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 Forum index » DIY Hardware and Software » The layout factory
Asking function & name of the NPN seen here in schematics
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rogerlatur



Joined: Dec 22, 2012
Posts: 116
Location: france

PostPosted: Sat Jul 27, 2013 11:01 am    Post subject: Asking function & name of the NPN seen here in schematics Reply with quote  Mark this post and the followings unread

I am taking a better look at certain OTAs and I was wondering if you know:
- what is the NPN function in the circuits seen here, like in Figure 7 on page 8 (document page number 72) for instance:
http://www.idea2ic.com/LM13600/UsingOTAs2.pdf

- can I simply take 2N3906, considering that nothing is mentionned in the all document ?
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elmegil



Joined: Mar 20, 2012
Posts: 1341
Location: Chicago
Audio files: 14

PostPosted: Sat Jul 27, 2013 11:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Not clear what you mean about the NPN, and a 2N3906 is a PNP.

Do you mean the darlington pair on the output? For some OTA chips, notably the LM13700, those are onboard on the chip you just have to link them in.
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Mongo1



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

PostPosted: Sat Jul 27, 2013 2:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

The NPNs are a darlington pair. They are actually part of the 13700 chip.

If my memory is right, the OTA actually controls current. The darlington pair is this to be used as a buffer, so you can control voltage.

Gary
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rogerlatur



Joined: Dec 22, 2012
Posts: 116
Location: france

PostPosted: Sun Jul 28, 2013 4:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you !!!
First time I see schematics like this.
I understood 10 was a pin, though it is after 12 on the same line (this was also new for me) and same for few pins, but I was not sure about the NPNs as I saw them as a part of the OTA itself in the datasheet, but never seen parts of a component in the schematics lines / outside of the component itself (if you know what I mean).
Good to know for upcoming schematics with OTAs (more than 8 pins !).

Thank you for correcting my error too ! I had these ones in my NPN stock list. I am going to check all the stock list as I wrote this when I started with DIY.

Last edited by rogerlatur on Thu Aug 01, 2013 2:40 am; edited 1 time in total
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gdavis



Joined: Feb 27, 2013
Posts: 123
Location: San Diego

PostPosted: Sun Jul 28, 2013 4:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

The output of the OTA is a current source. Most audio circuits are voltage sensing, so the current output needs to be converted to a voltage (this is what R5 is doing, remember V=IR that I told you about in another thread).

The output impedance of a current source is naturally high. This can affect the voltage level when the input impedance of the circuit it's connected to is low (the output impedance and load impedance act as a voltage divider). In low power voltage based circuits you ideally want 0 output impedance and infinite input impedance.

If the input impedance of the load circuit is known and constant, you can compensate for it, but a more general solution is to buffer the output. This is the purpose of the darlington pair, it can optionally be hooked up as a buffer.

The key characteristics of the darlington pair that make it useful here are:
1. It has a very high input impedance (due to the 2 cascaded base pins) which has a minimal effect on the output voltage from the OTA + resistor.

2. It has a very low output impedance which minimizes the affect of the input impedance of the load circuit it is attached to.

3. The gain factor is close to 1, the voltage you get out is the same as what you put in.

Downside is there is a small voltage offset due to the cascaded base-emitter junctions.

The darlington pair is a cheap and easy buffer method that is easy to integrate on to the same chip as the OTA (though it's not part of the OTA), but there are other ways to handle the output from the OTA depending on the application, that is why it is made so that hooking it up is optional.

_________________
My synth build blog: http://gndsynth.blogspot.com/
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rogerlatur



Joined: Dec 22, 2012
Posts: 116
Location: france

PostPosted: Thu Aug 01, 2013 2:48 am    Post subject: Reply with quote  Mark this post and the followings unread

It looks like I am making something wrong building this (see picture).

Could you please tell me to which LM13700 pins these nodes are corresponding to, because I am for sure mixing internal OTA circuit and connections / components I should add myself (this will help me also understand further OTA circuits):

A =
B =
C =
D =
E =
F =
G =
H =
I =
J =
K =

Posted Image, might have been reduced in size. Click Image to view fullscreen.


I got results (mix of 2 signals, sending one different signal to each input) but I am sure these are not the expected ones for this amplitude modulation circuit.

Here is what I did (and then tried with different configurations without changes and without knowing exactly what are H, I, J, K connected to):

A = 14
B = 15
C = 13
D = 11
E = 6
F / G = 16
H =
I =
J =
K =

Transistors:
"those are onboard on the chip you just have to link them in."
"it is made so that hooking it up is optional."
Please, how do you hook up the Darlington pair if these are intern ? You mean connecting pins together then ?
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feggster



Joined: Sep 12, 2011
Posts: 50
Location: uk

PostPosted: Thu Aug 01, 2013 4:34 am    Post subject: Reply with quote  Mark this post and the followings unread

http://www.idea2ic.com/LM13600/Lm13700Sch.jpg

H = pin 12
I = pin 10
J = pin 11 (v+)
K = pin 9
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rogerlatur



Joined: Dec 22, 2012
Posts: 116
Location: france

PostPosted: Thu Aug 01, 2013 4:37 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you so much for the help !!!
This picture with the internal circuit is really what I was missing. Awesome !

Also realizing that the symbol at the output of the OTA is not for one connection, but also the bias input !

feggster wrote:
http://www.idea2ic.com/LM13600/Lm13700Sch.jpg

H = pin 12
I = pin 10
J = pin 11 (v+)
K = pin 9
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feggster



Joined: Sep 12, 2011
Posts: 50
Location: uk

PostPosted: Thu Aug 01, 2013 4:45 am    Post subject: Reply with quote  Mark this post and the followings unread

no problem,
Google is your friend for chip pin-out images Very Happy
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analog_backlash



Joined: Sep 04, 2012
Posts: 371
Location: Aldershot, UK
Audio files: 21

PostPosted: Thu Aug 01, 2013 3:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi rogerlatur.

Sorry if I'm stating the obvious here, but beware that IABC must be 2mA max. I'm only saying this, because I managed to fry a few LM13700s when I first started breadboarding them Embarassed . With the 30K resistor, even at 15V you should only get 0.5mA, so that's fine. However, check your wiring carefully to make sure that you haven't miswired anything to pin 16 (or 1) and that nothing is shorted to these pins. You don't even get "magic smoke" - they just stop working!

Also, if I'm not using one half of the chip, I connect both unused inputs to 0V (i.e. pins 3 & 4 in your case), as you will find elsewhere (e.g. on Ray Wilson's MFOS site).

Happy experimenting Very Happy

Gary
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rogerlatur



Joined: Dec 22, 2012
Posts: 116
Location: france

PostPosted: Fri Aug 02, 2013 8:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you very much for all advices here !!!
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