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Opamp non-inverting amplifier basic question (noise related)

Discussion in 'General Electronics Chat' started by caster.cp, Jan 1, 2010.

  1. caster.cp

    caster.cp New Member

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    I have stumbled today with a very simple and basic question.

    I used to put a resistor in series with the non-inverting input of the Opamp in order to "cancel" the input bias current effect (I learned that some time ago and use it till today as a rule of thumb: Rn-i = Rfb || Ri. Rn-i - resistor at the non-inverting input. Rfb - resistor at the feedback loop. Ri - resistor at the inverting input).

    But today, building a basic power amplifier with an Opamp pre-amp stage, I first thought about the noise that the resistor will add to the circuit.

    If anyone is familiar with noise analysis: to quantify the noise in the circuit, generally what's done is to compare the total noise present in the circuit with the input signal, "at the input". As this resistor IS at the input, ALL its noise will be clearly amplified too.

    My question is: in terms of noise, is it worth putting this resistor at the input? Is the "bias current cancelation" so important?

    I couldn't HEAR any difference in the noise with and without that resistor in the circuit I've built. But in really noise critical applications, what's worse, the noise or the input current?


    Castilho
     
    Last edited: Jan 1, 2010
  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    So why not bypass the series bias resistor with a capacitor? Is a capacitor more or less noisy than the resistor alone would be? AFAIK, it depends on the value of the resistor, what type of resistor (carbon vs metal film), etc, etc...

    Ultra-low noise design is yet another "black art" subset of electronics.
     
  3. Sceadwian

    Sceadwian Banned

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    As long as the current going through the resistor is low the noise should be low as well.
     
  4. dave

    Dave New Member

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  5. Mikebits

    Mikebits Well-Known Member

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    The noise created by an input resistor is proportional to its resistance and temperature. The formula for this noise is formula.gif so when input noise is a concern such as in a high gain amp, it is best to scale your input resistor to a small value.
     
  6. caster.cp

    caster.cp New Member

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    Thanks for the prompt replies.

    I know (basically) how to calculate and estimate the noise in a circuit. Have learned the basics of thermal (Johnson) noise, shot noise, flicker (excess) noise in resistors and so on... The problem isn't that.

    The point is another one, and I think I haven't made it clear because of the title. I want to know if leaving OUT the resistor will do any (and what kinds of) harm to the circuit. What's the influence of this "uncompensated" input bias current? Will it leave some DC level on the input of the amp, will it be solved with the normal compensation terminals that some op-amps have (like pins 1 & 5 of the TL071)?


    But rephrasing the question: what kind of problems can occur if I leave the resistor connected to the non-inverting input of a op-amp OUT of my circuit? If any, how they compare with the existence of noise in the circuit?

    Thanks again :)
     
  7. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Leaving out the input resistor will effect the offset at the output of the opamp. If offset is of no concern, or if it can be adjusted out by other means, then dont sweat it.
     
  8. unclejed613

    unclejed613 Well-Known Member

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    usually, resistors 10k and below aren't significant noise sources. if the op amp gain is reasonable (between 1 and 50) it's probably nothing to worry about. there are other ways to correct the offset, depending on what you're using the op amp for, and some of these also apply to audio power amps (big op amps using discrete components in most cases). the simplest way to balance offset is to balance the input currents as you're describing. you could also use a bias source with a potentiometer and a high value resistor. unfortunately the resistor you use to isolate the bias supply from the input impedance of the op amp is also going to generate some noise. another option is to (and this is an audio only option, as you will no longer have any gain below a certain frequency, usually between 1 hz and about 30hz, depending on component selection) use a DC blocking cap between the inverting input and ground, in series with the divider resistor. there's another option, but it uses up another op amp (which actually would be good if you have an unused op amp in a dual or quad), and that's a DC servo. so you would have an AC feedback loop, and a separate DC feedback loop that goes through a second op amp being used as an integrator, and providing DC correction for the first op amp. of all of the solutions, the DC blocking cap probably introduces the least noise, and is only a few cents more expensive than the balancing of input currents method.

    the bias nulling terminals on the TL071 are linked to the current mirror, and are used as in the picture below.

    another couple of things to consider are using low-noise op amps and op amps with very low bias currents. FET input op amps are usually very good in both categories. the best known of these being LF353 and TL072 op amps. there are newer improved versions of these, the TL2072 being faster, quieter and having even less input bias current than it's "grandpa" the TL072.
     

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    Last edited: Jan 1, 2010
  9. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Except possibly for old style carbon composition resistors, the noise generated by a resistor is independent of the current though it. As Mikebits stated, a resistor generates a thermal noise voltage which is proportional to the resistor's temperature and resistance.
     
  10. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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