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Non-Inverting Operational Amplifier Circuit

- a tutorial, overview or summary of the basics of the non-inverting operational amplifier circuit with the essential calculations and circuit guide information

The non-inverting amplifier configuration is one of the most popular and widely used forms of operational amplifier circuit. Operational amplifier circuits are one of the mainstays of the analogue circuit designers library because they are very easy to use and they offer a very high level of performance. In addition to this, operational amplifier integrated circuits are very cheap and easy to obtain. This makes them ideal for many applications, both in commercial use and for the hobbyist.

Operational amplifiers have been available in the form of integrated circuits for very many years and they are a favourite circuit block with all analogue circuit designers. From non-inverting amplifiers and inverting amplifiers to multivibrators, astables, integrators, filters and differentiators, as well as a vast number of other circuits, operational amplifiers are able to form the core element in many analogue circuits.

Operational amplifiers are available in a wide variety of packages. Not only are they available in dual in line packages with one, two, or more amplifiers in a single package, but they are also available in surface mount varieties as well.

Op-amp circuit formats

Op-amps can be used in an enormous variety of different types of circuit but one of the most common is within amplifier circuits themselves. Here they enable high performance circuits to be made with a minimum number of components.

There are two common formats for amplifiers using op-amp based circuits:

• Non-inverting amplifier circuit: - here the output is in the same sense as the input signal, i.e. when the input voltage goes positive, so does the output.



• Inverting amplifier circuit: - here the output is an inverted version (upside down) of the signal that enters the circuit, i.e. when the input voltage goes more positive, the output goes more negative.

Both the inverting and non-inverting amplifiers have their advantages and disadvantages, and as a result they may be used in different ways. Under some circumstances the inverting configuration is better, whereas at other times the non-inverting operational amplifier circuit is better.

On this page we look at the non-inverting operational amplifier circuit.

Non-inverting amplifier circuit

The basic circuit for the non-inverting operational amplifier is relatively straightforward. In this circuit the signal is applied to the non-inverting input of the op-amp. However the feedback is taken from the output of the op-amp via a resistor to the inverting input of the operational amplifier where another resistor is taken to ground. It is the value of these two resistors that govern the gain of the operational amplifier circuit.

Basic non-inverting operational amplifier circuit

Non-inverting amplifier gain

The gain of the non-inverting circuit for the operational amplifier is easy to determine. The calculation hinges around the fact that the voltage at both inputs is the same. This arises from the fact that the gain of the amplifier is exceedingly high. If the output of the circuit remains within the supply rails of the amplifier, then the output voltage divided by the gain means that there is virtually no difference between the two inputs.

As the input to the op-amp draws no current this means that the current flowing in the resistors R1 and R2 is the same. The voltage at the inverting input is formed from a potential divider consisting of R1 and R2, and as the voltage at both inputs is the same, the voltage at the inverting input must be the same as that at the non-inverting input. This means that Vin = Vout x R1 / (R1 + R2). Hence the voltage gain of the circuit Av can be taken as:

Av       =       1 + R2 / R1

As an example, an amplifier requiring a gain of eleven could be built by making R2 47 k ohms and R1 4.7 k ohms.

Non-inverting amplifier input impedance

It is often necessary to know the input impedance of a circuit. The input impedance of this operational amplifier circuit is very high, and may typically be well in excess of 10^7 ohms. For most circuit applications this can be completely ignored. This is a significant difference to the inverting configuration of an operational amplifier circuit which provided only a relatively low impedance dependent upon the value of the input resistor.

AC coupling a non-inverting amplifier

In most cases it is possible to DC couple the circuit. However in this case it is necessary to ensure that the non-inverting has a DC path to earth for the very small input current that is needed. This can be achieved by inserting a high value resistor, R3 in the diagram, to ground as shown below. The value of this may typically be 100 k ohms or more. If this resistor is not inserted the output of the operational amplifier will be driven into one of the voltage rails.

Basic non-inverting operational amplifier circuit with capacitor coupled input

When inserting a resistor in this manner it should be remembered that the capacitor-resistor combination forms a high pass filter with a cut-off frequency. The cut off point occurs at a frequency where the capacitive reactance is equal to the resistance.

Non-inverting amplifier summary

The non-inverting amplifier configuration using an operational amplifier is particularly useful for applications where a high input impedance is required. The non-inverting amplifier circuit is easy to build, and operates reliably and well in practice.