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Input stage to op amp

Discussion in 'Electronic Basics' started by Jon Slaughter, May 10, 2007.

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  1. http://www.geofex.com/Article_Folders/TStech/tsxtech.htm

    I'm curious to why in the above analysis and circuit that the input state is
    an emitter-follower instead of directly going into the non-inverting input
    of the op amp?

    It mentions that the input impedance is large enough to prevent loading of
    guitar pickups but surely the op amp has a higher input impedence?

    Is there any particular reason they did what they did here? Only thing I can
    see is because of the bypassing but surely using a SPDT switch would work
    just as fine?

    Thanks,
    Jon
     
  2. Some of the opamp circuits have fairly low impedance
    networks, like the tone control stage, though I don't think
    that one connects directly to the input.

    It may just be that the circuit evolved from earlier
    experiments, and the emitter follower was grandfathered in,
    without really being needed.
    You have to consider the network connected to the non
    inverting input, as well as the input impedance of the op amp.
     
  3. You don't know what it was like in the old days. Not very good specs on
    op-amps. You could get something better, but they cost more and were
    thus less in circulation. You could get "high impedance op-amps", but they
    were generally merely better compared to what was common.

    Hence you'd see all kinds of schemes to get better input impedance. FETs
    and even bipolar transistors, one to each of the inputs, and only when
    you really needed it.

    Then CMOS and bifet opamps appeared, and worrying about input impedance
    pretty much went away.

    Michael
     
  4. Rich Grise

    Rich Grise Guest

    Where does "dry side of bypass" go? That might need the buffering.

    Good Luck!
    Rich
     
  5. yeah, I forgot that. But in my case the op amps have FET inputs so surely
    this means that the inputs have huge impedences?

    I know in a bipolar amp that the configuration and biasing can effect the
    impedences but in an op amp I thought that the inputs were made to have high
    impedences no matter what? (no matter the configuration and close loop gain)

    Thanks,
    Jon
     
  6. The opamp inputs can usually be assumed to be high impedance
    nodes (that is one of the definitions of an opamp). But the
    effect of the connected network certainly cannot. For
    instance, in the inverting amplifier configuration, even
    though the opamp inverting input does not load the circuit,
    the output connected back to it, does. That feedback
    connection forces the input node to look like a short
    circuit to whatever voltage is applied to the non inverting
    input, a condition referred to as a virtual ground.

    But all the configurations on the web page you linked to are
    non inverting, and these generally have high impedance
    inputs, except for the impedance of the bias resistors
    connected to the input. I think these could certainly be
    designed to eliminate either the input follower or output
    follower, or to replace them with opamp versions of those
    followers. I think there should be at least one follower in
    the circuit for he total bypass case, where the downstream
    cable capacitance is driven by the follower, to prevent the
    high frequencies from being rolled off by the loading effect
    of that capacitance on the high source impedance.
     
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