# Why are inverting op amp amplifiers used in most cases instead of non inverting amplifiers?

Why are inverting op amp amplifiers used in most cases instead of non inverting amplifiers?

Why are inverting op amp amplifiers used in most cases instead of non inverting amplifiers?

## Popular Answers

Lutz von Wangenheim· Hochschule BremenExactly the opposite is true!

An amplifier without any feedback is stable by nature.

Negative feedback has many, many advantages and is used for nearly each practical amplifier - however, the price to be paid is: Dynamic stability is decreased.

General rule: Negative feedback improves DC stability (of the operating point) and decreases the stability margin.

## All Answers (36)

Arbab Masood Ahmad· University of Engineering and Technology, PeshawarRichard Dudley· English Business ClassYadhunandana R K· BMS College of EngineeringVasile Surducan· National Institute for Research and Development of Isotopic and Molecular TechnologiesAnup Kumar· C. V. Raman College of EngineeringVasile Surducan· National Institute for Research and Development of Isotopic and Molecular TechnologiesYadhunandana R K· BMS College of EngineeringAnup Kumar· C. V. Raman College of EngineeringBhupendra Desai· Indian Institute of Technology RoorkeeAnup Kumar Bandyopadhyay· Jadavpur UniversityLutz von Wangenheim· Hochschule BremenExactly the opposite is true!

An amplifier without any feedback is stable by nature.

Negative feedback has many, many advantages and is used for nearly each practical amplifier - however, the price to be paid is: Dynamic stability is decreased.

General rule: Negative feedback improves DC stability (of the operating point) and decreases the stability margin.

Anup Kumar Bandyopadhyay· Jadavpur UniversityJose-Ignacio Izpura· Universidad Politécnica de MadridThese two "terminals" can be viewed as two metallic ones at some distance d..... Two conductors at distance d use to bear an electrical capacitance in our actual world.... Thus, there is a capacitive coupling between the input and the output of the amplifier you are building. This coupling tends to be positive feedback in the non-inverting topology whereas it tends to be negative feedback in the inverting topology (e.g. the "well known" Miller effect.). From this, the inverting topology seems more prone to be succesful for unaware designers than the non-inverting one..... Nature helps those working with negative feedback, although electronic "artists" can "play" both...

Lutz von Wangenheim· Hochschule BremenI am not quite sure if your above explanation really applies.

For my opinion, there is ALWAYS a parasitic capacitance between output and inverting opamp input node - independent on the operating mode (inv. or non-inv.).

On the other hand - the feedback through the capacitance between output and non-inverting input is shorted, either because this node is grounded (inv. operation) or because this node is connected to an (ideal) voltage source (non-inv. operation).

I am aware that I have assumed idealized conditions - however, this reasoning applies in principle also for real circuits, do they not?

Vaibhav Bharatrao Vaijapurkar· Pune Institute Of Computer TechnologyI know Negative feedback is responsible to assure a stability to circuit.Inverting amplifier has usually voltage shunt (transresitance)feedback (if RL is connected) or current series (transconductance) (without RL), while non inv is voltage series. now u compare their AC performance parameters as( AV, Ri, Ro by Millers and Negative feedback approach).

Jose-Ignacio Izpura· Universidad Politécnica de MadridI´m going to quote my answer using your own words, namely your sentence:

"For my opinion, there is ALWAYS a parasitic capacitance between output and inverting opamp input node - independent on the operating mode (inv. or non-inv.)."

Let us call C- and C+ respectively to these two capacitances, and let us consider that due to their geometry of conductors in space, both are of similar value.

Let us consider next that we are driving the input of the amplifier with a matched generator of impedance Z=50 ohms +j0 ohms (e.g. through a 50 ohms coaxial cable).

When the inverting configuration is used, C+ is grounded as you said and C- forms with the 50 ohms "viewed towards the generator" a high pass feedback filter with cut-off frequency fc=1/[2pi(50)C-] Hz. At those frequencies f>fc, this filter feeds-back signal to the inverting input with low phase lag, thus being negative feedback because the output signal is sampled and feedback with zero phase (roughly) to the inverting pin.

However, when the non-inverting configuration is used, C- is grounded and now C+ forms with the 50 ohms "viewed towards the generator" a high pass feedback filter with cut-off frequency fc=1/[2pi(50)C+] Hz. At those frequencies f>fc, this filter feeds-back signal to the input with low phase lag, thus being positive feedback because the output signal is sampled and feedback roughly with zero phase to the non-inverting pin.

Therefore, for high speed (broadband) amplifiers, the inverting configuration has negative feedback "by design" (Nature: metallic conductors in space) whereas the non-inverting configuration has positive feedback for the same geometrical reason, being this the idea I was trying to put forward in my first answer following yours.

Moreover, the small phase advance of the aforesaid high-pass, feedback filter to the non-inverting input tends to be counterbalanced by the phase lag expected for the amplifying electronics within the amplifier that will be band-limited by some cut-off frequency. This way, positive feedback with zero phase can appear in the non-inverting stage but NOT in the inverting one.

In summary, considering in the space the two input terminals and the output one, there is a "feedback assimmetry" between inverting and non-inverting topologies.

Lutz von Wangenheim· Hochschule Brementhank you for your reply and your explanation.

You may be surprised - but I completely agree with your analysis.

However, please take into account that my previous answer was based on a certain assumption (explicitly mentioned) : Ideal voltage source (zero input resistance). Thus, I see no conflict between us.

On the other hand - I have the feeling that the discussion is more or less an academical one. I was engaged in analog circuit design since more than 30 years and I must confess that the decision "inverting vs. non-inverting gain" never was governed by the parasitic influences as described in your last contribution.

The main aspect - of course - always was on the system level:

What is needed with respect to the overall performance?

More than that, as you know there always are other unwanted effects (caused by other parasitic capacitances/inductances), which (very?) often overshadow the effects as described by you.

Nevertheless, thanks again. I think your explanation was a big support to clear the situation.

Regards

L.

Lutz von Wangenheim· Hochschule BremenPlease read in the 5th line: (zero source resistance).

Thank you.

Cyril Mechkov· Technical University of Sofiahttp://en.wikibooks.org/wiki/Circuit_Idea/Walking_along_the_Resistive_Film#Varying_both_V1_and_V2:_a_resistive_summer

I remember the analog computers from my student years; they had only single-ended-input op-amps.

Regards, Cyril.

Cyril Mechkov· Technical University of SofiaCyril Mechkov· Technical University of Sofiahttp://en.wikibooks.org/wiki/Circuit_Idea/Revealing_the_Mystery_of_Negative_Impedance#Op-amp_implementation

And this is another example how to make two undesired quantities cancel each other.

Lutz von Wangenheim· Hochschule BremenThank you for the nice and interesting examples.

However, I have a general question:

The first original question implies (now supported by you: "Maybe another reason to prefer the inverting configuration...") that the inverting configuration would be preferred over the non-inverting topology. Is this really the case?

Can such a claim be confirmed by your experience?

For my opinion - as I have mentioned before - the decision inverting/non-inverting is governed primarily by system aspects.

Example: Some filter topologies need non-inv. and some other need inverting amplifiers.

Anup Kumar Bandyopadhyay· Jadavpur UniversityCyril Mechkov· Technical University of SofiaThe more general problem is (was) to create a voltage follower. We already know from our routine (Harold Black also-:) how to solve it - by "active copying" (formally, "negative feedback"). So, we have to build an NFB voltage follower. For this purpose, we have first to produce some output voltage, e.g. by an amplifier. Then, we have to compare (subtract or sum with an opposite sign) the output with the input voltage and to change the output voltage until it becomes (almost) equal to the input voltage. So, we need a subtractor (a summer). We know two possible ways to do it - by connecting the two voltage sources in series (KVL) or in parallel (KCL). Thus, depending on the used summer , we obtain two possible arrangements - noninverting and inverting; each of them has advantages and disadvantages.

IMO the role of the summer is crucial; it predetermines the other elements and the circuit properties. Thus, if we have chosen a series "summer" (simply a loop), it needs an op-amp with a differential input. Also, the output voltage has to have the same polarity as the input voltage with the purpose the two voltages to be subtracted (compared); in many cases the circuit can be supplied only by one power supply. So, the circuit is noninverting since the summer "needs" it to be noninverting-:)

If we have chosen a "parallel" summer (the two resistors); it needs a bare single-ended op-amp input (inverting). And if we have only op-amps with differential inputs (today's situation), we ground the needless noninverting input-:) Now, the output voltage has to have an opposite polarity regarding the input voltage with the purpose the two voltages to be subtracted (compared); so the circuit obligatory needs a split supply. Again, the circuit is inverting since the summer "wants" it to be inverting-:)

We may observe this duality (series and parallel) in circuitry ever since the Kirchoff's times and it gives this circuit variety...

Regards, Cyril.

Lutz von Wangenheim· Hochschule BremenSorry to say, but this statement is confusing (in fact, it is not true).

It is correct, that negative feedback is necessary - however, not "to ensure" stability but to fix a bias point within the linear operating range.

In contrary, negative feedback REDUCES dynamic stability.

However, these considerations are independent on the choice of the input node (inverting or non-inverting operation).

Anup Kumar Bandyopadhyay· Jadavpur UniversityLutz von Wangenheim· Hochschule BremenI never have mentioned something like to "swap the input terminals" .

This would lead, of course, to POSITIVE feedback.

In my contribution I was referring to NEGATIVE feedback only!

And it is a well-known fact that such a feedback reduces stability - in contrary to your claim (..."to ensure stability"). And the original question was about the use as inverting/non-inv. amplifier. However, in both cases, of course, only negative feedback is applied.

Regards

LvW

Anup Kumar Bandyopadhyay· Jadavpur UniversityRegards

AKB

Lutz von Wangenheim· Hochschule BremenQuote: "..could you please explain how for such systems "it is a well-known fact that such a feedback reduces stability " I understand by this statement that negative feedback will reduce the negative component of the pole."

Well, let me explain:

An amplifier with lowpass characteristic and without any feedback is always stable (open-loop poles negative-real).

Such an amplifier is equipped with a fixed bias point and the desired gain by applying negative feedback. It is a well known fact (documented in every relevant textbook) that - as a consequence - the stability margin is reduced (phase margin PM=180 deg without feedback). That means, depending on the amount of negative feedback as well as the open-loop pole location, the PM of the amplifier with feedback is smaller than 180 deg. (the closed-loop poles can be even conjugate-complex).

This can be observed by analyzing the step response of the closed-loop, which exhibits peaking or ringing - a typical indication of a reduced stability margin.

Remember: There are opamps, which are unity-gain stable, however, some other types (partly compensated) must NOT be operated with lower gains (heavy feedback), because they become unstable.

In fact - each real opamp with negative feedback will turn into an amplifier with POSITIVE feedback for higher frequencies (observations formulated by H. W. BODE, long time ago), however, in most cases the loop gain for this "critical" frequency range is already below unity. Thus, no instability will occur. But, in any case: Negative feedback reduces dynamic stability properties (smaller gain and phase margin).

Abdelhalim Zekry· Ain Shams UniversityPlease allow me to look at the answer from other point of view.

It is the circuit configuration itself which makes the the inverting amplifier op amp more popular than the non inverting op amp.

The non inverting terminal in the inverting amplifier configuration is grounded. This makes the inverting op amp terminal at virtual ground. This basically isolates the input from the output. Practically, there is no common circuit element between them.

Therefore,the input current is determined by the input voltage and the impedance connected between the input and the virtual ground. This current continues to pass in the feedback impedane and the voltage drop across it will be equal to the negative of the output voltage.Therefore due to this virtual ground, one can easily convert current sources into voltage sources and vice verse.One can also implement easily the integration and differentiation. Also ,one can implement the addition process.

The non inverting configuration is called the potentiometric configuration where the input voltage is a fraction of the output voltage as is well known for us.

The main advantage of this configuration is its relatively high input impedance.

In conclusion, the inverting op amp configuration is suitable for performing operations.

Thank you.

Cyril Mechkov· Technical University of SofiaAll these inverting circuits consist of four elements connected in a loop - two voltage sources (input and output) and two passive elements. The two voltage sources are connected in the same direction so that their voltages are summed. The output voltage source (the op-amp output) and the second passive element neutralize each other. So these two elements disappear (negative + positive impedance = 0) thus creating the illusion that there are only two elements in the circuit - the input voltage source and the first passive element, and only they define the current.

Maybe, you have noted that I have not used the virtual ground concept here and yet I have managed to reveal the phenomenon. The virtual ground is a concept closely related to the real ground concept; "virtual ground" is a point; it implies a potential of a point, not a voltage difference between two points. But if you have not a ground at all? Simply, your circuit is not grounded... My explanation is based on the whole loop, not only on the common point. The power of this explanation is that it is ever true, even in the case of ungrounded circuit; it is more universal... and it shows how to create virtual elements by adding an additional voltage...

I would like to ask you what you mean when saying "This basically isolates the input from the output. Practically, there is no common circuit element between them." IMO exactly the opposite is true for the inverting configuration - the input and the output are connected by the network of two series connected passive elements (a kind of a "bridge") and the same current flows through all these elements. Conversely, in the noninverting configuration there is no connection between the input and output and no current flows. This is a fundamental difference between the two configurations even in the case of a positive feedback (inverting and noninverting op-amp Schmitt trigger). Because of this difference, only inverting circuits can exhibit negative impedance since the output can affect the input current...

About "...This current continues to pass in the feedback impedane and the voltage drop across it will be equal to the negative of the output voltage..." I would reverse the causality:

This current continues to pass in the feedback impedance and "the output voltage will be equal to the negative of the voltage drop across the feedback impedane"

(the first is the voltage drop across the feedback impedance and the second is the op-amp output voltage)

About "...The non inverting configuration is called the potentiometric configuration where the input voltage is a fraction of the output voltage as is well known for us..." I would say that it is not obligatory to apply a fraction of the output voltage. You can apply the whole output voltage (voltage follower) or even the amplified output voltage (active voltage divider). Really, the last sounds guite odd and paradoxical (it means to connect an amplifer instead a voltage divider in the negative feedback).

Also, it is possible to connect an active element, e.g., a bipolar transistor in the negative feedback of the inverting configuration; the so-called "transdiode" is a good example. It also shows how to reverse a transistor (to make the output collector current - an input, and the input base-emitter voltage - an output). But this topic deserves our special atention...

Best regards, Cyril.

Cyril Mechkov· Technical University of SofiaBest regards to all of you, Cyril

Lutz von Wangenheim· Hochschule BremenIn his last contribution Prof. A. Zekry gave some good examples for my opinion (as I have already earlier formulated) that operational requirements dominate the decision inverting/non-inverting configuration (instead of parasitic influences).

Examples: I-V conversion, integrators, active filters with good active sensitivity figures, ...).

Cyril Mechkov· Technical University of SofiaAll these circuits can be implemented by the noninverting configuration as well with the advantage of the high input impedance. But the problem is that the output voltage is not the "pure" voltage across the second passive element (as it is in the case of the inverting configuration); here it includes the input voltage as well...

Abdelhalim Zekry· Ain Shams UniversityThank you for your complements. And thank you too for the the comments raised on my answer. The concept of virtual ground is highly recommended in the the op amp circuit analysis and we debate on it in a separate discussion. I think that at the end of discussion one is convinced with the logic answers.

I see the inverting op amp from my perspective and you see it from your perspective. BUT when coming to quantitative results we will agree.

There is a difference between isolated and independent. The two voltages are isolated in the sense there is no common element between them assuming virtual ground other wise they physically share the input impedance of the amplifier. Therefore i said practically. The dependence comes from the other main op amp characteristics which is negligible input current. Therefore, the current flowing in the input resistance continues to pass in the output impedance.And this which makes this circuit superior. It is simple but genius circuit..

Best regards.

Zekry

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