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 Forum index » DIY Hardware and Software
OTA - Operational Transconductance Amplifiers
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Sethkaine



Joined: Sep 26, 2015
Posts: 16
Location: Suisse
PostPosted: Mon Sep 28, 2015 3:00 am    Post subject: OTA - Operational Transconductance Amplifiers
Subject description: I want to know everything about OTA		 Reply with quote  Mark this post and the followings unread
Hi OTA Lovers,

I will talk about OTA - Operational Transconductance Amplifiers.
I have build the VCO-1 from Thomas Henry. Without the 2k thermistor but with 2x 1k in series around the expo converter transistors.
I have breadboarded the Thomas Henry VCF-1 without the PNP expo converter for now. But with one 100k pot to -/+ 12v center tap binded with 22k to each two OTA from LM13700. For now the VFC-1 sound nicely with a touch of resonance as I like.

ref. here:
http://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html
http://www.birthofasynth.com/Thomas_Henry/Pages/VCF-1.html

My questions :
1. I want to know if with a different expo (like the one from R. Schmitz) the VCF will sound the same ?

2. Someone has already build the NPN expo converter from René Schmitz ?
ref. here:
http://schmitzbits.de/expo_tutorial/index.html
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m.o



Joined: Jul 05, 2014
Posts: 44
Location: Sweden
PostPosted: Mon Sep 28, 2015 1:51 pm    Post subject: Reply with quote  Mark this post and the followings unread
Quote:
1. I want to know if with a different expo (like the one from R. Schmitz) the VCF will sound the same ?


The (static) sound itself would be the same, what might differ slightly is the response to CV (you might hear a difference when applying an envelope), but in theory: expo is expo...

Quote:
2. Someone has already build the NPN expo converter from René Schmitz ?
ref. here:
http://schmitzbits.de/expo_tutorial/index.html

Yes you'll find if you study other VCOs (sawtooth type) that this is pretty much a standard configuration.
(And if you rearrange the components slightly in the TH design you'll see it's also is similar).
NPN configurations like this will 'sink' current into the collector of the output transistor, while PNP configs such as the one in TH VCO 'sources' (pushes out) current - different directions.

The expo in the TH VCO needs a positive current to control the 3080 (OTA), which in turn emits a positive or negative current at it's output depending on the state of it's +/- inputs - this output current 'size' is what is controlled by the expo (it's the same mA amount wether it's positive or negative).
This current is what makes the op-amp IC3a to charge/discharge the C3 capacitor at a steady rate.
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Sethkaine



Joined: Sep 26, 2015
Posts: 16
Location: Suisse
PostPosted: Sat Oct 03, 2015 1:03 pm    Post subject: Reply with quote  Mark this post and the followings unread
thank you for your very interesting explication. I will study it closer and make tests.
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PHOBoS



Joined: Jan 14, 2010
Posts: 4826
Location: Moon Base
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PostPosted: Sun Oct 04, 2015 6:29 am    Post subject: Reply with quote  Mark this post and the followings unread
Just came across this on my harddrive, maybe it's useful.

UsingOTAs2.pdf
 Description:
Understanding and using 'OTA' op-amp ICs by Ray Marston

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L´Andratté



Joined: Sep 23, 2012
Posts: 135
Location: Hamburg, Germany
Audio files: 5
PostPosted: Tue Oct 06, 2015 9:58 pm    Post subject: Reply with quote  Mark this post and the followings unread
Mucho gracias, Phobos!
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bugbrand



Joined: Nov 27, 2005
Posts: 846
Location: Bristol, UK
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PostPosted: Wed Oct 14, 2015 5:57 am    Post subject: Reply with quote  Mark this post and the followings unread
For people interested in OTAs, this could be an interesting/useful doc, recently published::

Minimizing distortion in OTAs

http://www.openmusiclabs.com/files/otadist.pdf
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