About Op-Amp

Discussion in 'General Electronics Chat' started by OSOO, Mar 31, 2014.

  1. OSOO

    Thread Starter Member

    Feb 19, 2014

    I have two questions:

    When we want to design an op-amp circuit to be a non-inverting amplifier ,according to what do we put the values of the input and output resistances ?
    are there any limitations ?
    if I want to to put a gain of 10 times the input signal , can I choose any values that lead to 10 gain ? for example I want to amplify a 10mv ,so I put the values 100k and 900k for the input and output resistances respectively.

    or I can choose another values ,1k and 9k ,which also gives a gain of 10 !!?

    Also, I want to ask about he Vcc and what is the impact of it on the amplified signal ?

  2. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    At first, it might look like there are hundreds of resistor combinations that would calculate a gain of 10, and that is correct in theory. However, some opamps do not like low value feedback resistors, and some do not like high values. Also, if you need to keep DC errors (input offset voltage, input bias current, etc.) to a minimum, this can affect which resistor combinations to use. So other circuit factors like errors and input impedance and the opamp you select usually work to reduce the number of choices you have for the resistors.

    Sometimes it's a business decision. If many of the other resistors in the application are one particular value, such as 10K, and you particular application does not present a lot of restrictions, then it makes sense to make one of the feedback resistors 10K and go from there, rather than use some other value and have it be the only one on the board.

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  3. LvW

    Active Member

    Jun 13, 2013
    The classical gain formula for the non-inverting opamp (1+R2/R1)
    can be applied only
    - if the open-loop gain can be assumed to be (nearly) infinite (in most cases to be fulfilled for frequencies up to the mid kHz range), and
    - if the input and output impedances of the opamp unit can be neglected (input assumed to be infinite; output to be assumed zero).

    This last requirement can be best fulfilled for resistors in the lower or mid kohm range.
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  4. MrChips


    Oct 2, 2009
    Don't make resistances too big or too small, just right, as Goldilocks would prefer.
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  5. crutschow


    Mar 14, 2008
    The value of the op amp input and output resistances affect:

    The frequency response of the circuit as affected by the stray capacitance interaction with the resistances.

    The DC offset due to the opamp input bias currents.

    The amount of power dissipated in the feedback resistance.

    For typical applications the resistors usually fall within the range of a few kohm to several hundred kohm but certain applications can use resistors outside this range.
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  6. OSOO

    Thread Starter Member

    Feb 19, 2014
    okay ,this is really helpful
    thank you all

    but I couldn't find an answer to the second question

    what is the purpose of the power supply ,do we need it when the input signal is a direct voltage
  7. t06afre

    AAC Fanatic!

    May 11, 2009
  8. shteii01

    AAC Fanatic!

    Feb 19, 2010
    The op-amp power supplies are there to provide the ADDITIONAL energy needed to AMPLIFY the input.

    In other words, you can not get amplification out of your a$$, it has to come from somewhere. You are putting 1 volt in. You expect 10 volts out. 10-1=9 volts. Where did the 9 volts come from? Effing magic? No. The 9 volts came from the power supplies connected to op-amp to power it.