Why do op-amps have a high input resistance?

Discussion in 'General Electronics Chat' started by deki, Oct 1, 2011.

  1. deki

    Thread Starter New Member

    Sep 6, 2011
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    So op-amps are meant to have a high input resistance right? Typically 10^5 - 10^13 Ω according to my university studies. I don't understand exactly why?

    One explanation I can think of: because op-amps amplify a signal, therefore you'd want to lower the signal as much as possible before amplifying? But this doesn't make sense to me, since why not just amplify what you're already getting at the input? Then you have inverting amplifiers..

    I know that op-amps have a low output impedance (10-100Ω) and this is pretty obvious why...I'm just stuck with the high input impedance.
     
  2. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    That wasn't always the case ... have a look at their history in wikipedia!

    FET based opamps, which are basic stock components these days have very high input impedance because the gates are Field Effect Transistor based, although the types still vary.

    I am no expert so cant give you the lowdown on the basic types but it is safe to say if you look up FET gate types, JFET, MosFET for example you will get a handle on things.

    Very generally you would use an OpAmp when you wanted to measure a voltage differential without having to worry about any input current affecting the signal you are trying to measure. (There is always a tiny current but it is usually so small that you can ignore it for most practical purposes.

    There is a good tutorial on op-amps on this site in 'Vol. III - Semiconductors' I think.

    Output impedance is not always low in fact if you were to look at a transconductance ( possibly transductance ) you will see that it actually controls impedance as opposed to voltage
    Useful for making voltage controlled resistors with effectively isolated inputs, but again I am no expert.

    Op-amps are a huge subject in their own right so having just said 'in general' I am now going to say that there is such a diverse range that generalities will not get you all that far.

    Hope that gets you started looking further
    Al
     
  3. #12

    Expert

    Nov 30, 2010
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    Your question indicates that you think input impedance has something to do with signal level. It doesn't in this case.

    You almost never lower the signal before amplifying to avoid wrecking the signal to noise ratio.

    Op-amps have high impedance so they will not become a load on the signal, whether the signal is microvolts or whole volts. In the positive configuration, an op-amp is a dial-a-gain device. Pick a gain and calculate the resistors. Then, because the op-amp has high input impedance, you don't have to compensate for the loss into the input for the gain resistors or the signal.

    In the inverting configuration, you think in terms of input signal through a resistance causes a current. The feedback resistor allows the same amount of current and the op-amp supplies the voltage to drive that much current. Meanwhile, because of the high impedance of the input, you don't have to calculate a compensation for the signal loss into the op-amp input.
     
  4. nigelwright7557

    Senior Member

    May 10, 2008
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    The high input impedance is to make the op amp more useful at amplifying in precise amounts with external resistors. The high impedance also doesnt load high impedance signal sources.
     
  5. deki

    Thread Starter New Member

    Sep 6, 2011
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    Thanks fellas!

    So all it has to do is with the loading effect (?).
     
  6. nigelwright7557

    Senior Member

    May 10, 2008
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    Yes. An ideal op amp has infinite input resistance and zero output impedance.
     
  7. #12

    Expert

    Nov 30, 2010
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    You can't buy an "ideal" op-amp, but you can get really good ones. You must have rather special needs before the input impedance of a modern op-amp becomes a factor in the math. I've been there, but it's rare, and mostly because I was working with op-amps 40 years ago. They are a lot better now!
     
  8. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Remember the original point of op amps was to perform mathematical operations....hence the name.

    At any rate.....any amplifier with negative feedback is going to have a higher input impedance. Kirchoff's current law guarantees it.....the current leaving a node is equal to the currents entering the node. If you're feeding current back into the node from the output, less current is going to be admitted from the "outside'.....which is, by definition, higher impedance.

    This also works with Krichoff's voltage law. Amazingly consistent universe we dwell in!

    Eric
     
  9. dlhylton

    New Member

    Jan 24, 2012
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    Op amps need high input impedance because they are voltage-gain devices. In order for voltage to drop across the input, the impedance has to be very high, as ohm's law states, V=IR.

    It's also important to prevent the loading effect. If the impedance were small, the current draw would be high. This is undesired because it would produce dangerous amounts of current and introduce more noise into the circuit. To prevent this, op amps have high input impedance.

    Check out

    http://www.learningaboutelectronics...gh-input-impedance-and-a-low-output-impedance

    for a full explanation of this
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Why did you think high input impedance would lower the signal level? :confused:
     
  11. studiot

    AAC Fanatic!

    Nov 9, 2007
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    hello dexi,

    The short answer is they don't!

    Most electronic signal processing is conducted in terms of voltage and high input impedance makes this easy.

    However if you wish to work in current terms or at high frequency or low noise high impedance can be an embarrassment.

    High frequency op amps often work at transmission line impedances.

    There is a type of op amp called a transconductance amp specifically for this situation. Google has plenty of references to this.

    The official term is operational transconductance amplifier.

    go well
     
  12. #12

    Expert

    Nov 30, 2010
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    This thread is 6 months old. deki probably isn't looking at it anymore.
     
  13. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Well spotted mate!

    ;)
     
  14. #12

    Expert

    Nov 30, 2010
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    As one of the helpers on this thread 6 months ago, it was easy to spot. I just noticed the cobwebs in my mind being disturbed. ;)
     
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