From: bill.sloman@ieee.org   
      
   On 18/02/2026 5:18 am, Phil Hobbs wrote:   
   > Gerhard Hoffmann  wrote:   
   >> Am 17.02.26 um 18:34 schrieb Phil Hobbs:   
   >>   
   >>> I'm using a square-root version of this one in a demo for a circuit   
   >>> whose bandwidth wants to be quadratic in the supply voltage.  It's fine   
   >>> inside a feedback loop, and may eventually get put in an ASIC.   
   >>> (Unfortunately they don't make the CA3096 anymore.)   
   >>   
   >> Renesas  HFA3046, HFA3096, HFA3127, HFA3128   
   >>   
   >> I got them even space-proof, but not really cheap..   
   >>   
   >   
   > I know about them, and tried making a laser noise canceller out of an   
   > HFA3046 once. Unfortunately their R_ee’ is horrible, like 2 ohms iirc,   
   > which trashes their log conformity.   
      
   There is a way of compensating for that - EMI Central Research had a   
   hyperbolic function generator which relied on a transistor to feed an   
   exponentially decreasing current into a capacitor, and it used a bit of   
   positive feedback to take out the base resistance.   
      
   My name is on the relevant patent  U.K. patent 2028503 "Improvements in   
   or relating to ultrasonic apparatus"; assigned to EMI Ltd in 1978. I   
   didn't invent it - my name got on the patent (somewhat to my surprise)   
   for the digital scheme for generating hyperbolically increasing digital   
   numbers.   
      
   You've got to increase the voltage applied to the base terminal of the   
   transistor to compensate for voltage drop across the base resistance.   
      
   That's actually very easy to do if you can put a current mirror between   
   the collector of the transistor and the rail. The current going into the   
   mirror collector current is proportional to the base current, so you can   
   use it to develop and extra voltage drop of the right size between the   
   base of the hyperbolic transistor and the negative rail. There's got to   
   be a pot in there somewhere to be twiddled reflect the actual base   
   resistance and current gain of the transistor making the hyperbola.   
      
   I don't think that is the way we did it - it got bundled into the system   
   for starting the hyperbolic voltage at right point - but whatever we did   
   was pretty simple.   
      
   --- SoupGate-Win32 v1.05   
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