[sdiy] Comparator high frequency hysteresis

[Oakley Sound]

 > I've read about Tony Allgood's 'transistor starving trick' in the 
Oakley Sound dual comparator 2 documentation.

Well 'starvation' seemed a good term for it.

Basically it's deliberately using a high resistance for the collector 
resistor on the output of the comparator. This reduces the speed at 
which the output can transition between low to high thanks to the 
combination of the now high output resistance and stray capacitance. 
Combined with the standard two resistor hysteresis network this helps in 
cutting down the possible problems of a noisy transition.

Normally one uses a low resistance on the output of a comparator like 
1K. This gives mostly symmetrical fast rise and fall times and 
symmetrical hysteresis. But it also creates perturbations on the power 
supply rails which can be picked up elsewhere. Using a high resistance 
on the open collector output, I use 100K, reduces the rise time 
dramatically and reduces the impact of current pulses. Of course, you 
then need to tidy the output which is done in the Oakley Dual Comparator 
by a transistor switch and finally an op-amp for some level shifting and 
buffering.

But getting any comparator to be both precise (ie. low hysteresis) and 
to deal with very slow moving CVs is not trivial.

Tony

www.oakleysound.com