[sdiy] OTA question

John Richetta jrichetta at earthlink.net
Thu Sep 2 09:09:21 CEST 2010


Hi Ian,

Would it be possible to post a (preferably calibrated) picture of the  
waveform?  Does the waveform simply round off, or do you mean that it  
inflects downward (or...)?  What supply voltage(s) are you using?   
What op amp is being used for the V-I converter?  And what  
transistor?  Have you put a scope on the V-I op amp output?  Is *that*  
op amp using adequate pull-down/up?  Is there any capacitance anywhere  
in the V-I circuit (possibly amplified by Miller effect??)?  I assume  
not, but...

-jar


On Aug 29, 2010, at 1:56 PM, Ian Fritz wrote:
> I'm working on a Tri core VCO based on a CA3080 OTA.  It's similar  
> to the EN thru-zero VCO, which is up at Bernie's site.  Currently,  
> its output is swinging 0 to 6 V.
>
> When I drive the OTA bias current from a pot across the PS and a 22k  
> series resistor it works fine.  But when I drive it with a linear V- 
> I converter the waveform at low frequencies is distorted.  The  
> distortion makes the waveform dome shaped, ie, it is always curved  
> downward.  It appears that there must be some kind of leakage which  
> increases with increasing output signal.  But where exactly is it  
> and why would it only show with the active drive?
>
> I've tried a couple of versions of the linear V-I converter, both of  
> which are common and include a PNP transistor driving the control  
> input pin from its collector.
>
> Any thoughts?
>
>  Ian





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