Discharge Switches (was: Re: J105 )

Rene Schmitz uzs159 at uni-bonn.de
Sat Jan 23 22:21:28 CET 1999


Ian Fritz wrote:
>An interesting looking device, indeed. I haven't been able to find these
>in small quantities, though. Do you have a source? It would be very
>interesting to see what kind of performance you could get. One other
>point: the input capacitance is about 10x that of the 2N4391.  As Terry
>pointed out, this capacitance causes some spiking. I don't think
>switching speed is much of an issue as both devices are fast compared to
>the time required for discharge.

>Magnus Danielson wrote:
>>
>>
>> There is also a number of possible improvements over the 2N4391, if
>> you are willing to trade switch speed for lower resistance and thus
>> have shorter reset time of the oscillator.
>>
>> Without much deeper thoughts, I picked out the J105 which has an
>> rDS(on) of maximum 3 ohm as compared to the 30 ohm of the 2N4391.


Don't overlook the leakage effects on the lower end of the scale.
It would gain nothing if you find a FET with one halve the R(on) and double
the leakage. 

And two more thoughts on the subject of discharge switches:
(...also a very recyclable thread...) 

I found an interesting idea in Horowitz & Hill, they show how to make a
high precision integrator, as switch they use two Fets in series (though
they use MOSFETS).One has the larger portion of the voltage across it, and
the other a very small Vds. The middle of this arrangement is grounded via
a 1M Resistor, so the leakage current of the one with the high voltage
flows to ground. Kind of a guarding technique. Now what I think is this can
be done similarly with JFETS (then we would have essentially only gate
leakage contribution which is 1-10 pA). 
One can use a smaller cap value to bring the speed up at high frequencies.

And the second thing, charge injection from the gate-channel capacitance
can probably eliminated by adding the inverted discharge signal to the signal 
thru a tiny cap. 

Bye
 Rene

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