Glide circuit of the ProphetV (PART 3)

[Harry Bissell]

JH is correct. The diodes do nothing (save protection).  The OTA input is saturated,
and only comes out once the inputs are within about 150mV of each other. With the
resistor in place, theres a divider and you get out of saturation earlier.

I did see some offset at very low levels, the diode (linear) version is too small to
care. The
expo would throw tuning off, but it looks like you could adjust the vco to
comepnsate...
If you have a ProOne... (now if its a ProV its a little harder...)

Maybe this ends the thread... NOT !!!

H^)

Harry Bissell wrote:

> Hey Hey JH:
>
> Now I remember... The trick was to remove the diodes and substitute a
> resistor
> across the inputs of a substantially lower value. I think the diodes do
> "clamp" the
> feedback, so the differential arcoss the inputs is .7volts, and
> reasonably constant
> until the input and output are within the diode drop.
>
> Replacing this with a resistor of small value will limit the
> differential voltage
> (its a divider) but the difference will be a proportion, instead of a
> constant.
> Eventually, in either case the DC level will be reached.
>
> My model of course doesn't have any nasty offsets etc. But it does show
> the idea of
> the linear/expo operation. This is using the LM13700 model from National
> Semiconductor...
>
> And a "perfect" op-amp. ;^)
>
> Damn I had to think for an hour about this one... I remembered it was
> "so easy" but
> I never did it... 'cause I don't like expo glide anyway.. LOL!!!
>
> H^)  harry
>
> Harry Bissell wrote:
>
> > 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.
>
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