This article provides an in-depth look at the non-inverting configuration of operational amplifiers, as a natural continuation of the introductory guide to op amps. In a non-inverting amplifier, the signal is applied to the + input, while the − input receives a fraction of the output voltage through a resistive feedback network. The result is an amplifier with positive gain, high input impedance, and an output signal that remains in phase with the input signal.
Quick review: op amp terminals and the idea of feedback
An ideal operational amplifier is often described by assuming infinite input resistance, zero output resistance, infinite open-loop gain, and zero input currents. In real devices these parameters are never ideal, but the model is extremely useful for understanding circuits based on negative feedback.
The main terminals are:
- Non-inverting input (+): a signal applied here is transferred to the output without phase inversion.
- Inverting input (−): often used as the feedback node.
- Output: the terminal from which the amplified signal is taken.
- Power supply pins: generally indicated as VCC+ and VCC−, or V+ and V−.
In linear operation with negative feedback, the op amp tends to reduce the voltage difference between its two inputs to a very small value. This leads to the so-called virtual short approximation: V+ ≈ V−, even though there is no direct electrical connection between the two input terminals.
Structure of the non-inverting configuration
The non-inverting configuration consists of:
- an operational amplifier;
- a resistor R1 between the inverting input and ground, or a reference voltage;
- a resistor R2 between the output and the inverting input;
- the input signal Vin applied directly to the non-inverting input;
- any required decoupling, filtering, or compensation capacitors, depending on the application.
Where: R1 = resistor between the inverting input and the reference node R2 = feedback resistor between the output and the inverting input Vin = signal applied to the non-inverting input Vout = amplified output signal
The ideal voltage gain of the non-inverting configuration is:
\( \frac{ V_{out}}{V_{in} } = 1 + \frac{R2} {R1} \)This formula highlights an important characteristic: the theoretical minimum gain of a non-inverting amplifier is 1. When R2 = 0 and R1 is absent or equivalent to an open circuit with respect to the feedback path, the circuit becomes a voltage follower, also known as a unity-gain buffer.
Why can the gain not be lower than 1?
In the classic non-inverting configuration, a fraction of the output voltage is fed back to the inverting input. Since the circuit attempts to make V− equal to V+, the output must assume a value equal to or greater than the input signal, depending on the resistor ratio.
If a gain lower than 1 is required, a pure non-inverting amplifier is usually not the right choice. A resistive divider before a buffer, an inverting attenuator, or a specifically designed active attenuation stage may be more appropriate.
Gain examples
Example 1: gain Av = 2 Component choice: R1 = 10 kΩ R2 = 10 kΩ Av = 1 + (R2 / R1) Av = 1 + (10 kΩ / 10 kΩ) Av = 2 If Vin = 0.5 V: Vout = 0.5 V × 2 = 1 V
Example 2: gain Av = 11 Component choice: R1 = 10 kΩ R2 = 100 kΩ Av = 1 + (100 kΩ / 10 kΩ) Av = 11 If Vin = 100 mV: Vout = 100 mV × 11 = 1.1 V
Example 3: unity-gain buffer Component choice: output connected directly to the inverting input Vin applied to the non-inverting input Av = 1 Function: it does not amplify the voltage, but it increases the signal driving capability and provides high input impedance to the source.
Application fields
Impedance matching
One of the main uses of the non-inverting configuration is impedance matching. A weak signal source, such as a sensor, pickup, high-resistance divider, or RC filter, may not be able to directly drive a load. A non-inverting buffer allows the signal to be taken without excessively loading the source.
Audio signal amplification
In audio applications, the non-inverting configuration is widely used because it preserves signal phase and allows accurate gain to be set with only two resistors. The TL072 and RC4558 are both dual operational amplifiers commonly found in audio circuits, preamplifiers, active filters, mixers, guitar effects, and signal-conditioning stages.
Sensor signal conditioning
Many sensors generate small signals or signals coming from high-impedance sources. The non-inverting configuration is suitable when the signal must be amplified without changing its polarity. For very high-impedance sensors, a JFET-input op amp such as the TL072 may be preferable to a bipolar-input device such as the RC4558, because it requires much lower input bias current.
Active filters
The non-inverting configuration is also used in active low-pass, high-pass, band-pass, and notch filters. In many topologies, the op amp provides gain, isolation between stages, and reduces the loading effect on the RC network.
Voltage-reference buffers
In single-supply circuits, it is common to create a reference voltage, for example Vref = Vcc/2, to be used as a virtual ground. A unity-gain non-inverting op amp stage can buffer this reference, provided that the selected device is compatible with the required voltage and current range.
Advantages of the non-inverting configuration
- High input impedance: the signal is applied directly to the non-inverting input, so the source is only lightly loaded. This is one of the main advantages compared with the inverting configuration, where the input impedance is tied to the input resistor.
- No phase inversion: the output is in phase with the input. This simplifies its use in audio chains, data acquisition, and sensor conditioning.
- Simple gain formula: the gain depends on the R2/R1 ratio.
- Buffer operation: at unity gain it becomes a very useful impedance adapter.
- Good gain accuracy: in many cases the actual gain depends more on resistor tolerance than on the op amp open-loop gain, as long as the device operates within its limits.
Disadvantages and limitations
- Minimum gain equal to 1: it is not the ideal choice if the signal must be directly attenuated.
- Dependence on the input common-mode range: the signal applied to the non-inverting input must remain within the common-mode voltage range allowed by the device.
- Not all op amps are unity-gain stable: many general-purpose op amps are, but the datasheet must always be checked.
- Output limitations: the TL072 and RC4558 are not rail-to-rail op amps. The output cannot reach the supply rails exactly, so voltage headroom must be provided.
- Possible oscillation: poor layout, capacitive loads, lack of bypass capacitors, or very high resistor values can introduce instability.
Comparison with other configurations
| Configuration | Ideal gain | Phase | Input impedance | Typical use |
|---|---|---|---|---|
| Non-inverting | 1 + R2/R1 | Not inverted | Very high | Buffers, sensors, audio, signal conditioning |
| Inverting | −Rf/Rin | Inverted by 180° | Approximately Rin | Summing amplifier, precise gain, attenuation, signal mixing |
| Buffer | 1 | Not inverted | Very high | Impedance matching |
| Differential | Depends on the resistor network | Depends on the inputs | Medium, resistor-dependent | Measuring the difference between two signals |
| Instrumentation amplifier | Adjustable | Non-inverting or differential | Very high | Sensors, strain-gauge bridges, small signals in noisy environments |
Compared with the inverting configuration, the non-inverting amplifier is preferable when the source must not be loaded and when phase must be preserved. The inverting configuration, on the other hand, is often more convenient for summing multiple signals, creating analog mixers, implementing active filters with a virtual-ground node, or obtaining gains lower than 1.
Practical example with TL072: non-inverting audio preamplifier
The TL072 is a dual operational amplifier with JFET inputs, widely used in audio applications and in circuits where very low input bias current is useful. It is suitable, for example, for a line-level audio preamplifier or an input stage for musical instruments, provided that power supply, dynamic range, and noise performance are properly designed.
Indicative data for TL072 / TL07x family - always check the exact part number and datasheet revision: Manufacturer: Texas Instruments Type: dual operational amplifier Input stage: JFET / FET-input Typical supported total supply voltage: up to approximately 30 V for the classic TL072 Gain bandwidth product: approximately 3 MHz for the classic TL072 Typical slew rate: approximately 13 V/µs for the classic TL072 TL072H / TL07xH version: typical slew rate specified up to approximately 20 V/µs Voltage noise: family described as low-noise; typical values depend on version and frequency Output: not rail-to-rail Typical applications: audio, active filters, buffers, signal conditioning
Design specification
Objective: amplify a 200 mV RMS audio signal up to approximately 1 V RMS. Required gain: Av = 1 V / 0.2 V = 5 Formula: Av = 1 + (R2 / R1) Component choice: R1 = 10 kΩ R2 = 39 kΩ Actual gain: Av = 1 + (39 kΩ / 10 kΩ) Av = 4.9 Expected output: Vout = 0.2 V RMS × 4.9 = 0.98 V RMS
Simplified application schematic
Recommended supply for audio testing: ±12 V or ±15 V Connections: Vin audio -> coupling capacitor -> TL072 non-inverting input R2 = 39 kΩ between output and inverting input R1 = 10 kΩ between inverting input and ground Bypass capacitors: 100 nF close to the power supply pins + optional 10 µF / 47 µF on each supply rail
Design notes
- Use 1% metal-film resistors to reduce gain error and noise.
- Keep R1 and R2 within a reasonable range: values that are too low load the output, while values that are too high increase noise and sensitivity to input bias currents.
- With the TL072, resistor networks between approximately 4.7 kΩ and 100 kΩ are often a reasonable choice, depending on the noise/current/load trade-off.
- Do not use the TL072 expecting rail-to-rail behavior: with a single 5 V supply it is generally not the most convenient choice.
- For single-supply audio circuits, create a virtual ground Vref and bias the inputs around Vref.
Practical example with RC4558: non-inverting audio amplifier
The RC4558 is a dual general-purpose bipolar operational amplifier, very well known in analog audio circuits. It has been used in many preamplifiers, active filters, and guitar effects. Compared with the TL072, it has bipolar inputs and therefore higher input bias currents, but it can be a suitable choice in low- to medium-impedance circuits.
Indicative data for RC4558 - always check the exact part number and datasheet revision: Manufacturer: Texas Instruments Type: dual general-purpose operational amplifier Architecture: bipolar Total supply voltage: typically 10 V - 30 V Gain bandwidth product: approximately 3 MHz on the product page, 4 MHz in Rev. H datasheet Typical slew rate: approximately 1.7 V/µs on the product page, 2.2 V/µs in Rev. H datasheet Voltage noise: approximately 8 nV/√Hz at 1 kHz on the product page; datasheet values depend on frequency and test conditions Input bias current: much higher than in a JFET-input device; typically in the tens/hundreds of nA range Output: not rail-to-rail Typical applications: audio, active filters, buffers, general-purpose stages
Design specification
Objective: amplify a 100 mV RMS signal up to approximately 1 V RMS. Required gain: Av = 1 V / 0.1 V = 10 Formula: Av = 1 + (R2 / R1) Component choice: R1 = 10 kΩ R2 = 91 kΩ Actual gain: Av = 1 + (91 kΩ / 10 kΩ) Av = 10.1 Expected output: Vout = 0.1 V RMS × 10.1 = 1.01 V RMS
Quick slew-rate check
For a sinusoidal signal, the minimum required slew rate can be estimated as follows:
SR_min = 2 × π × f × Vp Where: f = maximum signal frequency Vp = output peak voltage
Example: Vout = 1 V RMS Vp = 1 V RMS × 1.414 = 1.414 V f = 20 kHz SR_min = 2 × π × 20000 × 1.414 SR_min ≈ 177715 V/s SR_min ≈ 0.178 V/µs Conclusion: an RC4558 with a typical slew rate in the 1.7 - 2.2 V/µs range is sufficient for this example. However, if the required output increased significantly, for example to 10 Vp at 20 kHz: SR_min = 2 × π × 20000 × 10 SR_min ≈ 1.26 V/µs The available margin would become smaller, especially when tolerances, load, temperature, and distortion are considered.
Design notes
- Avoid excessively high feedback-network impedances, because the RC4558 input bias currents can generate noticeable offset errors.
- For audio circuits, values such as R1 = 10 kΩ and R2 between 10 kΩ and 100 kΩ are often a good starting point.
- Use 100 nF decoupling capacitors close to the power supply pins.
- Always check the output-voltage margin relative to the supply rails: the RC4558 does not swing rail-to-rail.
- For high-impedance applications, consider the TL072 or a modern CMOS/JFET-input op amp.
Single-supply operation and virtual ground
Many educational schematics use a dual supply, for example ±12 V or ±15 V. In this case, the signal can swing around 0 V. If a single supply is used instead, for example 9 V, 12 V, or 15 V, an intermediate reference must be created.
Example with a single 9 V supply: Vcc = 9 V Vref = Vcc / 2 = 4.5 V The audio signal is AC-coupled through a capacitor and biased around Vref. The op amp "sees" the signal as an oscillation around 4.5 V, not around 0 V. Important: Vref must be stable and low impedance. A resistive divider followed by a buffer is often used.
This point is particularly important with the TL072 and RC4558, because they are not rail-to-rail op amps and are not always suitable for operation close to ground when powered from low single supplies. For modern 3.3 V or 5 V circuits, a low-voltage rail-to-rail op amp is often a better choice.
Main parameters to monitor in a datasheet
When selecting an op amp for a non-inverting configuration, looking only at the theoretical gain is not enough. The real circuit depends on many electrical parameters. The most important ones are listed below.
Supply voltage – Check the minimum and maximum supply voltage. Some op amps work well at 5 V, while others require higher supply voltages. The TL072 and RC4558 are better suited to dual supplies, or to relatively high single supplies, than many modern low-voltage op amps.
Input common-mode voltage range – This is the voltage range allowed at the inputs. In the non-inverting configuration it is critical, because the signal is applied directly to the + input. If the signal falls outside the common-mode range, the output may saturate, distort, or behave non-linearly.
Output voltage swing – This indicates how close the output can get to the supply rails. A non-rail-to-rail op amp powered at ±15 V may reach, for example, only about ±12 V or ±13 V depending on the load. With low supply voltages, this limitation becomes very important.
Gain bandwidth product – The gain bandwidth product determines the available bandwidth for a given closed-loop gain. As an approximation:
Useful bandwidth ≈ GBW / Av Example: GBW = 3 MHz Av = 10 Theoretical bandwidth ≈ 3 MHz / 10 = 300 kHz Note: the theoretical bandwidth does not automatically guarantee low distortion or sufficient phase margin.
Slew rate – The slew rate indicates how quickly the output voltage can change. It is essential for high-amplitude audio signals, pulse-like waveforms, active filters, and high-frequency signals.
Formula for a sine wave: SR_min = 2 × π × f × Vp If the required slew rate exceeds the available slew rate, the output cannot follow the signal properly and slew-rate distortion occurs.
Input offset voltage – This is a small equivalent differential voltage between the inputs. In the non-inverting configuration it is amplified by the noise gain, which is equal to the non-inverting gain. It is important in DC circuits and precision sensor applications.
Input bias current – This is the current required by the inputs. In a JFET-input op amp such as the TL072 it is very low; in a bipolar-input op amp such as the RC4558 it is higher. Bias currents flowing through external resistors generate voltage errors.
Voltage noise and current noise – In audio, measurement, and sensor applications, noise is a critical parameter. Total noise depends on the op amp, resistors, circuit bandwidth, and source impedance.
CMRR and PSRR – CMRR indicates the ability to reject signals common to both inputs; PSRR indicates how much supply-voltage variations affect the output. High values are preferable, especially in noisy environments or when using imperfectly regulated power supplies.
Output current and load-driving capability – Check the maximum output current and the recommended minimum load. An op amp is not always able to drive headphones, speakers, long cables, or capacitive loads without an additional output buffer.
Unity-gain stability – If the op amp is to be used as a buffer, the datasheet must specify unity-gain stability, or equivalent operating conditions. Otherwise, the circuit may oscillate.
Package, pinout, and power dissipation – The TL072 and RC4558 are often available as dual op amps in 8-pin packages, but the specific pinout, variant, operating temperature, dissipation, and component availability should always be checked.
In conclusion, the non-inverting configuration is one of the most important op amp topologies. It offers positive gain, high input impedance, simple gain calculation, and the possibility of operating as a buffer. It is therefore ideal for preamplifiers, active filters, sensor conditioning, and impedance matching.
The TL072 is particularly interesting when high input impedance and higher slew rate are required. The RC4558, on the other hand, remains a historic and practical component for many general-purpose audio circuits, especially when impedances are moderate and the supply voltages are appropriate.
The key point is not to stop at the formula Av = 1 + R2/R1: a correct design always requires checking supply voltage, common-mode range, output swing, GBW, slew rate, noise, offset, input bias currents, stability, and load-driving capability.
