A non-inverting amplifier is an operational amplifier (op-amp) circuit that increases the strength of an input signal while keeping the output signal in phase with the input. If your input voltage swings positive, the output swings positive too, just larger. The gain is always 1 or greater, meaning this configuration amplifies but never attenuates a signal.
How the Circuit Works
An op-amp has two input terminals: a non-inverting input marked with a plus (+) sign and an inverting input marked with a minus (−) sign. In a non-inverting amplifier, you connect your input signal to the plus terminal. A pair of resistors then forms a feedback loop from the output back to the minus terminal. This feedback is what controls how much the signal gets amplified.
The op-amp itself has enormous internal gain, far more than you’d ever want in practice. The feedback network tames that gain down to a precise, predictable level. Because the signal enters through the non-inverting terminal and the feedback goes to the inverting terminal, the output stays in phase with the input. An inverting amplifier, by contrast, flips the signal 180 degrees.
The Gain Formula
The voltage gain of a non-inverting amplifier is set by two resistors in the feedback path. The formula is:
Gain = 1 + (Rf / R1)
Here, Rf is the feedback resistor connecting the output to the inverting input, and R1 is the resistor connecting the inverting input to ground. Because the formula adds 1 to the resistor ratio, the gain can never drop below 1. If you want a gain of 10, for example, you’d choose resistor values where Rf is 9 times larger than R1.
This is a key difference from the inverting configuration, where the gain equals the negative ratio of the two resistors (−Rf/R1) and can be less than 1 to attenuate a signal. The non-inverting version only amplifies.
Input and Output Impedance
One of the biggest practical advantages of the non-inverting amplifier is its very high input impedance. Because the input signal connects directly to the op-amp’s non-inverting terminal rather than through a resistor, the circuit draws almost no current from the signal source. This minimizes the “loading effect,” where a circuit drains energy from the source and distorts the signal.
In an inverting amplifier, the input impedance is roughly equal to the input resistor, which you can control but which is inherently lower. Both configurations provide low output impedance, meaning they can drive downstream loads effectively without losing signal strength.
High input impedance matters whenever your signal source is weak or has high internal resistance. Sensors, microphones, and other delicate sources can be degraded by a circuit that draws too much current from them.
The Voltage Follower: A Special Case
If you set the feedback resistor to zero (a direct wire from output to the inverting input) and remove R1 entirely, you get a circuit with a gain of exactly 1. This is called a voltage follower or unity gain buffer. The output voltage equals the input voltage: Vout = Vin.
That might sound useless, but the voltage follower is one of the most common op-amp circuits in practice. It isolates a high-impedance source from a low-impedance load. The input impedance in this configuration is extremely high, often well above 1 megaohm, while the output impedance is very low. Think of it as a signal relay station: it passes the voltage along without changing it, but shields the source from being loaded down by whatever comes next. This makes it ideal for impedance matching and circuit isolation.
Output Voltage Limits
The gain formula tells you the theoretical output voltage, but the op-amp’s power supply sets a hard ceiling. The output can never exceed the positive supply voltage or drop below the negative supply voltage. In practice, most op-amps can’t even reach those rails. The output typically tops out a volt or two below the positive supply and a volt or two above the negative supply, depending on the chip’s internal design. Some “rail-to-rail” op-amps get much closer, but even they have small margins.
If the calculated output voltage (gain × input voltage) exceeds these limits, the output simply clips at the supply rail. The signal becomes distorted, flattening out at the maximum or minimum voltage the op-amp can deliver. When designing a non-inverting amplifier, you need to make sure the gain you’ve chosen won’t push the output into saturation for the expected range of input signals.
Bandwidth and Gain Tradeoff
Every op-amp has a fixed gain-bandwidth product, which means higher gain comes at the cost of lower bandwidth. If an op-amp has a gain-bandwidth product of 1 MHz and you set the gain to 50, the amplifier’s usable bandwidth drops to 20 kHz. Set the gain to 10, and you get 100 kHz of bandwidth. At unity gain, you get the full 1 MHz.
The non-inverting configuration actually has a slight edge here. At the same gain setting, a non-inverting amplifier preserves the full gain-bandwidth product of the op-amp, while the inverting configuration loses some of it. For example, at a gain of 10 with a 1 MHz op-amp, a non-inverting amplifier has a bandwidth of 100 kHz compared to 91 kHz for the inverting version. At a gain of 2, the difference is more noticeable: 500 kHz versus 333 kHz. This makes the non-inverting topology a better choice for applications where you need to push the frequency response as far as possible.
Noise and Common-Mode Performance
Non-inverting amplifiers typically exhibit higher common-mode rejection, which is the ability to ignore unwanted signals that appear equally on both inputs (like electrical noise picked up from nearby wiring). In the inverting configuration, both inputs sit at ground or virtual ground, so common-mode voltage isn’t really a factor. But the non-inverting configuration sees the full input signal as a common-mode voltage, which means the op-amp’s ability to reject common-mode interference directly affects signal quality. High-quality op-amps handle this well, and the non-inverting configuration generally produces lower noise overall due to how the feedback network is arranged.
When to Use a Non-Inverting Amplifier
The non-inverting configuration is the go-to choice in several situations:
- Buffering sensor outputs. Sensors like thermocouples, pH probes, and piezoelectric elements have high source impedance. The non-inverting amplifier’s high input impedance prevents signal degradation.
- Preserving signal polarity. Any application where you need the output to track the input without flipping it, such as level shifting a signal before feeding it into an analog-to-digital converter.
- Impedance matching. The voltage follower variant is used extensively to bridge high-impedance sources to low-impedance loads without signal loss.
- Simple gain stages. When you need clean, predictable amplification with a gain of 1 or more and don’t need to combine multiple signals.
The inverting configuration is better suited for mixing multiple signals together (as in an audio mixer), applying precise weighted summation, or when you specifically need signal inversion or attenuation below unity gain. Its virtual ground at the inverting input makes summing multiple inputs straightforward.