Differences in OTAs?

macumbista · Posted: Sun Oct 30, 2011 1:01 pm Post subject: Differences in OTAs?

Been looking at some of the simple VCA threads in the forum here. When a schemo calls for an OTA, could the following be used interchangeably?

LM3900 (is this actually an OTA?)
LM13600
LM13700
Others (CA3080...)?

Not so interested in minutia regarding performance, it's just I have a random selection of chips in the parts box and I'm curious what I can get away with using. Thx!

EDIT: looks like what I've got is a lot of LM13700 and CA3080. Dual and single of the same thing pretty much, right?

Mongo1 · Joined: Aug 11, 2011 Posts: 411 Location: Raleigh NC

Hi -
The 3900 is a norton amp - not the same as an ota really.
The 13700/13600 are OTAs and are pretty much interchangeable - the 700 has somewhat better performance.
The ca3080 is an OTA but is not interchangeable with the 13x00s.

Gary

EdisonRex · Posted: Sun Oct 30, 2011 1:49 pm Post subject:

LM3900 isn't an OTA. It's a Norton amp (transresistance), which although useful, they aren't OTAs (transconductance). This refers to the input, not the output, of the opamps as they are all voltage output devices.

13600 and 13700 are basically 2x 3080s with a few extra tweaks (linearizing diodes, output darlingtons). The CA3280, now mostly unobtainium, outperforms them all, but basically they all do the same basic thing, transconductance, and for most of the circuits we deal with, are mostly interchangeable with adherence to pinouts, etc.

Anyway, 3080, 3280, 13600 and 13700 are basically compatible for all intents and purposes, although there are undoubtedly some circuits which would notice the difference and need tweaks. That said, 13x00s are readily available and none of the others really are now.

Note the pinouts are different between the single OTAs and the dual ones.

macumbista · Posted: Sun Oct 30, 2011 1:54 pm Post subject:

Thanks much for the clarification! I wasn't sure about the 3900, but I thought maybe there was a reason the 13700 was twice as big as the 3080 tongue

EdisonRex · Posted: Sun Oct 30, 2011 2:28 pm Post subject:

The difference between the 13600 and 13700 (the only difference I know of) is that the output buffer bias is adjustable on the 13600 and fixed on the 13700.

This has no consequence if you set the 13600 to a fixed output buffer bias or don't care about it in the first place.

Clack · Posted: Sun Oct 30, 2011 2:36 pm Post subject:

you can almost drop in any OTA or VCA IC in some circuits and in others its a little tougher.

Check out this study by Achim Gratz, skip past the maths part it shows you detail between different OTA models.

http://synth.stromeko.net/diy/OTA.pdf

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

Hi macumbista

This link should be of interest for you

http://hem.bredband.net/bersyn/VCA/vca_shootout.htm

EdisonRex · Posted: Wed Nov 02, 2011 7:13 am Post subject:

Paradigm X · Posted: Wed Nov 02, 2011 7:57 am Post subject:

Hmm... doh!

I bought the Thomas Henry 3080 book on the (naive) basis that LM13700s, of which i have a number, are a directly compatible replacement (not in physical size/pinout etc but in terms of schematics.

doh.

Oh well, plenty of other uses for lm13700s.

StephenGiles · Joined: Apr 17, 2006 Posts: 507 Location: England

And then there was the CA3094 which was a CA3080 with an extra darlington pair, with which Electroharmonix got up to all kinds of tricks!

frijitz · Posted: Wed Nov 02, 2011 12:17 pm Post subject:

Paradigm X · Posted: Wed Nov 02, 2011 12:27 pm Post subject:

Ok, cool! So everything in that book will work as is (sorry, im well new at all thi).

And BTW i meant ' assumption ' not basis; i dont want to imply i was midlead etc.

Cheers, thanks ian (still need to pick up a threeler BTW !)

Cheers, Ben

Mooger5 · Posted: Sun Nov 06, 2011 3:37 pm Post subject:

EdisonRex · Posted: Fri Nov 11, 2011 5:40 pm Post subject:

CA 3094 definitely has a different spec. it can supply much more output current. 100x more or so. If you are not building an amplifier and need to drive transistors, it's fairly equivalent. Input wise they are not different.

Clack · Posted: Sat Nov 12, 2011 6:32 am Post subject:

Mooger5 · Posted: Sat Nov 12, 2011 2:53 pm Post subject:

It was part of a VCF+VCA module for the Juno 106. I tried several. It happened some time ago but I must have followed the datasheet.
All it was needed was a simple buffer after the OTA so, could it have been done with R2 of a lesser value (than the 47k) ?
The best I could sounded crude, which is a shame since a 3080+buffer in 8 DIL seems indeed so perfect for a number of applications.
The BA6110 sounds really great and it´s my current favourite. Easier to find and cheaper too Smile

Sebo · Joined: Apr 27, 2007 Posts: 564 Location: Argentina

Paradigm X · Posted: Mon Nov 14, 2011 2:28 am Post subject:

Mooger5 · Posted: Mon Nov 14, 2011 4:29 am Post subject:

Sebo · Joined: Apr 27, 2007 Posts: 564 Location: Argentina

Oh, I alway thought that the BA662 didn't have buffers, seem I was wrong.
Great reading the Oakley pdf, thank you.