SEM riddles (was: AW: Opinions: On op amp replacement

[Rene Schmitz]

At 12:53 02.02.99 +0100, you wrote:
>
>	>And if one connects the gate of the upper fet to the drain of the
>lower
>	>fet, then you have a nice and very linear inverting amp. The signal
>is then
>	>fed to the other gate.
>
>I didn't know that - sounds interesting !

Look at the mic-preamp on my homepage! This circuit was invented in the
late 60's by a japanese. It was originally build with triodes. Triodes have
almost a square law like the FETs (I think Ia ~ Ug^(3/2)). With well
matched FETs it is easy to get gains from 50 to 500. Better matched FETs
will allow higher gains.

>
>	>What I was referring to was the FET buffers in the filter, where
>there are
>	>a few
>	>FET-resistor source followers followed by a 741.  Here I would
>simply use a
>	>jFET opamp.
>
>I wouldn't, at least not generally. If you look at the SEM circuit, LP
>filter
>output, you have a two stage buffer made of a FET and an opamp.
>But only the FET (Q40) is inside the filter's feedback loop, not the
>opamp (A16). I think that Oberheim did that deliberately, in order to
>avoid the delay of the opamp, which would make the Q factor of the
>filter less independant of cutoff frequency. 

Nah! I don't think so, since the slewrate and therefore "delay" of a 3080 
is almost a linear function of Iabc. And here the 3080 drives a capacitive
load! And the unity gain bandwidth of a 741 is some 1 MHz! 

>A modern JFET-input
>may be faster than a 741, but hardly faster than the FET alone.
>The downside of Oberheim's circuit is a little nonlinearity from the
>simple FET source followers, of course.

We all know that a little nonlinearity here and there can be quite pleasant.

>But what really puzzles me is that the other 741, the one that buffers
>the BP output, is taken *inside* the second feedback loop of the
>filter ! Now why is that ?? And look how the circuit is drawn. This
>"smells" like the opamps were added to a working VCF later, to buffer
>the outputs (you need very low impedance for the potentiometer that
>selects the filter mode !) ... and then he chooses to place the LP
>buffer outside the loop, and tries what's best for the BP buffer, and
>places it inside the loop.
>Ok, that's all speculation of course. In case I am on the right track,
>what would be the reason for these decicions ?

>	>A second thing is that the dual-FET follower is powered from 18.5V,
>so
>	>there are no saturation effects when the integrating cap gets
>charged to 15V.
>	>(I ran into this problem with my last VCO design....)
>
>Same for me. Learned it the hard way, too (;->).
>
>Many other designs use an auxiliary voltage (like 10V) here, so the
>FET can be powered by the "normal" 15V.
>(Korg MS synths have the 18V version, too.)

That is the scheme I will adopt, till then I will have a little bit of
clipping.
In a new design I will use an opamp-integrator and a schmitt-trigger to
have trip points of 5V and 10V.

Sorry for the late reply, but I couldn't check my email for a while.
And sorry if this has been already covered here. (Ieek, 300 mails ;) 

Bye 
 Rene

             ,   : (uzs159 at uni-bonn.de)
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