AW: Using 1K Tempcos with Electronotes designs

[Haible Juergen]

	>For those who do not want to change resistor values in the
Electronotes
	>VCO, another (but more expensive) way of making use of your tempco
	>resistors is to use two 1k tempco resistors in series.  I tried
this once,
	>it works.
	>
	>Terry Michaels

Sure, but a little expensive.

If you want to keep these 100k summing resistors 
(and there's good reason for that!), I'd suggest
the following:
Connect a low resistance voltage divider to the
output of the opamp: 91R from opamp_out to
aux_node and 100R from aux_node to GND.
Then connect your 1k feedback resistor between
aux_node and the inverting opamp input.

Because of the low voltage swing at the opamp output, 
the 180R load  should be no problem. The equivalent
feedback resistor would be something like 
1.9k tempco and 100R normal resistor in series connection, 
which is close enough to a 2k tempco resistor, IMO.
(With a 3500ppm 1k tempco resistor, this reduction
of tempco might even be desirable.)

Haven't tried it myself (I use 560R tempco resistors,
because I have some of these in the drawer ...),
but it looks nice in simulation.

JH.