impedance matching

Discussion in 'General Electronics Chat' started by u-will-neva-no, Nov 19, 2011.

  1. u-will-neva-no

    Thread Starter Member

    Mar 22, 2011
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    Hi everyone, I was thinking about a recent project and was wondering what the solution might have been. I had a signal and fed it through a MC33171 Op-amp. Looking at the characteristics for an op-amp, it can be seen that it has a high input impedance and low output impedance. I connected the output to a high input impedance stage (arduino input pin). How could I have increased the impedance at the output of the op amp to match the high input next stage?

    Thanks!
     
  2. crutschow

    Expert

    Mar 14, 2008
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    3,227
    Why do you want to match the input impedance? I know you read something somewhere about impedance matching, but that is typically only done in high frequency circuits to prevent signal reflections and/or to match a source impedance to achieve maximum RF power transfer. Neither condition applies here.

    In low frequency circuits you normally want the load impedance to be much higher than the source impedance to maximize the signal voltage.
     
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  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    In general, this kind of interface would not usually be impedance matched. It is usual to feed a low impedance source into a high-impedance load, at least where the connection distance is short relative to the frequencies involved.

    Physically long connections or higher frequencies can require matched transmission lines. These could for instance be coaxial cable, or defined-impedance PCB tracks (strip-line). In this case the impedance at the receiving end, and possibly the send end as well, would require matching to the line impedance. In the case of a high impedance input, this might be done with a simple parallel resistance. Greater efficiency may be possible with the use of transformers, or certain types of tuned circuit, but this restricts the frequency range, severely so in the case of a tuned arrangement.

    My guess would be that if your circuit failed to give the results hoped for, it was due to some other reason than lack of impedance matching.
     
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  4. u-will-neva-no

    Thread Starter Member

    Mar 22, 2011
    230
    2
    Thanks for the replies!
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    Impedance matching is not restricted to high frequencies only. There are many situations where you may want to match the impedance of the source and the load, such as microphones, loud-speakers, telephone, attenuator pi and T-circuits, etc.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    I forgot about things like microphones, telephone lines and passive attenuators,:rolleyes: but matching is not used for loudspeakers. The output impedance of modern solid-state amps is much less than the loudspeaker impedance. It's true the impedance rating is listed for such amps but that's because the voltage and current output of the amps is optimized for a particular impedance, not because the impedances are "matched".
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Microphones are never impedance matched. The input impedance of the preamp is always higher than the impedance of the microphone so that the level is not cut in half.

    Modern amplifiers have an extremely low output impedance (0.04 ohms or less) so that the resonances of a speaker are damped and so that the output level is high. But old vacuum tube amplifiers used an impedance matching output transformer because vacuum tubes have high output impedances.
     
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