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Inverting Operational Amplifier Circuit

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

Although operational amplifiers can be used within a variety of different types of circuit, one of the most common is as an amplifier itself. There are two common formats for operational amplifier based circuits:

• 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.



• 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.

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 inverting operational amplifier circuit.

Basic inverting amplifier circuit

The basic circuit for the inverting operational amplifier circuit is quite straightforward and only needs a few components beyond the operational amplifier integrated circuit itself. The circuit consists of a resistor from the input terminal to the inverting input of the circuit, and another resistor connected from the output to the inverting input of the op-amp. The non inverting input is connected to ground.

Basic inverting operational amplifier circuit

In this circuit the non-inverting input of the operational amplifier is connected to ground. As the gain of the operational amplifier itself is very very high and the output from the amplifier is a matter of a few volts, this means that the difference between the two input terminals is exceedingly small and can be ignored. As the non-inverting input of the operational amplifier is held at ground potential this means that the inverting input must be virtually at earth potential (i.e. a virtual earth).

Gain of the inverting amplifier circuit

One of the main features of the inverting amplifier circuit is the overall gain that it produces. This is quite easy to calculate. The voltage gain is actually the output voltage (Vout) divided by the input voltage (Vin), i.e. it is the number of times the output voltage is larger than the input voltage.

It is also easy to determine the equation for the voltage gain. 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. Using ohms law Vout /R2 = -Vin/R1. Hence the voltage gain of the circuit Av can be taken as:

Av       =       - R2 / R1

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

Inverting amplifier input impedance

It is often necessary to know the input impedance of a circuit, and in this case of the inverting amplifier. A circuit with a low input impedance may load the output of the previous circuit and may give rise to effects such as changing the frequency response if the coupling capacitors are not large.

It is very simple to determine the input impedance of an inverting operational amplifier circuit. It is simply the value of the input resistor R1. This is because the inverting input is at earth potential (i.e. a virtual earth) and this means that the resistor is connected between the input and earth.