[sdiy] Filter Structure Question

[Scott Gravenhorst]

=?ISO-8859-1?Q?Ren=E9_Schmitz?= <uzs159 at uni-bonn.de> wrote:
>Hi all,
>
>harrybissell wrote:
>> The technique of using a spare inverter to estimate the bias point
>> is an old (and useful one).  If I followed correctly, you want to tie the
>> output of that inverter to "real" ground ?  (to the center of a pair
>> of resistors connected to the positive and negative rails).  I don't
>> know about that part. The output of the inverter would have only a
>> very limited ability to move that point to the correct one.
>
>Besides that, the supply to the 4069 wouldn't be very stiff, and this 
>could lead to "interesting" if not disturbing side effects, since the 
>current consumption of the gates varies with the signal.  It would lead 
>to severe crosscoupling between the gates, something which is already 
>present to some extent, since the internal traces and bonding wires were 
>never designed for the amount of current present in a linear application 
>of these chips.

This is why my original idea was to make this filter a self powered single
4069UB device.  Also, using a single 5 volt supply lowers the current.  I
may even go as low as 3 volts (heh heh, 2 D cells? - just kidding).  In that
configuration, the power supply resistors are not required which will make
the span across Vss and Vdd quite stiff.  However, the signal ground may
move around somewhat with respect to Vdd and Vss, I'm not sure how much and
I'm not sure how much it will matter, probably the lower the input swing,
the less it will matter.  My first experiments (after I buy a new battery
for my DMM) will be with this single 4069UB filter.  If that works
sufficiently well (i.e., sounds interesting), I may do other things with this.



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