AD844 CFOA

I want to purchase AD 844 current feed back operational amplifier, But i am facing a problem while doing simulation part on PSPICE simulator. It is written in data sheet that in case of CFOA current at pin 5 Iz=+ or - of Ix.

With the help of AD 844 i can use it as Iz=+Ix  but i want to know how it can be used for Iz= - Ix.

Because i am using both configuration in my project work.

Thanks.

  • Hello divyanakra,

    the current flowing into the low impedant pin 2 of the OpAmp will always be mirrored to node 5 of the simplified schematic in fig. 31 of the data sheet. So in your words: Iz will always follow Ix in magnitude and in sign (unless you drive the OPV beyond its limits). 

    Node 5 has an internal capacity of several pF, you may add external capacitor or a network at pin 5 to influence the frequency behaviour of the OpAmp. But the sign of the current through pin 5 should always follow the sign of the current through pin 2. In your words: you can not get Iz = -Ix.

    I assume, that you want to use the AD844 in inverting and in non inverting configuration (this is how I read your statement: "Because i am using both configuration in my project work."). But the inverting or non-inverting configuration is not related to the sign of the current to node 5. The sign of the current depends on the direction in which the output of the OpAmp is changing: for a rising slope Iz (and Ix) have a different sign than for a falling slope. When the output voltage does not change, then Iz = 0. 

  • Thanks for reply

    you think i want to use CFOA in inverting and non inverting configuration,but that is not correct actually.

    I want to make a filter(Band pass filter) which can be used for the purpose of image frequency rejection.

    I am attaching the paper here which i am following for my project work. you read highlighted text and see figure 1. That circuit contain two CFOA one contain a positive sign and one contain a negative sign and in theory they have written Iz=+-Ix so, that circuit i am unable to make because both CFOA are not identical and i have only one IC in pspice that is AD 844.

    Except this one more thing i want to ask that circuit drives with current so how much amount of current i can apply in this circuit to obtain correct response.

    Thanks

    MainPaper.pdf
  • ok, that's a more involved setup than I would have expected from your first description ;-)

    The paper is somewhat involved, and I can't go through it in detail. But I believe, that you misinterprete the statments in the paper about Iz=+- Ix and the fact, that they put a plus and a minus in the two OpAmps. I believe, that in their simulation they used two identical AD844. The + and - in the OpAmps do not describe different kinds of OpAmps. It is a symmetrical setup with differential inputs and outputs, and you just have to put names to the two lines to distinguish them. I think this is what the + and the - in the OpAmps are standig for.

    Then why do they talk about Iz=+-Ix in the paper? I'm not sure either, but I believe that they somehow want to describe the cross-coupling of the OpAmps via the Tz pin. Maybe one has to read it in the following way: the currenct flowing out of Pin 5 of the upper OpAmp (Iz=+Ix) is then flowing into Pin 5 of lower OpAmp. So in some sense one might say, that a positive Ix of the upper OpAmp leads to a positive Iz at the upper OpAmp, which is then transfered to a negative Iz at the lower OpAmp due to the external connection.  I'm not sure about this, but that would be my first explanation.

    In Chapter IV of the paper they describe their simulation setup. Did you already try to run this in Spice using two identical AD844? I expect, that you will see the same results as described in the paper.