Input offset current/voltage

Discussion in 'General Electronics Chat' started by mentaaal, Oct 2, 2008.

  1. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Hey guys, i am trying to find the difference between input offset current and input offset voltage in opamps...

    In the art of electronics it says:
    Which seems to suggest the two are the same thing or at least related.. But in my notes my lecturer treats them as two distinct errors as he eventually adds the errors together?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Lets call the inverting input as 1 and the non-inverting input as 2.

    The bias current is defined as Ibias=(I1+I2)/2 (1)

    The offset current is defined as Ioff=I1-I2 (2)

    If you combine the two above equations you will find that:

    I1=Ibias+Ioff/2 (3)

    I2=Ibias-Ioff/2 (4)

    These is the equations which relate this quantities. Now, by bias current we mean the input current the transistor (internal transistor of the input differential stage) needs to produce an amount of output voltage. On datasheets you find the average bias current (equation 1) of the two inputs. If you want to calculate the actual bias current of each input you have to use equations 3 and 4. Now, the offset current which appears in the equations is due to the fact that the input transistors (internal) does not have exactly the same gain and each one draws a bit different amount of current. The difference between these two input currents is called the offset current (equation 2). Your teacher is right because the error voltages created by the slight different bias currents of each input are not the same as the offset voltage. The offset voltage is an internal voltage created on the output of the differential input stage because the gain of the transistors are not exactly matched (gain - base-emitter diode not the same). You can treat this offset voltage as a DC voltage on one of the inputs off the amplifier (depends). Because the offset voltage (DC source on the input) and the offset voltage created by the offset current are in series you can add them together to get the total error voltage.
     
  3. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    That was a such an informative reply i am actually going to keep that in my notes!

    Thanks for going to the trouble. It is not going to get wasted. I will be using that for our next asessment
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    I am happy my information are useful to you :)
     
  5. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Opamps are the devil, i have spent so, so many hours trying to understand them!

    In an opamp question we are given:

    Iio = 10nA
    Ib = 50nA
    Vos = 2mV
    gain = 10
    Aol = 100dB
    Rf = 10k (negative feedback resistor)
    In working out the steady state errors: Vos just gets added to Rf*10nA = 2.1mV which later gets added to the gain error factor for a total error.

    Whats bother me here is that the art of circuits refers to Vos as:
    Then by this definition doesnt that make Vos an input error that still has to be amplified by the opamp? Why do we just assume that this means that there will be an extra 2mV at the output?


    Another thing i am confused about is in the calculation of the steady state errors in an integrator.

    Vout is defined to be: -1/RC∫Vin dt ± 1/c∫Ib dt ± 1/RC ∫Vio dt + Vio

    Ok before I go nuts:
    Why is the bias current only 1/c∫Ib dt and not 1/Rc∫Ib dt?
    And the same question as the first one: why is vio dealt with as: 1/RC ∫Vio + Vio??
     
  6. mik3

    Senior Member

    Feb 4, 2008
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    No, this is an error which has to be added to the other errors because the offset voltage exists internally. The art of the circuits just says a way to minimize this offset voltage by adding a voltage source between one of the inputs and the relative input resistor. The polarity of the voltage source depends on the internal offset voltage polarity. Imagine that you have an ideal op amp (without errors), to create this offset voltage error you add a voltage source (with its voltage equal the offset voltage) between one of the inputs and the relative input resistor. To eliminate this offset voltage (voltage source) you have to put another voltage source in series with it and with opposite polarity. Thus this another voltage source is has an equal voltage (magnitude) to the offset voltage and is that the art of circuits mentions.
     
  7. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Thanks for that Mike. What i am still confused about is, if this is only an error in the output voltage, why did my lecturer, for the integrator, integrate this error as well? ( 1/RC∫Vio dt) If you have to do this, do you not have to do something similar with Vos for ordinary resistance inverting opamps?
     
  8. mik3

    Senior Member

    Feb 4, 2008
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    Yes sure, you have to add the errors on the output voltage in all cases of operation of the op amp.
     
  9. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    ARGGGHHh then why didnt my lecturer say so! Its so hard to study my notes lemme tell you!

    Ok well then what up with the dealing of the bias current? Why does it not take into account Rin? (for first example - the output error was Rf*Iio and for the integrator : -1/C∫Ib )

    And my lecturer seems to have omitted Iio for the integrator... is that intentional or perhaps just a mistake do you think?
     
  10. mik3

    Senior Member

    Feb 4, 2008
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    Well, i am not sure because i havent analyzed an integrator in such depth until now to know how the offset current affects it but i believe he just omitted it for simplicity. The depth of analysis depends on how accurate you want your circuit to be. If you want a very accurate analysis you have to take into account that the values of the resistors, for example, vary with temperature and have tolerance and so on.......
    In electronics you dont have to design every time the perfect design, you design with the accuracy your application needs. Remember, the greater accuracy and performance of any circuit the greater the cost of the design and manufacture.
     
  11. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Well believe me mike, i have absolutely no intention of designing anything at the moment... I am just one of those people that, cannot tolerate not understanding my notes.

    I just want to understand whats going on here thats all...

    Which reminds me, you might wanna take a look at this little funny:
    http://xkcd.com/356/

    Thanks for your help though, tis greatly apprectiated.
     
  12. mik3

    Senior Member

    Feb 4, 2008
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    I am sorry but i am studying at the moment my university lectures so i cant analyze it for you. Maybe another time i will post what you want to know.
     
  13. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    No problem, its a forum, not an IM and I expect nothing from anyone ;-)

    Although i do believe i found the explanation i am looking for. Perhaps it will help you too if you are interested: (interesting part in bold)

     
  14. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Ok, in trying to further understand the effect of bias current in a circuit, i re-read the above quote a couple a million times. Unfortunately, when i copied it, the formulae didnt copy so instead i have uploaded the relevent section here in a pdf form. The author describes the voltage inputs to the circuit as being 0 volts. And using this, he shows how the output voltage depends on Ib*rf. what i dont understand is how this line of thought applies to input voltages that are not 0 volts. The reason why i dont follow this is that he says that when the current going into the non-inverting input is allowed to be zero, the voltage at the inverting terminal will be zero too (virtual ground) and so no current flows through R1. But this does not follow for input voltages != 0... does it?

    He clearly shows that it can be done like this as he does in an example in the pdf.
     
  15. mik3

    Senior Member

    Feb 4, 2008
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    For an ideal op amp the voltage difference between the inputs is zero because it has infinite gain. In a real op amp, because the gain is not infinite, you have a small voltage difference between the inputs but it is still very small assuming the amplifier works in the linear mode. This is true for every voltage and not only for zero. That is the non-inverting input is held at 3 volts for example, the voltage on the inverting input will be very close to 3 volts if the op amp works in its linear region.
     
  16. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Analogue circuits is starting to make me feel incredbly stupid.

    I agree with what you are saying Mike but my impression of the idea the author is trying to put forward is that on the inverting pin, because the input before R1 is 0 and because of virtual ground, no current flows through R1. Thats all fine and dandy but what if the input before R1 is not 0 volts, then the there would be a voltage difference between the terminals of R1.
     
  17. mik3

    Senior Member

    Feb 4, 2008
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    In reality even with zero volts on the input before R1 a small current flows through R1. But if we assume an ideal op amp then for current to flow through R1 a non zero voltage has to be applied on the input before R1.
     
  18. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    But if the there is a voltage across R1 then it cant be said that no bias current will flow through R1. Thats the basis of the author's argument, that is because Vin was 0 volts, the bias current would not flow through r1. (both terminals at same potential)

    Am i just looking at this wrong or is there another explanation?
     
    Last edited: Oct 17, 2008
  19. mik3

    Senior Member

    Feb 4, 2008
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    It can flow through Rf and the output will be slightly negative as to have zero volts on the inverting input.
     
  20. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    ok.. perhaps one last question to annoy you with and i will understand... what if vin was negative.... is it possible that the bias current could flow through r1?
     
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