# OpAmp: inverting and non-inverting amplifier both amplify the signal, how to know which one to use?

Both the inverting amplifier and non-inverting amplifier use negative feedback. Both provide gain. But one inverts the signal while the other does not. How to know which one to use? Are there applications where only one of the two can be used (provides both positive and negative supplies exist)?

Is it true that single supply op amp can only create non-inverting amplifier?

• It depends on what the desired functionality is. Do you want the signal inverted or not? Commented Feb 16, 2016 at 13:31
• This is a general question. When does it matter if the signal should be inverted or not? Inversion means 180 deg phase shift. I don't think it matters in audio applications. I wonder where it does matter. Commented Feb 16, 2016 at 13:33
• Isn't that basically answered by this question already? electronics.stackexchange.com/questions/37227 Commented Feb 16, 2016 at 13:44
• I'd be more careful with the generalization "Inversion means 180 deg phase shift", because it actually doesn't mean that. It just looks like it does if your waveform is continuous & symmetrical. A real phase shift requires some time delay. Commented Feb 16, 2016 at 14:41

There are a couple reasons (at least) for choosing a particular configuration:

1. If you need a high input impedance, then you are forced into a non-inverting configuration. This is commonly required in a buffering situation. An inverting configuration has an input impedance equal to the input resistor which may load the source circuit.

2. If you need a summing amplifier, then inverting is the way to go as the inverting input is the summing junction.

Is it true that single supply op amp can only create non-inverting amplifier?

No. With a DC offset (on the non-inverting input) you can have an inverting configuration although the input signal will need to be ac coupled under most circumstances.

• A high input impedance in non-inverting configuration comes with its own problems: input bias current and input noise current have nowhere to go and significantly impact the input voltage, input capacitance may significantly influence the signal, THD for large input impedance differences tends to quite larger than under more controlled conditions. Generally, it exposes the input stage implementation constraints a lot more. Commented May 30, 2023 at 9:31

There are some specific applications which require (a) non-inverting or (b) inverting gain stages. This is true - in particular - for active filters and harmonic oscillators.

• Examples to (a): Sallen-Key-filter stages; WIEN type oscillators.
• Examples to (b): Multi-feedback filterstages (MFB); Phase-shift oscillators.

Opamps with JFET inputs tend to have an input capacitance depending on the input voltage. In inverting configuration, both inputs are at ground potential (one by being connected to ground, the other by being a virtual ground). That gives comparatively constant capacitance on the inverting input. The non-inverting configuration follows the input signal, and a signal-dependent capacitance will lead to harmonic distortion unless the input is driven with a rather low impedance.

Just recently I read a warning about this in the data sheet for the OPA2134 while the data sheet for the OPA1642 explicitly mentions having stabilised input capacitance essentially independent of the input voltage.

• Do you use op amps a lot on your job? Commented May 29, 2023 at 19:02
• Nah, just hobbyist pimping amps. And spec sheets are, all-in-all, just one data point. Of the named two, it happened that the OPA2134 made the race in spite of somewhat worse noise specs because it didn't have rail-to-rail output facilitating an obnoxious switch-off thump while the other specs were already in the "not significant in the context of the surrounding circuit" camp. But I still like understanding things in theory. Commented May 29, 2023 at 21:28

# The common idea

Through both configurations - inverting and non-inverting, the same negative feedback idea is implemented as follows:

The input Vin and (part of) the output voltage Vout are compared by subtraction and the result is applied to the (differential) op-amp input so that the difference to be zeroed.

# The difference

It is only in the method of subtraction - in the inverting configuration, it is implemented in a parallel manner and in the non-inverting one in a serial manner. This leads to significant differences in the properties of the two circuit solutions.

## Inverting amplifier

Configuration. In the inverting amplifier, the two voltages are of opposite polarity (referenced to ground) and summed through a R1-R2 resistor network. The output (difference) voltage is also referenced to ground and therefore no differential input is required (the unused op-amp non-inverting input is grounded). So an inverting amplifier can be made by an amplifier with a single ended input but it has to be supplied by a voltage with an opposite (to the input voltage) polarity. This means that if the input voltage is bipolar, the op-amp has to be split supplied.

Gain. It is exactly equal to the ratio of the two resistances (R2/R1) because the two voltages being compared are separate.

Input resistance. Looked at from the other side, inside the loop the two voltages are of the same direction, therefore they are summed. As a result, the current increases and the input resistance decreases (Miller effect). In other words, the op-amp output voltage neutralizes the voltage drop across the resistor R2 and only R1 is left to operate; so the circuit part after R1 can be thought as a short connection.

## Non-inverting amplifier

Configuration. In the non-inverting amplifier, the two voltages are of the same polarity (referenced to ground) but are connected in the opposite direction in the loop (- +, + - or + -, - +) so they are subtracted. The difference voltage is "floating" so an amplifier with differential input is required... but it can be single-supplied.

Gain is one unit greater than the ratio of the two resistances (R2/R1 +1) because the output voltage includes the input voltage within itself. If we use the voltage drop across R2 as an output voltage, then the gain would be exactly R2/R1 as in the inverting amplifier but the load would be floating.

Input resistance. Since the two voltages are subtracted, the current enormously decreases and the input resistance increases ("bootstrapping" effect). Seen from the input source side, the circuit is an open circuit.

I have illustrated these explanations by CircuitLab simulations in my answer to a similar question.

You must always connect both input terminals of an opamp to thought-out and considered points. What you connect them to determines what function the opamp will provide.

If you connect +ve to a ground, and -ve via a feedback impedance to the output terminal, you create a 'transimepdance amplifier'. Any current you now push into the -ve input must also be balanced by a current from the output terminal through the impedance. Use a feedback resistor to convert current to volts. Use a feedback capacitor to create an integrator. The gain will be inverting.

If you connect your input voltage to +ve, and the -ve to a fraction of the output voltage (create a fraction with a voltage divider to ground, usually resistors, but you could use other impedances or even a transformer), you create a non-inverting amplifier, where the gain is the reciprocal of the feedback voltage divider.

You can do more complicated things as well, but the two above are the basics.

Some arguments I haven't seen mentioned in other answers:

• Inverting configuration with IN+ at GND, greatly reduces the importance of the op amp CMRR, allowing generic parts. Also you won't get nasty cross over distortion in rail-to-rail inputs. This is one reason, why this is popular in Audio. You can get pretty low distortion with low supply voltages and cheap components.
• Non-inverting needs no input resistor, so this is the lowest noise configuration.

I made primarly an error in my schematic. I apologize. I am getting old.

Always simulate to being sure !

If unsure of inverting or not inverting for general use, this schematic is ok. for pcb. Simply add possibility of short input+ to gnd, and left of R5 or R8 also to gnd.

Drop of solder. R7 may be lower. R10 may be ommitted. This schematic here is for Gain -1 or +1.

For general use, draw the wires on pcb for using drop of solder (or a jumper) to choose your configuration. It is only a little trick when you use a pcb for testing.

• A word of caution: while this will make an ok inverting amp, it is a quite poor non-inverting amp. attenuting signal to 1/2 , and regain it 2x is bad from noise perspective. 100k feedback is bad from noise perspective, gain will be not exactly 1 in non-inverting case. 100k feedback is often unstable without feedback C. Input impedance is very low for many non-inverting applications. instead of the switch, it is better to unsolder some parts, e.g. R1 & R4, and change values of R2 & R3. And add another footprint from IN- to GND Commented Jun 15, 2021 at 7:43
• I apologize. I made a error ...
– user288518
Commented Jun 15, 2021 at 11:37