Output impediance

Discussion in 'General Electronics Chat' started by jmsheehan, Jun 28, 2013.

  1. jmsheehan

    Thread Starter New Member

    Mar 31, 2013
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    How would I find it in this circuit? I want to know what the impediance is so I can match it better to either an amplifier or an antenna. I was told I am going to have to design a LC pi filter and tune it for best results. Also, I am using the SS9018HBU transistor for both sections. *Impedance* Realized I spelled it wrong.
     
  2. rahdirs

    New Member

    May 22, 2013
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    Hi,
    Look like you are using Multisim,use a multimeter & measure your o/p impedance with 'ohm'in it.
    or
    If you are using Multisim 11, select File>>Open Samples>>LabVIEW Instruments>>Impedometer>>Impedometer.ms11, copy/paste the impedance meter from that circuit to your circuit.
    I recommend using Impedometer
     
    Last edited: Jun 28, 2013
  3. LvW

    Active Member

    Jun 13, 2013
    674
    100
    I am not familiar with Multisim - however, independent on the simulator you can feed a current of 1A into the output node (ac analysis) and measure the corresponding voltage at that node. It is identical to the output impedance.
    Don`t forget to signal ground the input during this simulation.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    It looks like you have a Colpitts oscillator with a buffer stage. Why did you choose that buffer topology?

    You need to be more explicit about your requirements/specification or design goals.

    In principle you could use the simulation model to apply a variable load resistance at the output terminal (collector or emitter?). If the output is to be modeled as a voltage source with an internal impedance, you simply need to simulate two load conditions (one of which can be the unloaded output voltage) to determine the output resistance. Keep in mind a simulation is an indication only of real physical performance. What you obtain in practice - if that's your goal - may differ significantly from what your model tells you.

    If you know the Early Voltage parameter for the particular transistor mentioned you could possibly predict the output resistance at the Q2 collector terminal - if that's where you are taking the output signal.

    Are you intending to use the LC pi network for a specific impedance match (such as 50Ω) with an ongoing stage or antenna? You would need to know the input impedance of the following stage or the antenna impedance (usually complex) to complete a design.

    Again, if you are planning to make a working circuit coupled to a radiating antenna there are most likely government restrictions on what you can legally do with respect to RF radiation.
     
  5. LvW

    Active Member

    Jun 13, 2013
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    Because t_n_k has mentioned the output stage:

    For my opinion, it contains is a rather bad (unconventional) method for stabilizing the bias point of the transistor. At the same time you provide current feedback (R5) AND voltage feedback (R7). This does not work satisfactorily. For example, the voltage feedback with R7 works best with a fixed dc base voltage. Thus, choose only one method.
     
  6. jmsheehan

    Thread Starter New Member

    Mar 31, 2013
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    I didn't design this circuit but it was just a redesign on another oscillator that was not working. This one works great on a copper clad pcb. So the buffer stage is bad? And I should just choose to either delete R5 or R7? Considering as I have the output and the PLL on the collector of Q2 I'm thinking it may be better to get rid of R7? Sorry for being vague, RF is rather newish to me and I'm still learning. Also, I already know the input impedance of some of the rf amplifiers that I want to use and I'm not making this thing at all high power, maybe 100mW and that's it, it's more a learning experience than anything else.
     
  7. jmsheehan

    Thread Starter New Member

    Mar 31, 2013
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    Also on another forum I was told to drop the 470ohm from the buffer stage and make it an inductor, though he hasn't said what the value of it should be.
     
  8. LvW

    Active Member

    Jun 13, 2013
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    If you want a buffer at the output drop the collector resistor and also R5.
    Use a classical resistive divider instead for biasing purposes..
     
  9. jmsheehan

    Thread Starter New Member

    Mar 31, 2013
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    I was most likely going to drop it anyways. Replace the 470 with an inductor? If so what value. Resistive divider?
     
  10. jmsheehan

    Thread Starter New Member

    Mar 31, 2013
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    0
    Okay, nevermind.. Now I know what a resistive divider is.
     
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