[sdiy] Need help with simple VCO design

[Tim Stinchcombe]

Hi Julian,

> This is the schematic of a VCO I'm playing with:
> 
> http://farm3.static.flickr.com/2488/4217027179_c41d9bedd3_o.png
> 
> It is (very) loosely based on a circuit that appeared in
> "Applied Ideas" in Electronic Engineering magazine in Jan 1975.

...which looks like a modified version of this fairly standard one, sheet
13:

http://cache.national.com/ds/LM/LM324.pdf

> However, I don't get the range from the Log input: it
> flattens off too quickly, and I'm not sure why. I basically 
> just bolted on the log current generator without really 
> thinking too hard about it.

Usually for that arrangement of NPN+PNP the output is the NPN collector
*current* - by taking the NPN emitter, all you really have is a
voltage-follower, so you'll just have another linear response (a DC sweep on
your simulation should show this...). Flipping the whole thing over by
swapping the NPN and PNP is tempting, but then I think you'll run into
biasing problems with either or both the transistors (i.e. keeping the input
nicely referenced to ground, rather than the positive rail/whatever). If I
have any other thoughts on this I'll get back to you.

No idea about the slow starting!

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk