Groups > sci.electronics.design > #452244

Re: Parasitics?

From	rickman <gnuarm@gmail.com>
Newsgroups	sci.electronics.design
Subject	Re: Parasitics?
Date	2017-03-17 04:11 -0400
Organization	A noiseless patient Spider
Message-ID	<oag5i4$fja$1@dont-email.me> (permalink)
References	<oaf7ro$efu$1@dont-email.me> <oag0c1$1m7$1@dont-email.me>

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On 3/17/2017 2:42 AM, Tim Williams wrote:
> 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.

Isn't that what I said, its the gate-drain charge?


> 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).

Thanks for the info.  I'm not actually building a circuit from 
discretes.  So I'll just use a voltage source to model this for now and 
check it on the bench.

-- 

Rick C

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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