Inverting operational amplifier circuit

- circuit and design details for an operational amplifier, op-amp inverting amplifier configuration

Op-amp circuits selection includes:
    •  Operational amplifier basics
    •  Op-amp circuit gain
    •  Inverting op-amp
    •  Non-inverting op-amp
    •  Op-amp high pass filter
    •  Op-amp low pass filter
    •  Op-amp bandpass filter
    •  Op-amp variable gain amplifier
    •  Op-amp notch filter circuits
    •  Operational amplifier multivibrator
    •  Op-amp bistable
    •  Op-amp comparator
    •  Op-amp Schmitt trigger

Operational amplifiers can be used in a wide variety of circuit configurations. One of the most widely used is the inverting amplifier configuration. It offers many advantages from being very simple to use, requiring just the operational amplifier integrated circuit and a few other components.

Circuits are available for an inverting amplifier, and a non-inverting amplifier. This version details the inverting amplifier, where the output is the mirror image of the input.

Basic inverting circuit

The basic circuit for the inverting operational amplifier circuit is shown below. It 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.

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).

As the input to the op-amp itself draws no virtually 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:

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.

Reminder
It is worth mentioning at this point that for high levels of gain, the gain bandwidth product of the basic op amp itself may become a problem. With levels of gain of 100, the bandwidth of some operational amplifier ICs may only be around 3 kHz. Check the data sheet for the given chip being used before settling on the level of gain.

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