[time-nuts] Questions about TAC frontend, and some measurements

[Bruce Griffiths]

Fabio Eboli wrote:
> Hello, Bruce
>
>> Using saturated transistors as switches in the current source and
>> elsewhere isn't conducive to fast switching.
>> The traditional arrangement using current mode switches is much
>> faster and more predictable.
>
> This is something I'd like to understand better.
>
> I'm referring to this schematic here:
> http://www.flickr.com/photos/14336723@N08/8293076065/
> Q2 and Q5 are saturating toward the end of the
> ramp pulse, when the ramp capacitor C1 starts
> to go up.
> I was prepared to see the circuit I designed
> fail miserably on switch time, but it seem
> to be working, as far as I could see on the DSO.
> As far I can understand, the fact that Q2 and Q6
> don't saturate, saves the circuit, since
> at the end of the ramp, when Q1 and Q5 are
> into saturation, Q6 is able to steer the
> current to ground, and reverse bias BE (and CB)
> of Q5. Is this correct, or I was only
> lucky with the specific parts I used?
The simulation indicates that the TAC capacitor charging current is far 
from constant whilst charging.
This is due to the use of saturated switches rather than current 
steering switches.
The capacitor charging current is poorly controlled.
So in this respect your circuit fails miserably.

bruce