To amplify a signal, non-inverting opamp is superior to inverting opamp?

Discussion in 'General Electronics Chat' started by tpny, Nov 1, 2012.

  1. tpny

    Thread Starter Member

    May 6, 2012
    Because with non-inverting you can tie signal directly to opamp's input without a resistor. (But you do with inverting if gain > 1.) And since resistor adds noise to siganl why would you ever use an inverting configuration?
  2. panic mode

    Senior Member

    Oct 10, 2011
    what do you know about the noise? what makes noise? where does the noise come from?

    every device produces noise (even a tiny piece of wire or diode or transistor etc.). but just what do you think the opamp is made of? it has few dozens of such components already inside of it. and now you are ignoring all those many components inside it and choose to freak out because of one that is added externally?

    how much noise is ok?

    it depends on application, sometimes we purposely allow it. next time you use the phone, compare the amount of noise to a CD player. does that make the phone a useless invention? obviously we can make phone that would have far better quality, why don't we?

    there is something called cost. not just cost of phones but cost of bandwidth and switching.

    note that every signal can be viewed as composition of magnitude and phase (like vector). and phase is very important. in circuits there is often need to change phase of the signal to produce desired effect.

    next time you see a fighting scene in some movie, think about how much beating one would get if he could move away in a same phase as blows are delivered. what if the movements are such that one just happens to place his face of whatever body part in the path of each blow? beating somoene or being beatten is a huge difference, isn't it? or is it just a matter of phase?

    phase is what makes the difference between an amplifier and an oscillator. both have gain and both are used but for different reasons. being able to change phase is obviously great advantage. the simplest way to change phase is to invert it. are the inverting inputs still useless? are the amplifiers with gain =1 useless too?
    Last edited: Nov 1, 2012
  3. crutschow


    Mar 14, 2008
    A non-inverting op amp configuration is slightly better for noise and also has a higher input impedance than an inverting configuration.

    But if you want to sum more than one signal, than an inverting amp configuration is better since there is no interaction from one input signal to the next as there is with the non-inverting configuration. Also for doing various functions like integration, differentiation, etc. the inverting configuration is easier to use.
  4. mlog


    Feb 11, 2012
    Even in non-inverting applications with a bipolar opamp, it is a good practice to include compensation resistors on the inputs in order to cancel out the effects of the input bias current. For example, if the impedance seen by the inverting input is 100 kΩ, then a 100 kΩ resistor would be added to the non-inverting terminal. You would not simply connect the input signal to the opamps non-inverting input.
  5. crutschow


    Mar 14, 2008
    That depends somewhat on the application and whether you are concerned about noise. You would not likely have to do it for a MOS or JFET input amplifier since they have very low bias currents. And the resistor does add some noise thermal noise to the circuit. It can also add a high frequency rolloff pole due to stray input capacitance, which may be important in high frequency amplification applications.
  6. ramancini8

    Active Member

    Jul 18, 2012
    The noise generated by a single resistor is not an issue in the vast majority of applications because op amp noise, power supply noise, stray noise picked up through the wiring, other resistor noise, etc. usually swamp out the noise generated by a single resistor. If the resistor is a low value its generated noise is minimal.

    Other considerations such as impedance, signal polarity, etc. normally overshadow the resistor noise issue.
  7. crutschow


    Mar 14, 2008
    In general true. But there are op amps that have a noise level comparable to a 100Ω resistor and for very low noise applications you don't want to add any more resistance at the input than you absolutely need. For a first stage with reasonable gain, the input resistor values are the most critical and contribute the most noise relative to the signal.