LM13700 vs. CA3080

[Joachim Verghese]

On 19 Jun 1996, Christopher List wrote:

> Is there any difference between 1/2 of an LM13700 (not counting the buffer)
> and a CA3080?  Is the decision to use one or the other just based on
> whether you need one or two and whether or not you can use the buffers?

While the data sheet specs are very similar for the 3080 and 13700,
there are some differences that might be significant for your particular
application.

First, the transconductances of a 13600/13700 are relatively well matched.
For example, in a 2-pole VCF application, you typically want to have good
cut-off frequency tracking between the two stages. This is easy to achieve
with a 13700, but not necessarily when using 3080s, since the latter might
have a device-to-device transconductace spread as large as 2:1. 

Mis-tracking between VCF stages alters the cut-off curve, and degrades
'maximun usable Q' performance. (Incidentally, the 'VCF Balance' trim
in the early ARP Odyssey was used to compensate for this mis-tracking
effect, not for cancelling CV feedthrough, as one might expect).

Second, the current mirrors in the 3080 are said to be far from ideal,
adding distortion to the processed signal. For synth applications, this
distortion might not be all that bad, though.

As has been pointed out before on this list, it seems the 3080 has
gotten better over the years, with improved manufacturing processes,
etc, at least in terms of noise and offset performance.

-joachim