[sdiy] Need help with simple VCO design

[Julian Bunn]

Hi Tim,

Thanks for the reply. I am sending you, off-list, the Spice
model I am using.

I spent some time looking for a straightforward Voltage to
Log current setup, and the 2N3906/2N3904 seemed to fit the
bill. In the Spice model, the behaviour of the output frequency
vs. the input voltage is (very) approximately log ... it's not
at all linear :-)

Regarding the circuit: in fact I have two photocopies of Applied
Ideas of about the same vintage (1975) that are similar to this.

Julian

PS I read your analysis of VCF ladder networks recently: great
stuff!


Tim Stinchcombe wrote:
> 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
> 
> 
> 
> 
> 
>