Compensated Exp generators and VCO's

Don Tillman don at till.com
Thu Jun 8 01:47:49 CEST 1995


Some random comments on the VCO...  (I've been away for a while.)

You don't need the opamp after the LM394.  In fact it makes things
worse in that you have the LM394 collector current being turned into a
voltage with the opamp and being turned back into the same current by
the integrator.  In the process low notes get affected by any small
offset voltages.  I like the simpler ARP and MOOG approach where you
charge the cap directly with the exponentional generator output
current.  Then you can use a vfet or bipolar transistor to discharge
the cap real fast so you don't need the discharge compensation (which
screws with the amplitude and gives the sine waves a bit of an edge).

You can probably get away with a regular pfet for the precision
current source. 

I'd also recommend using a transistor array where you can heat the
substrate directly with very little current (instead of aiming a
blowtorch at the LM394).

It seems that a more accurate comparator can be built by wire-or-ing
two comparators together; one detects the positive trip point, one the
negative.  You see this configuration in lots of application notes.

  -- Don







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