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two exponential converters in one ca3046?

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okelk
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two exponential converters in one ca3046?

Post by okelk »

Is it ok to pack two exponential converters from 2 different VCOs, or one VCO and one VCF into one ca3046?
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Post by fitzgreyve »

Q1 and Q2 (pins 1-5) are already wire internally for use as a diff pair - I see no reason in theory why Q3 (6,7,8) and Q4( 7,10,11) couldn't be used as a second one.

I'd avoid using Q5 (12,13,14) in a diff pair as pin 13 is the substrate - "The substrate (Terminal 13) must be connected to the most negative point in the external circuit to maintain isolation between transistors and to provide for normal transistor action."

If doing this, you can't use a heater method of temperature compensation - not enough transistors (in addition to the diff pair, you need one to heat and one to temperature sense).
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roglok
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Post by roglok »

keep in mind that the differential pair (Q1/Q2) is 'matched' to:

VBE: +/-5mV
IIO: 2uA (Max)

http://www.intersil.com/content/dam/Int ... a-3046.pdf

while Q3-Q5 are not. you could of course go through a bunch of 3046s manually and select those that are within your desired specs...
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Post by Jack Gull »

wait a sec... i thought that since they were all from the same substrate, frabrication process and all they would have been really simillar if not "perfectly" matched
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Post by roglok »

Jack Gull wrote:wait a sec... i thought that since they were all from the same substrate, frabrication process and all they would have been really simillar if not "perfectly" matched
well, it may help that they are on the same substrate and real life specs may be better than in the datasheet but if they were 'perfectly' matched there probably would be no such products as MAT02, SSM2210 or THAT300...
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Post by Jack Gull »

yeah but i thought that the main point of makng a transistor array was to not select them.. just put the chip and forget about it :)
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okelk
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Post by okelk »

roglok wrote:keep in mind that the differential pair (Q1/Q2) is 'matched' to:

VBE: +/-5mV
IIO: 2uA (Max)

http://www.intersil.com/content/dam/Int ... a-3046.pdf

while Q3-Q5 are not. you could of course go through a bunch of 3046s manually and select those that are within your desired specs...
Ah! Interesting.
I read over that part of the datasheet...

but I suspect that the second pair is still good enough for my purposes...

I just happened to be looking at the schematic of the rhodes chroma voice board. There they use one ca3086 for the exponential converters of the two VCOs.
...so I guess doing the same thing with a ca3046 shouldn't be too bad either...
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Post by Don T »

Jack Gull wrote:yeah but i thought that the main point of makng a transistor array was to not select them.. just put the chip and forget about it :)
The main point was to save space, and reduce cost.
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Post by guest »

on page 2 of the datasheet, they list the magnitude of the offset voltage for the remaining transistors, and its the same spec as the "matched pair". not the greatest matching, but probably good enough (current match is not spec'd for the other transistors, however). my only concern would be heating from one set causing drift in the other. but, if Vce is kept small, that probably wouldnt be a big issue.
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The Real MC
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Post by The Real MC »

It's been done before.

The Oberheim SEM/OBX VCOs share a 3086 for expo conversion.

The Minimoog oscillator card uses two 3046s between the three VCOs for expo conversion. The interesting thing is VCO 1 and 3 use the diff pair with common emitter but not VCO2.

The Minimoog originally used the SG3821, pin-for-pin compatible to 3046 but much better matching.
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Post by mskala »

Jack Gull wrote:yeah but i thought that the main point of makng a transistor array was to not select them.. just put the chip and forget about it :)
All ICs are to some extent selected. The chips come off the assembly line and they get tested for specific parameters. The ones that meet the spec get sold, the ones that don't meet the spec get sold by Radio Shack, I mean, discarded. Only the parameters that are officially guaranteed will be tested, because testing anything else is going to cut into the yield. So maybe you'll be lucky and the other transistors not specified to match will happen to match just because of the nature of the manufacturing process, but it's definitely not something you can count on.
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