[sdiy] Overdriving 4069UB

[Harry Bissell Jr]

Hi Seb
  
  A good shottky might be the 1N5817
  
  I have done input overdrive with 100K series resistor. Two notes...
  I don't intentionally overdrive at that level... and in your linear case
  (I assume you have a feebback resistor) making the input much higher
  will make the feeback value much higher as well...
  
  why not a low impedance voltage divider to reduce the opamp swing first
  to below the 15V limit ?  Knock the gain down first, then go through the
  22K input resistor. 
  
  28V p-pk sounds like the opamp is +/-15V supply. Then you go to a 0-15V
  on the CMOS ???
  
  Are you AC coupling into the CMOS ??
  
  H^) harry

Seb Francis <seb at burnit.co.uk> wrote:  Hi Harry,

Can you give an example part no. of a suitable schottky clamp diode?  
Would a 1N4148 do or would it be better to use something with a lower 
voltage drop?  I guess I need them between input and VDD and between VSS 
and input (i.e. in parrallel with the internal protection diodes).

What would you consider a safe value of input resistor to drop the 
voltage from 28V ptp to 17V ptp (I guess the max current flowing in 
either protection diode would be (28-17)/2 / R, so about 0.5mA 
currently).  Unfortunatly they don't spec these things on the datasheet :-(

Seb


Harry Bissell Jr wrote:

> with a 10K resistor in the inout you might be over the edge. I'd be
> concerned. You might want to use shottky clamp diodes instead
> of allowinng the CMOS input diodes to clip.
>
> The input clipping 'may' be part of your sound. Who knows ?
> I'd expewriment a little more with it.
>
> H^) harry
>
> */Seb Francis /* wrote:
>
>     Well, I've been getting some really nice results with using between 1
>     and 3 stages of a 4069UB for distortion. The MOSFET clipping has a
>     really warm sound to it. (Incidently the 4049 really wasn't so good -
>     the clipping was way too asymmetrical.) Anyway, more on this later ..
>     I'll publish the final circuit in due course.
>
>     What I'm concerned about in my circuit design is whether it is
>     safe to
>     drive the 4069 beyond its absolute maximum input voltage. The quoted
>     absolute maximum range is -0.5V to VDD+0.5V (VDD=15 in my circuit).
>
>     The input current is limited by a 10K resistor (with a DC blocking
>     cap
>     in series before it), so although the opamp driving it can kick
>     out up
>     to 28V ptp, the voltage at the 4069 input gets clipped to approx 17V
>     ptp. This is a bit beyond the quoted absolute maximum, but I guess as
>     long as the current through the internal protection diodes is
>     limited by
>     the 10K resistor then this is ok?? Is this a safe assumption?
>
>     Seb
>
>
>



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