[sdiy] They aren't sawtooths, they're ramps

[Ian Fritz]

At 03:59 PM 11/5/2009, David G. Dixon wrote:
>I'm using 20k resistors to +/-15V on the + input, and a positive feedback
>resistor of 15.8k, which is giving exactly 5.000V output levels.  Sizing the
>speedup capacitor across the feedback resistor is relatively important.
>With a speedup capacitor of between 33 and 47p, it takes about 135ns for the
>output to cross zero from +/-5V.  If the speedup capacitor is above or below
>this optimal range, then the zero-crossing time climbs to about 170ns.
>
>The discrete version takes about 200ns to switch.  It takes about 150ns to
>make up its mind, but then the actual switching takes place very fast,
>within about 50ns.  However, it doesn't begin switching very swiftly.

Many thanks for that info.  I always had trouble accepting that the 
discrete version was slower, since everyone else seemed to think it was 
much faster.


>On balance, I think it will be easier to use the LM318 for designing
>rock-solid threshold voltage levels, and of course it begins to switch
>instantly once the threshold level is achieved by the integrator.  The
>discrete switch is a little bit dicey in this regard.

If you like, you might try looking at how other opamps do, say OPA134.


   Ian