MOTM & other voltage conlversions

[Paul Schreiber]

Ususally, it is fairly simple to convert designs from +-15 to +-12. Simple resistor scaling
is sufficient in most cases. However, you need to understand the purpose of the
supplies to scale the right parts in the right ways.

For example, in the MOTM-110 Ring Mod/VCA, the CEM3330 needs a baseline bias current
which decides the transfer (gain) function. The value I use is 100uA. With a +15 volt
supply, there is a 150K 1% resistor to set the current. For a +12V system, this resistor
must be changed to 120K 1% to get 100uA (12/120K).

John's comment about the gain scaling error is to be expected. The 3330 (and most other
OTA gain stages) are CURRENT based, not voltage based. The resistors are selected to
generate certain currents, and if you change the supply rails the current decrease.

For critical circuits, such as VCOs, I will NEVER use the supply rails as a reference. I use
the Analog Devices REF-02 or equivalent. This adds cost, but will keep supply voltage issues
at a minimum.

Also, most 'older' bipolar op amps (353, TL082, etc) use PNP current mirrors in the outputs, which
means you cannot swing + or - to the rail. More like 2 volts from each rail. A +-12V supply will
not give +-10V swings unless you carefully select the op amps (if you need that big of a swing).

If you plan to use a MOTM kit on +-12, please email me for resistor changes. However, this is
NOT recommended.

Paul Schreiber
Synthesis Technology