Colpits Oscillator Pspice design

Discussion in 'The Projects Forum' started by mamakuxa, Oct 7, 2015.

  1. mamakuxa

    Thread Starter New Member

    Oct 5, 2015
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    Hello, I am currently working on a wireless charger prototype for my senior design course. I am currently working on the oscillation portion so I will be able to create an inductive coupling with two coils.

    I have this circuit on the pspice but all I am getting is a flat DC output. Anyone have any suggestion to fix this problem? This is very similar to this thread, but that is an older thread. http://forum.allaboutcircuits.com/threads/common-base-colpitts-oscillator.72724/
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Previous experience tells me that your L to C ratio is way off; you do better to have a much lower L and much higher C to keep the same L*C product, but have a tuned circuit with a much higher Q. I am modelling a lossy inductor with a series resistance of 20mΩ.

    Also, your feedback (Xc10/Xc9) is much too high.

    So here it is running. Note the distortion, which is typical for a Colpitts.

    94.gif

    The start-up behavior is as shown here:

    94s.gif
     
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  3. mamakuxa

    Thread Starter New Member

    Oct 5, 2015
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    Thank you for the help! Its strange, im still getting the same problem of basically a DC output. I will have to continue testing the circuit with different values to see if that helps.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Sometimes an oscillator simulation will not start because the initial DC operating point part of the simulation finds a quasi-stable state with no oscillation (which would not likely occur in a real circuit due to intrinsic circuit noise).

    Try using the Use Initial Operating Conditions option (.uic) to see if that helps.

    Otherwise you can put a voltage source in series with the transistor base to generate a small single pulse (say 1V) at the beginning of the simulation to kick-start the oscillation.
     
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  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Try two things:

    First, set the total simulation run time to about 20 of the expected cycles of oscillation; in this case about 30us. When I do that in LTSpice using the .TRAN 2ou directive, this is what happens: (all other things being as they are in post #2)

    94b.gif

    Alternatively, leave the sim run time long (at 5ms, many, many cycles of oscillation), but force Spice to take little tiny steps (overcome Spice's time-step-control algorithm by specifying the minimum step command thus: .tran 0 5m 0 1u If you accept the default Spice time-step, it gets so big quickly that Spice misses the tiny perturbations that are taking place (math noise), which gets amplifed, which starts the oscillation.

    Yet another way of forcing a Spice sim of an oscillator to start is to put an initial condition inside the LC tank circuit. Adding this directive .IC I(L1) = 1f (set an initial current of 1femtoA in the inductor) creates enough of an initial perturbation to get it started.
     
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