Non-inverting operational amplifier circuit

- Circuit and design details for an operational amplifier / op-amp non-inverting amplifier circuit.

Operational amplifiers can be used in two basic configurations to create amplifier circuits. One is the inverting amplifier where the output is the inverse or 180 degrees out of phase with the input, and the other is the non-inverting amplifier where the output is in the same sense or in phase with the input.

Both operational amplifier circuits are widely used and they find applications in different areas. When an operational amplifier or op-amp is used as a non-inverting amplifier it only requires a few additional components to create a working amplifier circuit.

Basic circuit

The basic non-inverting operational amplifier circuit is shown below. 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

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.

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

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.