Op Amp

Discussion in 'General Electronics Chat' started by alphacat, Feb 7, 2010.

  1. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    I understand pretty well how an op-amp operates at small signal.
    You need to bias the op-amp in order to set the transistors at their constant current region, to achieve max output voltage swing, and to eliminate DC voltage/current at output.

    Once these are set, you can calculate the small-signal gain of the amplifier.

    However, how do you know what is the large-signal gain of the amplifier?

    Moreover, how can you tell the DC input and output resistances of the amplifier?
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    The only thing you need to do is ensure that there is a path for dc current that can be sourced or sinked by the opamp input.
    Rin depends on the specific circuit.
    And Rout is very low thanks to negative feedback
    Rout=Rout_open_loop/(1+Aol*K)
    Where
    Aol is a open-loop gain of a opamp (see datasheet)
    K - feedback factor ( feedback gain ), sometime we use letter "β" for feedback gain.
    http://forum.allaboutcircuits.com/showthread.php?t=24971&highlight=loop

    And for example this circuit
    [​IMG]

    Rin≈R5; Au=Vout/Vin=1+(R2/R1); F_cut-off=GBW/Au
    GBW - Gain bandwidth product (in the datasheet )
    And low corner frequency depends on C1 and R5, C3 and R1, C4 and Rload.
    And ideal opamp has
    Rin=∞; Rout=0; Aol=∞;
     
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    What is the difference between small-signal gain and large-signal gain.
    In opamp large-signal gain is Aol
    http://forum.allaboutcircuits.com/showpost.php?p=173559&postcount=9
     
    Last edited: Feb 7, 2010
  4. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    Thank you very much Jony.

    Well, the small-signal input resistance of a BJT-based Op-amp is 2*rπ.
    How do you calculate the large-signal input resistance?

    How do you reach the large-signal Rout_open_loop?
    I know how to calculate the small-signal Rout_open_loop but not the large-signal one?

    You are right, they are the same, my mistake.
    Do you calculate the large-signal gain by calculating first the small signal gain and placing ω=0?
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    When we design circuit with opamp we usually treat them as ideal opamp. And if we want to know Rin or Aol , Rout wee look to the datasheet, we don't need to do any calculations .
    For example for old op amp LM741

    http://pdf.elenota.pl/pdf/Fairchild/lm741.pdf
    Rin=250KΩ (Figure 2.Input Resistance and Input
    Capacitance vs Frequency)
    Rout_open_loop=100Ω (Figure 1. Output Resistance vs Frequency)
    And Aol=15000[V/V] ω=0
    http://www.national.com/an/AN/AN-A.pdf
    http://users.ece.gatech.edu/~mleach/ece4435/chap02.pdf

    And can you tell me the difference between small-signal vs large-signal resistance ?
    For me there is no such thing as large-signal resistance.
     
    Last edited: Feb 7, 2010
  6. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    Thank you very much Jony!
    I got it now.
    My problem was that i didnt know there was no difference between Rin, Rout, Aol in small signal (for low frequencies) and in large-signal.

    Theoretically, in large signal analysis, I can draw a 2*rπ resistor between the op-amp's input pins, in order to calculate DC voltage across the circuit?
     
  7. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Well, yes but we never do this. We treat op-amp as black box and assume that:
    Rin=∞, so input current is 0A. (No current is flow to the op-amp inputs)
    Rout=0Ω
    And the open loop gain is infinite large.
    And with this assumptions we get that in linear region non-inventive input voltage "V+" is equal inverting input voltages "V-" and we call it "virtual short". And that is why is this diagrams V+=V-
    [​IMG]

    [​IMG]

    http://focus.ti.com/lit/an/slod006b/slod006b.pdf
     
  8. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    Got it!

    Thank you very much! :)

    You taught me very important things.
     
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