[sdiy] More VCO fun

[Ian Fritz]

At 10:41 AM 12/3/2009, David G. Dixon wrote:
>Right.  I had actually read that paragraph quite a while ago and had
>forgotten.  I wish I would have remembered at about 11 o'clock lastnight,
>when I was freaking out about how to fix my VCO.  Actually, about two hours
>into the ordeal, I had the idea that temperature was affecting the
>on-resistance, and that's when I decided to fix the reset timing resistor.

Yeah, that had me stumped for quite a while, too.


>One thing I am finding with this updated circuit is that I need an even
>bigger reset RC to ground the integrator cap.  I'm up to 3.3us now (33pF *
>100k) and I'm still only getting to within about 100mV of ground (resetting
>a 1nF cap from +5V).  I am thinking of going to 47pF, but I know this will
>wreak havoc with high frequency tracking.  It also defeats the purpose of
>having a high-speed comparator (although LM311 isn't exactly "high-speed" as
>comparators go...).

Isn't it just that you need extra time to make up for the slew time that 
you had with the opamp?  Still, it's an unusually long time 
constant.  Remind me, what value is your integrating cap?


>However, r_DS also
>decreases with increasing V_GS(off), such that r_DS is about 90ohms at
>V_GS(off) of -8.0V at 25degC.  Does this mean that holding the gate at lower
>voltages when off gives lower channel resistances when on?

No.  (The device has no way of knowing where you had been holding the gate 
previously.)  V_GS(off) is the pinchoff voltage of the device (a device 
parameter), not the voltage you are holding the gate at.


>Also, the data
>table says that 2N5485 has a minimum V_GS(off) of -0.5V and a maximum of
>-4.0V.  From the chart, this would imply about a factor-of-four variability
>in the on-resistance.  Am I interpreting this correctly?

Probably. Just make sure to check if the curve is for a whole family of 
devices, in which case you have to use just the part of the curve that 
pertains to your particular device.  What values is your simulator using 
for these parameters?

For the 2N4391 device I use, the V_GS(off) ranges from -4V to -10V and the 
corresponding change in R_DS is about 50%.

Notice also, while we are at it, that the saturated drain current increases 
with increasing pinchoff voltage.

   Ian