Resistor values for op amp - does size matter?

Discussion in 'General Electronics Chat' started by Di11on, Dec 1, 2014.

  1. Di11on

    Thread Starter New Member

    Nov 26, 2014
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    Hi folks,

    In most op amp applications, it is the ratio of resistors that matters, not their absolute value. For example, in a unity gain differential amplifier, does it matter if R1=R2=R3=R4=1K or R1-4= 10K?

    Will high resistance values mean lower power consumption and/or greater precision?

    Thanks in advance
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Short answer is that it doesn't matter much for resistors 100K and lower ohms, for most op amps. The op-amp sees a voltage and doesn't care how it got there.

    No op-amp is ideal, however, and does require a finite current at the input. So you cannot use, say, 10MΩ input resistors with every op-amp and expect reliable results.

    You need to read the data sheet for the op-amp if you want to push the edge.
     
    Last edited: Dec 1, 2014
  3. MrChips

    Moderator

    Oct 2, 2009
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    Your input impedance will depend on the values of the resistances chosen.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,047
    Yes and no respectively. But a higher input impedance might help avoid distorting the input signal. I guess you could call that greater "precision".
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    How high is the frequency going to go? I designed a high gain high frequency op amp stage once to get a gain of 32 dB at 3MHz. For a non-inverting configuration I had to drop the input resistor something like 50 ohms. This is due to the simple fact that IC pins are made of metal and have a capacitance associated with each pin, and the input was rolling off because of this RC.

    Yep, driving the feedback network alone took considerable current as could be seen on the power supply line. There was some droop that needed to be carefully bypassed to make it tolerable.
     
  6. Di11on

    Thread Starter New Member

    Nov 26, 2014
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    Thanks all. I'm just implementing a differential amplifier to read a temperature signal, so no high frequency issues.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If the resistors in the feedback path are too low (1KΩ sounds too low to me), the opamp maximum output current spec can be exceeded. I usually use 10KΩ to 100KΩ. 1MegΩ starts getting into the range where opamp input bias current comes into play.
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    There are a number of factors that drive these decisions. If you truly only have constraints on the ratio of resistances, then try to keep your values in the 10kΩ to 100kΩ range (though coming down to 5kΩ or going up to 1MΩ will usually not cause major problems in most designs) and you will usually be pretty good. In general, higher values will result in less power consumption but will create worse performance in terms of bias and offset issues as well as noise issues. But you often don't have the flexibility that you think you do because of needing to meet input/output impedance specs or use reasonable size capacitors (or inductors) or other considerations that really narrow down your options, often to the point of having to add stages or otherwise rework your design to decouple these constraints sufficiently so as to be able to meet them all.
     
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