[sdiy] linear schottky TTL operation

[Roy J. Tellason]

On Saturday 18 June 2005 11:50 am, anthony wrote:
> Is it possible to use schottky based TTL gates in the linear mode? I
> figured since the transistors don't saturate that they could. I guess a
> better point to ponder is would there ever be a reason to want to? I was
> thinking about using the 74LS121 that I have for a pulse/ramp generator but
> I wouldn't need to run this in linear mode to do that at all. Using dual
> monostables seems like a cool way to do this too. I was also thinking about
> the 74LS06 and 74LS19 - used as oscillators they wouldn't need to be run in
> linear mode either (well definitely with the '06 but I suppose its having
> open collectors makes it special). And that led me to wonder about using
> NAND gates or inverters in something like the Wasp filter?
>
> Having bucketloads of old TTL gates makes you think about stuff you
> wouldn't normally think about.

Schottky-based stuff ain't *old*,  7400 (without the "LS" in the middle of the 
number),  now _that's_ old...   :-)

> And heck what have I got to lose? I don't care if I blow out a few chips. I
> think I'll whip something together that I think /shouldn't/ work in both
> schottky and non (I think I have some ceramic '04s). I'll check it out on
> the 'scope. Sometimes I wonder if I spend too much time thinking about
> things this way... but nah...

Do you have a copy of Don Lanaster's TTL Cookbook?  It'd probably be worth 
your while to get one if not,  as he gets into that stuff somewhat.

I've done very little experimenting along these lines.  I can say that the 
circuit given as some typical astable using a single schmitt inverter (or 
gate) didn't work right off when I tried it with TTL instead of the CMOS I 
was looking at in the schematic -- the impedances were all wrong,  and I had 
to go to a *way* lower value of resistor with correspondingly bigger value of 
capacitor to get the same effect.

TTL also generates noise spikes,  trying to run it in any sort of linear mode 
is gonna be worse in this respect.