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Another op amp question

  1. Mar 8, 2014 #1
    Hello folks,
    This is my first post here, I hope I am posting my question in the correct place/manner? I am a mechanical engineering student taking an electrical class. We are studying op amps. The thing that is confusing me is current flow in the circuits. I understand that there is infinite input impedance going into the op amp, and zero impedance out. I wonder how current can flow out of the op amp if none flows in? Am I correct in understanding that inside of an amplifier there is some sort of dependent voltage source? If there is no current flowing into the op amp how can the source inside depend on anything? Thanks in advance for any help somebody can give me in understanding op amps, I will appreciate it.
     
  2. jcsd
  3. Mar 8, 2014 #2
    Some diagrams of Op Amps leave out the voltage source (Vs+,Vs-). That is where the output current flows from.

    180px-Op-amp_symbol.svg.png
     
  4. Mar 8, 2014 #3

    sophiecentaur

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    A real OpAmp, of course, has a finite input current and a finite gain. (And a non zero output impedance). But, when incorporated into a suitable circuit, (using feedback) the circuit characteristics can be calculated by assuming that the OpAmp is 'ideal'. The results are 'near enough' for the purpose in a good design.
    As in all electronics, there is an accepted tolerance (depending on the spec of the requirement) for all component values and that includes amplifier characteristics. So 'near enough' is not a cop-out; it's a way of achieving a result with (as always) finite resources.

    If you are starting on a course on OpAmps, you will soon get involved with the idea of feedback and with the formulae involved. You find that there is (usually) negligible difference in performance in a feedback circuit with a real amplifier unit or an idealised one and the gain or other characteristics of the circuit are pretty well independent of the detailed performance of the amplifying component.

    If you are a Mech Eng, you will be familiar with the idea of assuming some parts of structure are 'ideal' and you will identify the critical elements and use only them in calculations. It's similar with electronics.
     
  5. Mar 8, 2014 #4

    rude man

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    This is probably your 3rd answer at least, but anyway the answer is the current emanates from either or both power supplies.
     
  6. Mar 9, 2014 #5

    sophiecentaur

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    Ah yes - I now see what the OP was confused about.
    The basics of all amplifiers is that they control the flow of power out, supplied by a power supply, by a small amount of input signal power. I always think that the first amplifying devices were given a good name - Valve. That implies controlling the flow through something with a control input. All amplifiers have at three or more connections for this reason.
    On OpAmp schematic circuits, they tend to leave out the Power Supply connections. Confusing but it reduces the 'clutter' on the diagram.
     
  7. Mar 9, 2014 #6
    Thanks for the replies everyone. I had seen the diagram with the Vs (+ and -) going into the sides, but it is confusing how usually they are left out. I guess I have been confused in that I have been thinking the V+ and V- input's are essentially voltage sources? But I guess maybe not. In fact my professor has given op amp homework problems where he included voltage sources that are connected directly to the V- and V+ terminals but the Vs terminals shown on the diagram above are left out. This confuses me, and I guess brings me back to my original question. I have actually been able to solve several of these circuits for gain and or resistor values, etc. using voltage and current division but none the less I find it confusing. I will go talk to my professor about it tomorrow I suppose.
     
  8. Mar 9, 2014 #7

    sophiecentaur

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    A voltage source means it will supply enough current to maintain it's specified voltage. Like a big battery.
     
  9. Mar 9, 2014 #8

    rude man

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    The +V and -V power supply contacts are often left out of a diagram, the assumption being that, in conjunction with appropriate feedback, the op amp is operating linearly. Their value becomes important only in dc analysis when inputs come close to, or result in, saturating the amplifier.
     
  10. Mar 9, 2014 #9

    dlgoff

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    There are integrated circuits with multiple operational amplifiers in one package. e.g. this dual-in-line package (DIP) of the LM324.

    lm324.jpg

    When drawing a schematic of a circuit that uses more than one op-amp from a package, you really only need to show the power input pins on one. e.g. this circuit using two amps of the quad op-amp package (each labeled as 1/4 LM324 with power pins 4 and 11 shown on only one).

    figure_02.gif
     
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