Exponential current source blues

[Steven Varner]

Speaking of EN PCC VCO option 2...

The difficult and touchy parts of the standard discrete VCO's and 
VCF's is the exponential current source.  The parts include 
matched dual transistors and tempco resistors as well as a 
regulator IC for the current reference.  Since I've been shopping 
for parts, I've found that these give me the biggest headache.  I 
know from archived posts that it is not a new issue.

1. Why do VCO's (and VCF's), that at heart are made up of simple 
op amp oscillators (like LFO's), need the exponential current 
source as given in most common circuits?  Is it mainly for scaling 
of CV's? (1V/oct to 18mV/oct).  Can another structure do this 
besides tempcos and transitors (either matched OR arrays)?

2. What are some of the latest sources of tempcos? Vishay 
Electronics is still apparently carrying them (See my next post 
for info).  Can someone check out the following web page and see 
if these parts might work too:
Koa Speer
http://koaspeer.com/surface/lt73_1.htm


3. I called several of the distributors of Analog Devices products 
and asked about MAT03's (and 02's).  After being transfered a lot 
because I wasn't a company, I would finally get a price.  
What is the difference in quality between the various versions 
(MAT03AH, MAT03EH, MAT03FH)?  If they are matched and the 
transitors have the same substrate should it matter much in a VCO 
circuit?  The prices range from about US$5.15 to US$16.00 
(although Wyle had them for $28.35!).  The FH's seem the cheapest.  
The 5.15 price was from the First Stop division of Pioneer 
Standard (1-216-349-9310, I talked to Mike Boyer).

Steve Varner