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doubt about the OTA
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Joelelectro



Joined: Oct 01, 2014
Posts: 9
Location: Xalapa

PostPosted: Tue Oct 07, 2014 9:17 pm    Post subject: doubt about the OTA
Subject description: Exist some OTA
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Hi,guys i have a question about the OTA. I search in the internet some OTA for example LM13700,CA3080,LM4250,NE5517 i can design filters only with capacitors, because i read the datasheets and for design filters use resistance and capacitors i think when i see some pictures about the OTAs don´t use much elements for create filters for example fifth order. Exist some OTA with this topology.

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JovianPyx



Joined: Nov 20, 2007
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PostPosted: Thu Oct 09, 2014 6:13 am    Post subject: Reply with quote  Mark this post and the followings unread

There is a difference between an OTA and an OpAmp. An OpAmp always uses a resistor with a capacitor to make an integrator. The integrator is a basic component of a filter because it forms a "time constant".

An OTA, however looks a bit like an OpAmp with a funny double circle thing at the point of the triangle. This circle thing represents a variable current drive. Note that resistors can change the amount of current flowing through them by Ohm's law. An OTA changes current in a current controlled manner, that is, a current (Iabc) is used to control the output current. This is why OTA filters don't look the same as OpAmp filters by the same name.

In an OpAmp, if you want to change the current flow to the integrator, you change the resistor that feeds current to it. For an OTA, you change a control current called Iabc (I for current - abc for Amplifier Bias Current). The change in Iabc current does a couple of things, 1) it changes the open loop gain of the OTA and 2) it changes the drive current available at the output. Note that in both cases, we are controlling the current into the integrating capacitor - opamp with a resistor, OTA with a current.

It is the OTA's ability to control the current feed to the integrating capacitor that makes it possible to create a Voltage Controlled Filter. All one needs is a voltage to current converter so that the input control voltage (CV) is converted to a current which is needed by the Iabc pin of the OTA.

Look at the LM13700/LM13600 datasheet examples - there are several voltage controlled filters shown there including a 2nd order filter (the State Variable Filter).

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