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Voltage Controlled Voltage Source

Discussion in 'General Electronics Chat' started by Mazaag, Feb 2, 2008.

  1. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    Hey guys

    I have a question about OrCAD...

    In the schematic Capture program, I want to create an Integrator model of an opamp..

    I set up a voltage controlled voltage source ,but my question is as follows.. If i want to set the gain to be a function of s , do i use the letter s ? as in 1000000/s for example ? if not , how do I make the gain a functino of frequency...

    thanks guys
  2. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    hmm...32 views so far, and no reply yet...

    I must be in the wrong forum :D
  3. scubasteve_911

    AAC Fanatic!

    Dec 27, 2007
    Not everyone that views is able to respond or perhaps does not have time to do so. I think you need to explain your problem better, it pretty much ensures a response here.

    It sounds like you are trying to use spice to make an integrator with variable gain? Do you plan on making a real integrator from this, or is it just a model? Why not use simulink for this? Better yet, why not check out the opamp implementation of an integrator and try to modify it ?

    Give us something to work with :)

  4. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    I need to do this in PSPICE..

    basically, I need to model an non-ideal opamp which has infinite input impedance, infinite output impedance and a gain A(s) which is wt/s ...

    So basically i need SOMETHING that will look at the input voltage (across an open circuit) , and provide a gain A(s) times that value... but the problem is that if i use a regular voltage controlled voltage source to do this, I do not know how to specificy the gain to be a function of s , ( to get the wt/s) ...

    I was thinking of using a voltage controlled current source, set the current equal to 1, and put a capacitor in parallel with c = 1/wt, that way the output voltage will be the current times 1/sC , which should give me the voltage i need...but in that case..would I need a buffer at the output ?

    Thats the best I can do in clarifying this question

    Thanks anyways guys
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Whether you need to buffer the output or not depends on your load. If you have a load, you need a buffer. A buffer is just a VCVS with a gain of 1.
    Why do you need to do this? Sometimes a little more info can lead to a lot more help.
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
  7. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    Okay guys,
    so here's what i did so far...

    Attached is a circuit schematic of a low pass filter using an Inverting opamp section...

    R3 is connected to the inverting input of the amplifier,

    C2 and R4 form the feedback network..

    for the opamp model.. I set the gain of the transconductance to = -1, that is , the current running into C3 should = Vin

    then , C3 is set to have a capacitance value = 1/wt , therefore an impedance of wt/s

    the voltage at C3 should then be Vin*wt/s .. which is the gain i need,

    I then put a VCVS as a buffer...
    when i run the simulation , i get this error

    ** Creating circuit file "assignment 1-assignment 1-frequency_response.sim.cir"
    * Local Libraries :
    * From [PSPICE NETLIST] section of C:\Program Files\OrcadLite\PSpice\PSpice.ini file:
    .lib "nom.lib"
    *Analysis directives:
    .AC DEC 100 10 100k
    .PROBE V(*) I(*) W(*) D(*) NOISE(*)
    .INC ".\assignment 1\assignment 1-Assignment 1.net"

    **** INCLUDING "assignment 1-Assignment 1.net" ****
    * source ASSIGNMENT 1
    E_U1 N00278 0 VALUE {LIMIT(V(0,N00216)*1E6,-15V,+15V)}
    V_V2 N00177 0 DC 0Vdc AC 1Vac
    R_R1 N00177 N00216 10k
    R_R2 N00216 N00278 10k
    R_R4 N00264 N00331 10k
    E_E3 N00331 0 N08335 0 1
    V_V1 N00215 0 DC 0Vdc AC 1Vac
    C_C1 N00216 N00278 1.98n
    C_C2 N00264 N00331 1.98n
    G_G1 0 N08335 0 N00264 1
    C_C3 0 N08335 1.59u
    R_R3 N00215 N00264 10k
    **** RESUMING "assignment 1-assignment 1-frequency_response.sim.cir" ****
    ERROR -- Node N08335 is floating

    First of all...Is my implementation correct ?

    Secondly..does anyone know what the error is about ?

    Thirdly.. having the circuit in this current configuration, do I need a buffer ?

  8. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    My bad....