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Desirable output resistance

Discussion in 'Electronic Basics' started by Scale, Jun 15, 2007.

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  1. Scale

    Scale Guest

    Hello. I have a simple question.

    I am currently taking an electronics course and I'm confused by one
    thing: When is output resistance desirable?

    We are studying various typed of amplifiers and transister based
    circuits (source followers, current sinks, differential amplifiers,
    etc, pretty common stuff I guess), and we're always finding Rin, Rout,
    Av, etc of the entire amplifier. Av is a given, but what exactly is
    the significance of Rout? I would have assumed that a low Rout would
    be desirable for the same reason that a low Rout is desirable in a
    voltage source (er... right?). Am I right? Are there times when a high
    Rout is desirable? Is it always desirable?

    Also, on that note, a high Rin IS a good thing, right? I assumed that
    a lower Rin means that connecting the amplifier has a stronger effect
    on the circuit and alters the results.

    Any help greatly appreciated! Thanks a lot!

    --MDL
     
  2. Bob Pownall

    Bob Pownall Guest

    Scale wrote:
    <Summary: When do you want an amplifier to have a high Rout vs. a low
    Rout. Ditto for Rin.>

    Basically, it depends on whether you're building a voltage amp or a
    current amp.

    Voltage amp: High Rin good (doesn't load down whatever's driving it),
    low Rout good (think of a voltage divider and where you want most of the
    voltage to drop)

    Current amp: Low Rin good (think current divider and where you want most
    of the current to go), high Rout good (Ditto, but from the other point
    of view).

    If I'm wrong, I'm sure someone will flame me...

    Bob Pownall
     
  3. Lord Garth

    Lord Garth Guest

    Consider that an audio amplifier supplies a typical speaker with a large
    current
    to create the loud sound you hear. An audio preamplifier would boost the
    voltage of its input in order to drive the power amp.

    Here's a link to a class D amplifier app note:
    http://www.irf.com/technical-info/appnotes/an-1071.pdf
     
  4. Eeyore

    Eeyore Guest

    For any circuit using voltage transfer it's never desirable.

    However for impedance matched (transmission line) circuits it's essential.

    Graham
     
  5. Eeyore

    Eeyore Guest

    Yes, a high Rin is generally a good thing.

    I can however think of an example in audio where you may not want the highest
    possible Rin.

    Dynamic microphones (moving coil) have complex electro-mechanical behaviour. In
    some cases a very high load impedance will result in undesirable under-damped
    behaviour which can result in frequency response anomalies.

    To take example, 'professional' microphones used in broadcasting, recording and
    live sound have typical source impedances in the 150-200 ohm region (but with
    500-600 ohms also considered 'normal'). The usual load impedance of a mic amp is
    2kohms (the 10:1 loading gives good voltage transfer for optimum signal to
    noise ratio) but a few simply 'sound better' with a classic 600 ohm load.

    Graham
     
  6. Guest

    Does anyone know how this company did this introduction on there
    website. www.empero.us it is so awesome.
    Please contact me if you can help!

    Ali
     
  7. Bob Pownall

    Bob Pownall Guest

    One not-so-minor correction to my earlier post.

    If you're concerned about maximizing power transfer (for example, if
    you're working with very small power levels), you'll want to match your
    input impedance to your source and/or your output impedance to your
    load, no matter whether the source and load are low or high impedance.

    Bob Pownall
     
  8. Lord Garth

    Lord Garth Guest


    Sure, like this:

    <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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    name="rainsize" value="0"><param name="rainfactor" value="20"> <param
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    :)
     
  9. Eeyore

    Eeyore Guest

    Power transfer is mostly irrelevant to good signal-to-noise ratio.


    Graham
     
  10. An Rout lower than necessary might be a problem if the output could be
    shorted. It might also create instability when driving a capacitive
    load. If Rin was very high in a circuit driven from a low impedance
    source, but through a long cable, it might be more sensitive to noise.
    There are generalizations you can make, but you have to look at the
    whole circuit.
     
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