makes things worse by substituting the capacitance of the 
diode through the 1k resistor for the capacitance of the transistor.

I was trying to use a very low power approach by injecting charge onto a 
capacitor which would have a very low drain from it.  But if low value 
resistors have to be added, I can just use a dithered pullup/pulldown 
instead.  That may also have current injection, but it should be 
approximately equal in both directions because of the push-pull output.

-- 

Rick C

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Thread

Parasitics? rickman <gnuarm@gmail.com> - 2017-03-16 19:44 -0400
  Re: Parasitics? bloggs.fredbloggs.fred@gmail.com - 2017-03-16 16:48 -0700
  Re: Parasitics? Sylvia Else <sylvia@not.at.this.address> - 2017-03-17 10:59 +1100
    Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-16 20:19 -0400
  Re: Parasitics? Sylvia Else <sylvia@not.at.this.address> - 2017-03-17 12:21 +1100
    Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-16 23:16 -0400
      Re: Parasitics? Sylvia Else <sylvia@not.at.this.address> - 2017-03-17 14:51 +1100
        Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-17 00:57 -0400
          Re: Parasitics? bloggs.fredbloggs.fred@gmail.com - 2017-03-18 01:36 -0700
            Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-18 05:45 -0400
              Re: Parasitics? bloggs.fredbloggs.fred@gmail.com - 2017-03-18 06:33 -0700
                Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-18 12:22 -0400
                Re: Parasitics? Sylvia Else <sylvia@not.at.this.address> - 2017-03-24 13:30 +1100
                Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-24 00:36 -0400
                Re: Parasitics? Sylvia Else <sylvia@not.at.this.address> - 2017-03-24 16:10 +1100
  Re: Parasitics? "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-03-17 01:42 -0500
    Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-17 04:11 -0400
  Re: Parasitics? Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-03-17 09:53 -0400
    Re: Parasitics? Cursitor Doom <curd@notformail.com> - 2017-03-17 18:52 +0000

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