[sdiy] DCO ramp core / capacitor discharge transistors

[Dave Manley]

re: analog switches - the octave cat schematics are a good example of 
using analog switches in a vco.

-Dave

harrybissell at wowway.com wrote:
> The transistor will eventually be limited by the voltage drop
> across it, and the FET will limit by on resistance. Eventually
> the FET will cause a complete discharge. Either one will work...
> analog switches are another possible choice.
> 
> I'd use the NPN if it will work, its cheap and probably less fussy than the FET.
> 
> H^) harry
> 
> 
> 
> 
> On Tue, 16 Dec 2008 22:00:47 +0000, Tom Wiltshire wrote
>> Hi All,
>>
>> I'm having one of my periodic "Should I build a DCO?" moments...
>>
>> Consequently, I've been looking at the Electronotes/ASM ramp core,  
>> and various schematics for DCOs, including the JX8P.
>>
>> I'd have thought that since the ramp core of both of these circuits  
>> is identical, you'd see similar components, but you don't. The ASM  
>> VCO uses a FET chosen for low on resistance (2N4856 seems favourite) 
>>  whereas the JX8P uses a normal NPN, the 2SC945. The JX8P also uses  
>> half the capacitor value (1n versus 2n2) so probably doesn't need  
>> such a low on resistance.
>>
>> Can anyone tell me why we need to use a FET for the VCO, if the DCO  
>> can get by with a NPN?
>>
>> Thanks,
>> Tom
>>
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> 
> 
> Harry Bissell & Nora Abdullah 4eva
> 
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