Op amp input impedance; inverting vs. noninverting amplifiers

Discussion in 'General Electronics Chat' started by colinb, Nov 3, 2011.

  1. colinb

    Thread Starter Active Member

    Jun 15, 2011
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    (This question comes from some comments made in the thread Easiest way to detect sound.)

    I have really one major question and one minor question.


    Question #1, Minor question:
    Is there such a thing as an “inverting op amp” and “noninverting op amp”?

    I understand that an inverting amplifier and noninverting amplifier are just two configurations of circuit constructed with an op amp (the op amp itself being potentially an identical device in either configuration). Audioguru keeps mentioning a inverting op amp but I don't understand. Now, perhaps “inverting op amp” is just an imprecise shorthand for “inverting amplifier build with an op amp”, but that is not clear to me. (For reference, I have read the AAC op amp chapter and specifically been looking at the divided feedback section.)
    Well, the AAC op amp chapter clearly does talk about noninverting and inverting amplifiers using divided negative feedback. If it “does not talk about inverting and non-inverting opamps”, then what exactly is an inverting or non-inverting opamp???



    Question #2, Major question:
    What is the input impedance of an op amp based amplifier circuit?
    Does an inverting amplifier have a lower input impedance than an inverting amplifier? After reading the AAC Op-Amp chapter, I did not find any mention of the fact that an inverting amplifier configuration of op-amp has a lower input impedance. In fact, I first thought that op amp amplifiers would have a very high input impedance, since it is a distinguishing characteristic of the op amp inputs that they have nearly infinite impedance (theoretically infinite in simple model).

    But looking at the inverting amplifier divided feedback circuit it is clear that the op amp output voltage has a finite impedance to the amplifier input signal, due to the two resistors in series between these points. That seems to indicate that with, for instance, the first amplifier circuit, a noninverting amplifier, on the Divided feedback page (Figure 1, see below) would have a very high input impedance (ideally infinite) because the input signal (the 6 V battery) is directly connected only to the op amp noninverting input.

    [​IMG]
    Figure 1: Noninverting amplifier circuit.


    On the other hand, an inverting amplifier appears that it would have a low input impedance due to the current that would flow from the input signal (again, the 6 V battery) to the op amp output through the two divider resistors (see Figure 2, below).

    [​IMG]
    Figure 2: Inverting amplifier circuit.


    It sounds like the AAC Operational Amplifiers chapter could benefit from a note about the significant difference in input impedance between noninverting and inverting amplifier circuits. This is likely going to be a critical factor in the choice of op amp circuit used for many applications.

    (On the topic of this AAC page, Vol III > Op amps > Divided feedback, I think that the battery symbol showing the input voltage to the op amp could be more clearly indicated as the “input signal” to be amplified rather than just a battery with “6 V” label.) When this input signal moves around e.g. between the inverting and noninverting amplifier circuits, it is to me not immediately clear from where the input signal is coming.)
     
  2. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    You have written more than I am willing to push thru. However some simple ( aproximate ) theory. Both input to amp have infinite impedance. When an amp is used as an inverter the inverting input is a virtual ground. Because of the virtual ground the input to the whole circuit is just the input resistor connected to the inverting input. With a non inverting input you can pretty much ignore the op amp and look at the resistor network to ground ( if any ).
     
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  3. crutschow

    Expert

    Mar 14, 2008
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    No.

    As you surmised, that is shortened terminology for an op amp connected as a inverting amplifier or a non-inverting amplifier. All op amps have a differential input with the (+) terminal in phase with the output and the (-) terminal 180 degrees out-of-phase with the output (at DC or low frequencies).
     
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  4. #12

    Expert

    Nov 30, 2010
    16,313
    6,817
    You have it all right. You just needed confirmation.
    You're doing just fine.
     
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  5. jimkeith

    Active Member

    Oct 26, 2011
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    99
    Yes, input resistance of inverting amplifiers are determined by the value of the source resistor. High input resistance is possible via the use of a voltage follower to drive the input resistor, or another inverting amplifier on the output. Since they come 2 or 4 in an IC, additional stages are not a big deal.

    The one amplifier not mentioned is the instrumentation amplifier--it has high impedance on both inputs--but that is an entirely different deal with the gain controlled internally--generally a factor of ten, hundred etc.
     
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  6. Audioguru

    New Member

    Dec 20, 2007
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    Yes, I meant an inverting opamp amplifier circuit.
    The AAC Opamp Chapter should mention that the inverting opamp amplifier has a fairly low input impedance that is the input resistor's resistance.
     
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  7. SgtWookie

    Expert

    Jul 17, 2007
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    You seem to have it covered, along with everyone's feedback so far.

    One thing that might help you understand opamps a bit better is, that when wired with negative feedback, the opamp will adjust its' output so that what it "sees" on the inverting input is equal to what it sees on the noninverting input.

    The input impedance of an opamp isn't exactly infinite, but it's pretty high. JFET-input opamps have a much higher input impedance than transistorized inputs. In many cases, you can be relatively safe in assuming that a voltage feedback opamp itself will have a near-infinite input impedance. What makes a big difference is how it's connected.

    Current feedback operational amplifiers are another story; the inverting input has a relatively low impedance, and it's much lower than the noninverting input. These type of opamps have very high bandwidth (hundreds of MHz), and are most frequently used in video signal processing.
    If you are interested in learning about current feedback amplifiers, Texas Instruments has this handy circuit collection:
    http://www.ti.com/lit/an/sloa066/sloa066.pdf
    At the very least, you should read the introduction to help understand the difference between voltage and current feedback opamps.
    National has another current feedback opamp circuit collection:
    http://www.national.com/an/OA/OA-07.pdf

    Here is a handy page of links to various application notes and guides for opamps, comparators, and various other circuits:
    http://my.ece.ucsb.edu/bobsclass/2C/Tutorials/application_notes.htm
     
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  8. Wendy

    Moderator

    Mar 24, 2008
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    There is a reason FET type op amps exist. Their input impedance is several orders of magnitude better than a BJT op amp. BJTs do pretty well though. I think (but am not sure of) in excess of 100MΩ.
     
  9. colinb

    Thread Starter Active Member

    Jun 15, 2011
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    I guess the surprise to me was that the inverting amplifier configuration has such a low impedance, no matter how high the op amp's input impedance.
     
  10. RickH

    New Member

    Dec 10, 2010
    21
    0
    High and low impedance is relative to the device driving it.
    An example would be a Microphone is 7K ohm output impedance. If you tried to amplify it with an inverting Amp and used a 10K input resistor 40% of the voltage would be dropped across the Mic and 60% would be at the amp.
    If you increased the input resistor too say 250K only 2% would be dropped across the mic.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    The thing here is the impedance is absolutely, completely predictable. You can measure the input resistor and have it down precisely. For a lot of electronics this is not possible, but it is one of the many reasons op amps are so popular.
     
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  12. SgtWookie

    Expert

    Jul 17, 2007
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    When an inverting amplifier configuration is used, you will frequently see a voltage follower/buffer just ahead of the stage. The voltage follower is an opamp with the input to the noninverting input, and the output wired to the inverting input. The voltage follower/buffer is a noninverting amplifier with a gain of 1.

    The advantage of this configuration is that the input has a very high impedance, and the output has a very low impedance. There is practically no loading effect on the input signal, yet the output can assert the input signal to the following stage.

    To use the voltage follower/buffer configuration, you need to be certain that the opamp being considered for use is unity-gain stable; as not all opamps are. If they are not, there will usually be a statement on the first page of the datasheet; something like "stable when av>3".
     
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  13. Adjuster

    Well-Known Member

    Dec 26, 2010
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    There are cases when the lower impedance available by working into the inverting input from low input impedance is desirable.

    For example, the bandwidth available from a photo-diode working into a load resistor is restricted by parallel capacitance. If large load resistance is used the output voltage will be relatively big, but the bandwidth will be small. If a small load resistance is chosen to extend the bandwidth, the output voltage will be low, making it smaller in comparison with noise in any following amplifier.

    The use of an inverting feedback configuration allows a relatively large load resistance to be used as the amplifier feedback resistance, potentially allowing a better compromise between bandwidth, gain, and noise. This is sometimes referred to as a transimpedance circuit.
     
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