My first VCF

[Paul Perry]

At 11:05 AM 15/05/98 -0400, Brian Towles  wrote:
>Paul Perry wrote:
>> Brian, with my LM13600 filters I have to use a fet follower
>> instead of the LM13600 buffer or the low freq is f*cked..
>What's the problem with the low-freq? Distortion or cutoff? I've haven't
>built anything, all my numbers are from HSPICE simulations using
>National's LM13700 model.  
>Basically, the output current of the OTA is forced through one passive
>component ( either a cap or resistor ) and the voltage across that
>component is buffered.  One problem could be if a cap was used as the
>"charging" element:  at low-freqs its impedance could get close to the
>input impedance of the darlington pair ( I would estimate 10M )
.....this is what happens... I think this has been discussed here before..
whether there is a difference between LM13600 and LM13700 in this regard
I know not.. I am using 3300pf (same prob with a similar config with 820pf).
I defy anyone to get the claimed range from the LM13600 oscillator in 
The Art of Electronics without a fet buffer..

paul perry melbourne australia