What to do with unused lm13700 buffers?

[slacker]

I've been playing around with the Oscar synth VCFs that Paul Perry posted recently and am going to build them on vero to experiment with the circuit a bit more.
I'm not building the VCA part so I've got 2 of the darlington buffers doing nothing at the minute. Do I need to do anything with the unused pins or can I just leave them unconnected?
I've had a look at some of Ray Wilson's designs and he just leaves them floating so I guess it's Ok, I just wanted to make sure there wasn't a better way to deal with them.

Thanks in advance.

[gez]

Personally, I'd leave them be.  However, why not make the Darlingtons work for you?  Wire up one as a high impedance emitter-follower to buffer the input of the filter? (I only gave the schematic a quick once over, but it seems to assume that the filter is being fed from a low impedance source...unless I missed something).

Another use might be a status indicator for bypass with DPDT switch (see RG's article on bypass).  LED and series resistor from emitter to ground?

[slacker]

Good idea Gez, no reason why they can't be made to earn their keep.
You're right about the low input impedance. I'm currently using an opamp input buffer but a transistor one would do the job just as well, and save on components and board space.

[gez]

I'm currently using an opamp input buffer but a transistor one would do the job just as well, and save on components and board space.

Indeed!  I'm sure you know this, but just use the same bias you've used for your op-amp.  It should work fine as the buffers in the chips are Darlingtons, so use little base current.  If you've used a huge resistor to the divider, however, it's worth checking the data sheet to see what you can get away with (I'm pretty sure they publish figures for the buffer by itself).  Alternatively, you can bootstrap for high impedance.

[JDoyle]

It should be noted that in the 13600 the output buffer's biasing was tied to the same bias current as the OTA, so using them outside of situations where they are connected to the OTA would be unadvisable.

The 13700 buffers are supposed to be independent so they should be good to go, and they are already there!

Regards,

Jay Doyle