Inverting operational amplifier circuit

- circuit and design details for an operational amplifier, op-amp inverting amplifier configuration with additional high impedance version

A variety of operational amplifier, op-amp circuits including:
[1] Operational amplifier basics
[2] Op-amp circuit gain summary
[3] Inverting op-amp
[4] Inverting op-amp
[5] Op-amp high pass filter
[6] Op-amp low pass filter
[7] Op-amp bandpass filter
[8] Op-amp variable gain amplifier
[9] Op-amp notch filter circuits
[9] Operational amplifier multivibrator
[11] Op-amp bistable
[12] Op-amp comparator
[13] 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.

Inverting amplifier input impedance

It is often necessary to know the input impedance of a circuit. 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.

High input impedance inverting amplifier

The standard inverting amplifier configuration is widely used with operational amplifier integrated circuits. It has many advantages: being simple to construct; it offers the possibility of summation or mixing (in the audio sense) of several signals; and of course it inverts the signal which can be important in some instances.

However the standard inverting amplifier circuit does have some drawbacks which can be important on some occasions. The main drawback is its input impedance. To show how this can be important it is necessary to look at the circuit and take some examples. 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.

The gain for the amplifier can be calculated from the formula:

If a high gain of, for example 100, is required this means that the ratio of R2 : R1 is 100. It is good practice to keep the resistors in op amp circuits within reasonable bounds. In view of this the maximum value for R2 should be 1 M Ohm. This means that the input resistor and hence the input resistance to the amplifier circuit as a whole is 10 k Ohm. In some instances this may not be sufficiently high.

To overcome this problem it is possible to modify the circuit, and add a couple of extra resistors. The feedback resistor R2 serves to limit the amount of feedback. The higher it is the less feedback, and hence the higher the gain. By adding a couple of additional resistors across the output to act as a potential divider and taking the resistor R2 from the centre point, the level of feedback can be reduced. The circuit for this configurations is shown below:

The gain for this amplifier can be calculated from the formula:

Again the input resistance is equal to R1, but this can be made higher for the same gain.

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.