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op amp basics

Discussion in 'Electronic Basics' started by lerameur, May 13, 2008.

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  1. lerameur

    lerameur Guest

    hello,

    I am trying to understand the fundamental of op-amp.
    I found this link to be very interesting:
    courses.ece.uiuc.edu/ece486/labs/lab1/386_op_amp.pdf
    I would like to know why that when the output of an opamp is
    connected to the input, like a voltage follower, the voltage is equal
    to the other input.
    The pdf I mentionned says that the 2 input tries to be the same
    voltage. So if you put 5 volt in one input, you will get 5v on the
    other input. Looking a the basic of opamp, v2-v1, that would have an
    output of 0, Then one input would be zero, It does not make sense that
    on the same wire, there are two voltages

    thanks

    k
     
  2. Chris

    Chris Guest

    Hi, K. The tutorial you mention doesn't include non-inverting
    amplifiers, of which the voltage follower is the simplest:

    | Voltage Follower
    |
    | Vin |\|
    | o-----|+\ Vout
    | | >----o---o
    | .--|-/ |
    | | |/| |
    | | |
    | | |
    | | |
    | '----------'
    |
    |
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


    Assume the gain of the op amp is arbitrarily high (a couple hundred
    thousand is very common, use 100,000 for this example), and you apply
    5.000V at the input. The output will be 99,999/100,000 of the input,
    using your equation

    Vout = Av (v2 - v1)

    Which means the output will be 4.99995V, essentially the same voltage
    as the input.

    The main source of error with an op amp configured as a voltage
    follower for low frequency or DC inputs is voltage offset (typically
    several millivolts or so).

    Hope this has been of help.

    Chris
     
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