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<DIV><FONT face=Arial size=2>hi,</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I've been studying expo converters for about 2 or
3 weeks now because I wanted to adapt an active CV summing stage to René
Schmitz's 4069 VCO. It also is going to use a PTC tempco resistor instead of the
NTC resistors. What I've come up with seems like it should work, atleast
mathematically, but I'm not sure about a few components which I've seen on other
expo converters.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>here is the schematic for the 4069 VCO with a
modified expo converter. Everything after the MAT-02 was copied from the
original 4069 VCO</FONT></DIV>
<DIV><FONT face=Arial size=2><A
href="http://www.sdiy.org/destrukto/4069_VCO.png">http://www.sdiy.org/destrukto/4069_VCO.png</A></FONT></DIV>
<DIV><FONT face=Arial size=2>this seems to be the common way of making an expo
converter using the MAT-02 as the differential pair. The part I'm unsure about
is the capacitor in the feedback part of the 2nd opamp (the opa2277). I picked a
100pF capacitor here although I have no idea what it should be, or how to
calculate what it should be. I've been told that it's purpose is to
prevent the opamp from oscillation but I just put it there because everyone else
does...</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>so, does anyone have any comments, suggestions,
explanations about this capacitor? I've seen it in many of your designs although
the value seems to range from 1pF to 680pF in different designs.
</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>one more question. I've chosen 100K pots for all
the pots.because I'm unsure how to pick the values of pots in a voltage
divider. Is there some rule of thumb to follow when choosing pot values?
It seems like the pot will affect the voltage seen at the input of the opamp so
this could cause a problem?</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>thanks...</FONT></DIV>
<DIV><FONT face=Arial size=2>ryan</FONT></DIV></BODY></HTML>