Operational Amplifier Questions

Discussion in 'General Electronics Chat' started by ELECTRONERD, Sep 30, 2009.

  1. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    Hey everyone,

    I have a few questions regarding op-amps and wondered if you could clarify them for me. According to the picture in the attachment, you can see that an op-amp is more or less a voltage amplifier, correct? What about current gain, how would you calculate that on an op-amp? Would the current going into the circuit be the same coming out? If you wanted to increase current, would you use a couple of transistors, which are current amps, to accomplish this?

    How do you define negative and positive feedback? Would some of the output going into the non-inverting input (+) be positive feedback, and some of the output going into the inverting input (-) be negative feedback?
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    896
    No.
    The circuit is missing a power supply, an input connection, an output load and negative feedback.

    Some opamps have Fet transistors or Mosfets at their inputs that use zero current. Then is the current gain infinite?
    An opamp can be a current amplifier when it amplifies the voltage across a resistor when the voltage is caused by current in the resistor.

    Of course not!

    Yes with additional resistors and diodes to bias the transistors.
    But you could also use an opamp with a higher output current like a power opamp or an audio amplifier IC.

    Yes.
    Positive feedback makes an opamp latch its output near the positive supply voltage or near the negative supply voltage or cause oscillation.
    Negative feedback reduces the gain, reduces the distortion, increases the high frequency bandwidth, increases the input impedance and decreases the output impedance.
     
  3. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    Well, I'm ignoring those connections at the moment, I just want to note the basic function of an op-amp; that would be amplifying the signal.

    How would you calculate the current amplification then?

    Thanks for the positive and negative feedback response, it was very helpful.
     
  4. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    I'm not sure you can talk about current "amplification". Since the current going into the op-amp is practically zero (very small at least).
    The current output depends entirely on the load resistance. The voltage is amplified to some output voltage. The current out depends on the output resistance of the op-amp and the load resistance.

    So Io = Vo / (Rout + Rload).

    But because the current going into the op-amp is extremely small because of the really high input resistance of the op-amp you could say that the current gain was really high.
    I'm not sure you can say that current gain matters in the usual voltage amplification op-amp configurations. Since it "depends" on the input resistance, output resistance, resistance of the load and the internal resistance of the power supply feeding the amp.
     
  5. peranders

    Well-Known Member

    May 21, 2007
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    Have you read the datasheet for the opamp you plan to use? There you can find maximum output currents, input bias currents etc.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    He got the specs for transistors mixed up on their datasheets so I think he will also get the spec's for opamps mixed up on their datasheets.
     
  7. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    Guess what! I'm not even using an op-amp, I just wanted to find out this out of curiosity. I'm not building anything, I just want information for future projects.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    Look at the definition of an opamp:
    1) Extremely high input impedance and extremely low input current.
    2) Extremely low output impedance and a very high output current.
    3) Extremely high voltage gain before negative feedback is added.
    4) Very wide bandwidth.
    Opamps are a compromise of these definitions.
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,962
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    Definition
    "op-amp" is an electronic circuit module (normally built as an integrated circuit, but occasionally with discrete transistors or vacuum tubes) which has a non-inverting input (+), an inverting input (-) and one output. The output voltage is the difference between the + and - inputs multiplied by the open-loop gain: Vout = (V+ − V−)*G_open_loop.

    When the voltage at the input "+" (non-inverting) rise,output voltage is also increases.
    Increase voltage at "-" input (inverting input) causes, decrease the output voltage.
    Decrease voltage at input "-" increases the output voltage.

    [​IMG]

    And some examples

    [​IMG]

    [​IMG]

    [​IMG]


    And Op-amps with negative feedback

    1. Non-inverting amplifier with close loop voltage gain
    Au=1+(R2/R1)

    [​IMG]


    2. Inverting amplifier with close loop voltage gain
    Au=-R2/R1

    [​IMG]
     
  10. ifixit

    Distinguished Member

    Nov 20, 2008
    639
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    If you're interested in current mode op-amps then take a look at the Norton amplifier. e.g. LM3900, LM359.

    Regards,
    Ifixit
     
  11. delmasli

    New Member

    Sep 17, 2009
    17
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    Negative feedback is a self-correcting feedback. It means; if one of the facts (cause or result) of a system varies, system resists to this variation and tries to return its previous condition.


    Positive feedback is self-reinforcing feedback. It means; if one of the facts (cause or result) of a system varies, intra-system interactions cause to increase these variations gradually.
     
    Last edited: Jan 31, 2010
  12. Audioguru

    New Member

    Dec 20, 2007
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    The unusual LM3900 is not made anymore.
    I have never seen an LM359.
     
    Last edited by a moderator: Oct 25, 2009
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