Glide circuit of the ProphetV

[Harry Bissell]

Hey JH:

thanks for the info. I'm not sure I agree about the diode function... maybe.
There was a mod that sequential wrote for "expo glide... I'll try to find it an
make this more accurate. Its been a long time and I don't remember it
perfectly...

H^)

jhaible wrote:

> > > And, when you use the 13700, you don't need the buffer, because you can
> use
> > > the one on the chip right?
> >
> > No... the darlington buffer can only "pull up" and has a passive
> pull-down. so
> > it can source lots of current, but sink is fixed... this would cause
> asymmetry
> > in the glide.
>
> No. Not if you add a small enough resistor from the buffer's output to the
> negative supply. An emitter follower has low output impedance regardless of
> polarity,
> as long as you don't steal *all* the current from the transistor.
>
> > The diodes clamp this and the resistor limits the voltage to the clamp. as
> a
> > side effect this makes the ramp linear for large steps (input differential
> is
> > clamped at .7 volts) and sort of exponential for small steps. In practice
> you
> > get a linear glide.
>
> The diodes are just for protection. They don't have anything to do with
> linear
> or expo glide. (when they start to conduct, the OTA's input stage is long
> saturated.)
>
> > If you remove the diodes and resistor and ground the negative input you
> will get
> > exponential glide
>
> No. That's the case if you remove the diodes and use small resistors to GND
> at the OTA inputs.
>
> (and problems of offset voltage which you must trim out for DC
> > accuracy)
>
> Yes. That's why expo glide with OTAs is a bad idea.
>
> > The feedback assures DC accuracy.
>
> Yes. What you see at the output is the offset of the input, i.e. small.
> In the expo case, the offset voltage is amplified according to the chosen
> amount of negative feedback. Small feedback -> high offset,
> strong feedback -> glide becomes more and more linear for larger
> intervalls.
>
> JH.