Non-Inverting Amplifier

non-inverting

Non-Inverting Operational Amplifier

A non-inverting op-amp amplifier is one of the most commonly used operational-amplifier configurations. In this arrangement, the input signal is applied to the non-inverting (+) terminal, while the inverting (−) terminal is connected to a feedback network made of resistors. As the name suggests, the output signal is in phase with the input (no polarity reversal).

Basic Configuration

In a non-inverting amplifier, the input voltage Vin is directly connected to the (+) input of the op-amp. The (−) input receives a fraction of the output voltage through a resistive voltage divider formed by two resistors:

  • R1: connected between the inverting input and ground
  • R2: connected between the output and the inverting input

This negative feedback forces the op-amp to adjust its output so that the voltage at the inverting input matches the voltage at the non-inverting input.

Voltage Gain

The closed-loop voltage gain of a non-inverting amplifier depends only on the resistor values and is given by:

Av = 1 + (R2 / R1)

Key observations:

  • The gain is always greater than or equal to 1
  • If R2 = 0, the circuit becomes a voltage follower (buffer)
  • The gain is positive, so the output is in phase with the input

Phase Relationship

Unlike the inverting amplifier, the non-inverting configuration does not invert the signal. A positive change in the input voltage causes a positive change at the output. This makes it very useful in applications where signal polarity must be preserved, such as sensor conditioning and audio preamplifiers.

Input Impedance

One major advantage of the non-inverting amplifier is its very high input impedance. Since the input is applied directly to the op-amp’s non-inverting terminal, almost no current is drawn from the signal source.

This makes the non-inverting configuration ideal for:

  • Interfacing with high-impedance sensors
  • Audio sources that should not be loaded
  • Buffering weak or sensitive signals

Output Impedance

Due to negative feedback, the output impedance of a non-inverting amplifier is very low. This allows the amplifier to drive loads effectively without significant voltage drop, as long as the load current is within the op-amp’s limits.

Voltage Follower (Unity Gain Buffer)

When the output is directly connected to the inverting input and R1 is removed, the gain becomes:

Av = 1

This configuration is known as a voltage follower. It provides no voltage amplification but offers excellent buffering by combining very high input impedance with very low output impedance.

Practical Considerations

In real-world circuits, several practical limitations must be considered:

  • The output voltage cannot exceed the op-amp’s supply rails
  • The gain is limited by the op-amp’s bandwidth (gain-bandwidth product)
  • Input common-mode voltage range must include the input signal
  • Noise and offset voltages may be amplified along with the signal

Typical Applications

  • Audio preamplifiers
  • Sensor signal conditioning
  • Voltage buffers
  • Instrumentation and measurement circuits

Summary

The non-inverting op-amp amplifier is a simple, stable, and versatile configuration. Its high input impedance, predictable gain, and in-phase output make it a preferred choice in many analog signal-processing applications, especially where signal integrity must be preserved.

An inverting amplifier is one of the most fundamental and widely used configurations of an operational amplifier (op-amp). In this configuration, the input signal is applied to the inverting (−) terminal through a resistor, while the non-inverting (+) terminal is connected to ground. The output signal is a scaled and phase-inverted version of the input.

Basic Concept

The key idea behind the inverting amplifier is the concept of negative feedback. A resistor connects the output back to the inverting input, forcing the op-amp to adjust its output so that the voltage difference between the two input terminals remains extremely small (ideally zero).

Because the non-inverting input is grounded, the inverting input is held at a voltage very close to 0 V. This point is known as a virtual ground. Although it is not physically connected to ground, it behaves as if it were at ground potential.

Circuit Description

The circuit consists of:

The input signal current flows through Rin, and because the op-amp input draws almost no current, the same current must flow through Rf. This current relationship is what determines the gain of the amplifier.

Voltage Gain

The voltage gain of an inverting amplifier is given by:

Av = − (Rf / Rin)

The negative sign indicates that the output signal is inverted, meaning it is 180° out of phase with the input. For example:

Input and Output Behavior

When a positive voltage is applied to the input, current flows into the virtual ground at the inverting input. To maintain equilibrium, the op-amp drives its output negative. Conversely, a negative input voltage results in a positive output voltage.

This inversion property makes the circuit especially useful in signal processing applications where phase reversal or precise gain control is required.

Input Impedance

The input impedance of an inverting amplifier is approximately equal to Rin. This is because the inverting input is held at virtual ground, causing the source to see Rin as the main load.

Unlike the non-inverting amplifier, the input impedance is not extremely high unless Rin itself is chosen to be large.

Advantages

Limitations

Typical Applications

Inverting op-amp amplifiers are commonly used in:

Conclusion

The inverting op-amp amplifier is a cornerstone of analog circuit design. Its predictable behavior, mathematical simplicity, and versatility make it an essential building block in electronics, from audio systems to precision instrumentation.