Non-Inverting Op-Amp Resistor Calculator entries are the gain, R1, the signal voltage and the output you want. It returns R2, and it adds R3 and R4 when the output needs an offset.
R1 runs from the - input to ground and R2 is the feedback resistor. Enter the output you want at the given signal input.
Offset, Supply and Tolerance
V2 is the fixed voltage that shifts the output, such as a reference or the supply rail.
Op-Amp Limits
Headroom is how far the output or the inputs must stay from each rail. Picking a part fills in its figures.
Resistors at Other Gains
| Gain | R2 | R3 | R4 |
|---|---|---|---|
| 1.500 V/V | 10.00 kΩ | 6.667 kΩ | 20.00 kΩ |
| 2.000 V/V | 14.00 kΩ | 7.000 kΩ | 35.00 kΩ |
| 3.000 V/V | 22.00 kΩ | 7.333 kΩ | 110.0 kΩ |
| 5.000 V/V | 38.00 kΩ | 7.600 kΩ | 190.0 kΩ |
| 10.00 V/V | 78.00 kΩ | 7.800 kΩ | 65.00 kΩ |
| 20.00 V/V | 158.0 kΩ | 7.900 kΩ | 49.38 kΩ |
| 50.00 V/V | 398.0 kΩ | 7.960 kΩ | 43.26 kΩ |
| 100.0 V/V | 798.0 kΩ | 7.980 kΩ | 41.56 kΩ |
Non-Inverting Op-Amp Resistor Calculator Setup
The Non-Inverting Op-Amp Resistor Calculator works on the standard non-inverting amplifier. The signal V1 goes to the + input. R1 runs from the - input to ground, and R2 runs from the output back to the - input.
Four entries describe the job. They are the gain, the value of R1, the signal voltage V1 and the output wanted at that voltage. The calculator compares the target output with the gain times V1. If the two match, only R2 is needed.
If they differ, the output needs an offset, and the calculator adds an offset source V2 with resistors R3 and R4. A reference voltage or the supply rail usually serves as V2. The line under the main result lists where each resistor connects.
Non-Inverting Amplifier Gain Formula
To calculate the gain of a non-inverting op-amp, divide R2 by R1 and add 1. The gain is always positive and never below 1, so the output has the same polarity as the input.
Design work runs the other way. With the gain and R1 chosen, the non-inverting amplifier gain formula rearranges to give the feedback resistor, which is the main result of the calculator.
The output voltage of a non-inverting op-amp is then the gain times the signal input, as long as no offset is added. The input and the output are both measured from ground.
A gain of 11 with R1 at 1 kΩ needs R2 = 10.00 kΩ. A 0.1 V input gives 1.1 V at the output, and the gain reads 20.83 dB. The table lists what the Non-Inverting Op-Amp Resistor Calculator returns for common gains with R1 at 10 kΩ and a 1 MHz op-amp.
| Gain | In dB | R2 | Bandwidth |
|---|---|---|---|
| 2 V/V | 6.02 dB | 10 kΩ | 500 kHz |
| 5 V/V | 13.98 dB | 40 kΩ | 200 kHz |
| 10 V/V | 20.00 dB | 90 kΩ | 100 kHz |
| 100 V/V | 40.00 dB | 990 kΩ | 10 kHz |
Adding an Offset With R3 and R4
Many jobs need the output shifted as well as amplified. Ron Mancini's TI application report on single-supply op-amp design treats this as a straight line, with the gain as the slope and the offset as the intercept. The calculator works out the offset b from the entries.
When the offset has the same sign as V2, both V1 and V2 feed the + input through R3 and R4. The op-amp then needs a higher gain N from its + input to the output, and that sets R2. The ratio of R4 to R3 sets how much of V2 gets through.
Take a gain of 2, V1 at 1 V, a 4 V target and a 5 V source for V2, with R1 at 10 kΩ. The offset b is 2 V and N is 2.4. The Non-Inverting Op-Amp Resistor Calculator returns R2 = 14.00 kΩ, R3 = 7.000 kΩ and R4 = 35.00 kΩ.
TI's own worked example gives a check. It asks for a gain of 3.535 and 1 V out at 10 mV in, from a 5 V reference. TI arrives at 183.16 kΩ for the offset resistor with a 10 kΩ signal resistor. The calculator shows 183.2 kΩ on its R4 if R3 Is Set Equal to R1 row, with R2 at 27.28 kΩ.
An offset in the opposite direction cannot be made at the + input. The alert then reads Offset Goes to the - Input, and V2 connects to the - input through R4. For a gain of 10 with 1.5 V out at 0.2 V in, the result is R2 = 89.00 kΩ and R4 = 890.0 kΩ.
Using the supply as V2 saves a part and costs accuracy. TI warns that noise on the supply is multiplied by the circuit gain when the supply serves as the reference. A dedicated reference voltage keeps the offset steady.
R3 in the Non-Inverting Op-Amp Resistor Calculator
R3 and R4 are not picked at random. They are sized so the + input sees the same resistance as the - input. Analog Devices explains in its MT-038 tutorial on input bias current that this match makes the bias currents drop equal voltages at both inputs, so the error cancels.
In the example, R1 and R2 in parallel come to 5.833 kΩ, and so do R3 and R4. With no offset, a single R3 equal to R1 in parallel with R2 does the same job. The card labels it optional, and Analog Devices notes that CMOS and JFET op-amps seldom need it.
The second row gives a simpler option. If R3 is set equal to R1, R4 becomes 50.00 kΩ in the example. The gain and the offset stay the same, and only the bias match is lost.
Standard 1% Resistor Values
Exact answers such as 6.970 kΩ are not stocked parts. The second card in the Non-Inverting Op-Amp Resistor Calculator swaps each resistor for the nearest standard value. Resistors at ±1% come from the E96 series, which has 96 values per decade in Vishay's decade table. Parts at ±5% use E24 and parts at ±0.1% use E192.
For the default example the E96 values are 14 kΩ, 6.98 kΩ and 34.8 kΩ. They give 1.999 V/V and 4.004 V against the 4.000 V target. With every part at its ±1% limit, the output can land anywhere from 3.931 V to 4.078 V.
Bandwidth and Noise Gain
A higher gain leaves less bandwidth. For a voltage-feedback op-amp, the closed-loop bandwidth is the gain bandwidth product divided by the noise gain, as Analog Devices sets out in its MT-033 tutorial. Noise gain is 1 + R2 ÷ R1.
In a plain non-inverting stage the noise gain equals the signal gain. An offset network at the + input raises it. The default example has a signal gain of 2 and a noise gain of 2.400, so a 1 MHz op-amp gives 416.7 kHz and not 500 kHz.
TI lists the LM358 at a typical 0.7 MHz and the LM358B at 1.2 MHz. Picking the LM358 under Op-Amp in the Non-Inverting Op-Amp Resistor Calculator brings the same circuit down to 291.7 kHz. The Gain Bandwidth Product field also takes any other figure from a data sheet.
The value of R1 is a free choice, since only the resistor ratios set the gain and the offset. TI notes that supply current, frequency response and the op-amp's drive capability decide the absolute values. A larger R1 scales every resistor up and draws less current from the output.
Supply Rails, VCC and Headroom
The output cannot go past the supply rails. VCC is the positive supply pin of the op-amp, and it carries a DC voltage that depends on the design. The Power Supply setting in the Non-Inverting Op-Amp Resistor Calculator covers dual supplies from ±5 V to ±15 V and single supplies from 3.3 V to 12 V.
Many op-amps stop short of the rails. The Op-Amp setting fills in how far the output and the inputs must stay from each rail. TI's figures for the LM358 put the output 2 V below the positive rail and the inputs 1.5 V below it, while both reach close to the negative rail. The default is a rail-to-rail part with no headroom.
The fourth card places the target inside that swing. A common job is fitting a ±1 V signal onto a 0 V to 3.3 V converter input. Set a gain of 1.65 with 1.65 V out at 0 V in, and V2 at 3.3 V. The result is R2 = 11.50 kΩ, R3 = 6.970 kΩ and R4 = 23.00 kΩ.
The card then reads 50.0% From the Bottom, which leaves equal room each way. The V1 Range Before Clipping row shows -1.000 V to 1.000 V, the full span of the signal. A target outside the swing turns the card red and shows where the output clips.
The same job fails on an LM358 from a 3.3 V supply. Its output tops out at 1.300 V, so the 1.65 V midpoint is already past the limit, and the card reads Output Clips at 1.300 V. When the output fits but the + input sits too near a rail, the card reads Input Past the Op-Amp Range.
Op-Amp Rules, Inverting Gain and Input Impedance
What are the golden rules for op-amps?
There are two, from Horowitz and Hill's The Art of Electronics as quoted in MIT's course notes. The output does whatever is needed to make the voltage difference between the inputs zero. The inputs draw no current.
Applied to this circuit, rule one puts V1 on the - input as well. Rule two means the current in R1 also flows in R2. A 0.1 V input across a 1 kΩ R1 is 100 µA, and 100 µA through a 10 kΩ R2 adds 1 V, for 1.1 V out.
The rules only hold with negative feedback, where R2 returns to the - input. If R2 goes to the + input, the stage snaps between the rails. That is the circuit worked out in the Schmitt Trigger Calculator.
How do you know if an op-amp is inverting or non-inverting?
Look at where the signal enters. In a non-inverting amplifier it goes to the + input, and the gain is 1 + R2 ÷ R1. In an inverting amplifier it goes through R1 to the - input, with the + input held at ground or at a reference.
The inverting op amp gain formula is -R2 ÷ R1, so 10 kΩ and 1 kΩ give -10 where the non-inverting stage gives 11. The Non-Inverting Op-Amp Resistor Calculator sizes the + input form only, and the Op Amp Gain Calculator covers both.
A differential op amp stage uses both inputs at once and amplifies the difference between two signals. It is the last stage worked out in the Instrumentation Amplifier Gain Calculator, where its gain is R4 ÷ R3.
What is the input impedance of a non-inverting amplifier?
It is very high, because the signal drives the + input directly and that input draws almost no current. An R3 in series for bias matching does not change that. This is the main reason to pick the non-inverting stage for a weak signal source.
The offset network changes the picture. With R4 tied to V2, the source sees R3 plus R4 on the way to V2. The third card in the Non-Inverting Op-Amp Resistor Calculator shows it as Resistance Seen by V1, which is 42.00 kΩ in the default example. The source has to drive that load without its voltage sagging.