negative resistance with conveyor current

Discussion in 'The Projects Forum' started by azerty1, Nov 5, 2013.

  1. azerty1

    Thread Starter New Member

    Apr 30, 2013
    Hello everyone,
    I'm working on a project on current conveyors and I want to test the operation of the latter in negative resistance. My question is that I have to attack the Y input conveyor with a current generator and a voltage generator? to have a 180 ° phase shift between the voltage and the current input .
    Thank you bcp.
    You will find attached the diagram of the negative resistance.
  2. crutschow


    Mar 14, 2008
    What you are trying to do is not very clear. Could you elaborate on exactly what that is? Where is this negative resistance coming from? What is a "current conveyor"?
  3. LvW

    Active Member

    Jun 13, 2013
    A current conveyor (CC) is an analog IC exhibiting a high-resistive current output (z) and two inputs, one which high resistive (y, non-inverting) and the other one (x, inverting) is low-resistive (10 ohms or so).
    For an ideal CC we have Vx=Vy and Iy=0 and Iz=+-Ix (CCII+ or CCII-).

    A very popular example is the AD844, which very often is called "current-feedback amplifier (CFA)" (because it has as a last stage an additional I-V converter on-board), however in 99% of all applications it is used as a CC.
    With other words:
    * A CC is a CFA with a current output, or it can be seen as
    * an OTA with a low-resistive inverting input.

    The above description applies to a 2nd generation CC (CCII).
    According to my knowledge there are only a few IC`s on the market: AD844, EL2082, OPA860.

    Current Conveyors are very versatile units - used, in particular, for filters and oscillators.

    The 3rd generation CC has in addition the capability to control the current-transfer ratio with an externally applied current (current-controlled conveyor - CCC).
    I don`t know if such a device is available as IC.

    It is known that the classical OTA can be used as a grounded resistor (Iabc can externally control the resistor value) using 100% feedback from output to the inverting input.
    A similar application is possible for an CC. In case of polarity change of the output current a CC unit can mimic a negative resistance. The value of the resistance is determined by the external control current.

    As to the question: Of course, a Voltage must be applied to the high-resistive y-input.

    Finally, I am not convinced that the shown circuit will function as a negative resistance. But this statement is a preliminary one because we have to know the exact transfer properties of the device (how is it realized?).

    EDIT: see next post.
    Last edited: Nov 7, 2013
    avee kumar likes this.
  4. LvW

    Active Member

    Jun 13, 2013
    I have simulated the given CCC circuit using the CCII model (AD844).
    I can confirm that - in principle - it can simulate a negative input resistance at the non-inverting y-input node.
  5. azerty1

    Thread Starter New Member

    Apr 30, 2013
  6. LvW

    Active Member

    Jun 13, 2013
    Azerty1, of course, I could post a picture from my simulation results - however, you will see only two sinusoidal signals shifted by 180 deg. Not too descriptive.

    Description of my simulation arrangement:
    (I know that the AD844 is a CCII and not a CCC, however, for our purpose it is sufficient to demonstrate the desired function.)

    Spice model AD 844.
    X-input (inverting, low-resistance) grounded
    Current output (transimpedance point, node "TZ" for AD844) directly connected with the Y-input (non-invertig, high-resistance).
    For measuring the current I have placed a small resistor (1 Ohm) between voltage source and the Y-input..
    Both, AC as well as Tran simulations show the expected phase shift (180 deg).

    Question: Did you convince yourself that your transistor-based circuit works correctly for other applications (amplification)?