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Re: Parasitics?

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Well gee, it's a simulation, you don't even have to clip instruments onto 
the thing to see the current and voltage everywhere!

If you look at the V and I versus time, at the collector, you will see that 
Ic / (dVc/dt) is proportional to Ccb.  In other words, the transistor is 
off, and the capacitance is being charged through the 47k resistor.

Simple solution: remove the charge!

Connect collector directly to the load.  To set charge current, shove a 
resistor between +V and emitter.

Now you have a transconductance stage: voltage in, current out.  Or, with a 
resistor from +V to base, a current mirror (current in, current out).  Or a 
diode-strapped transistor plus a resistor (same thing, less drift and 
offset).

You still get some charge injection because of the base voltage moving 
(because it's Ccb, not just C to +V).  This can be reduced by using a 
relatively small emitter resistor (for, say, 0.1mA output, you only need to 
tug on the base by 0.1V, if you use a 1k resistor).  Or it can be isolated 
by cascoding the collector current with a second transistor (whose base 
voltage is fixed somewhat below the lowest base voltage you want to use on 
the first transistor).

HTH,

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com


"rickman" <gnuarm@gmail.com> wrote in message 
news:oaf7ro$efu$1@dont-email.me...
> 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?
>
> -- 
>
> Rick C 

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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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