VCA chips

[Don Tillman]

Time passes...
(Sorry for the delay, my day job has been calling.)

On the topic of JH's suggestion of running the 3280 OTAs with an Iabc
of up to 10mA, enough current to eliminate the need for any LED drive
circuitry (just place the LED in series with the OTA's Iabc pin).

First a correction: I mentioned that the plots in the data sheet for
the 3280 didn't show an Iabc over 1 mA.  The data sheet I was looking
at was from an old Harris databook; if you grab the current datasheet
from the Harris website there's one curve of transconductance vs. Iabc
that does go up to 10 mA, and sure enough, the transconductance starts
slipping starting a little above 2 mA.

But I think the real issue is power dissipation.  Running a 3280 at an
Iabc of 10 mA means the total device current will be a little more
than 20 mA (due to the current mirrors) and with the standard +/- 15
volt supplies, that'll be a power dissipation of about 0.6 watts.  
That puppy is going to get hot!

  -- Don