[sdiy] Does Source = Drain???

[Ian Fritz]

At 12:18 PM 6/13/2010, Antti Huovilainen wrote:
>On Sun, 13 Jun 2010, Ian Fritz wrote:
>
>>My through-zero Saw VCO requires discharging both positive and negative 
>>ramps.  For this bipolar operation I use two MOSFETs in series so that 
>>only one protection diode is turned on at any time. This is the standard 
>>solution as far I have seen.  You increase the parasitic capacitances
>
>Don't you lose the advantage of the mosfet being able to pull the signal 
>to ground (without constant voltage drop like bjt's have)?

Hmmm ... I'm not exactly sure what you are getting at here.  The series R 
of the forward-biased protection diode?  Remember the diode curve goes 
through (0,0).

The ramps reset quite close to zero (within about +/- 30 mV), after 
compensation for the charge injection at reset.  Depending on the width of 
the discharge pulse they can actually overshoot 0V by a bit.

The final proper time-reversal waveform is derived from the two ramps by 
shifting the upramp down and the downramp up so they span +/-2.5V.  The 
adjustments for getting this waveform very close to perfect take care of 
any imperfect reset.

   Ian