# High impedance inverting operational amplifier circuit

### - circuit and design details for a high impedance version of the inverting operational amplifier configuration.

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 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.

Basic inverting operational amplifier circuit

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

Av       =       - R2 / R1

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:

High input impedance inverting operational amplifier circuit

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

Av       =       - R2 (R3 + R4) / (R1 x R4 )

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.

Rhett Evans | AndersDX
CPU choice for display based systems
Behind every display based system is the CPU which acts as the brain of the entire module creating the data that is presented as well as driving the I/O peripherals, display and touchscreen. There are many factors to consider when choosing the right CPU for your project, including whether you pick a complete Single Board Computer (SBC) or a stripped back core module (COM).
Online - RF and Microwave Connector Types & Connector Care
Know your connector types and avoid expensive damage

More training courses

Forthcoming Events

. . . . More Events

##### Channels
Radio-Electronics.com is operated and owned by Adrio Communications Ltd and edited by Ian Poole. All information is © Adrio Communications Ltd and may not be copied except for individual personal use. This includes copying material in whatever form into website pages. While every effort is made to ensure the accuracy of the information on Radio-Electronics.com, no liability is accepted for any consequences of using it. This site uses cookies. By using this site, these terms including the use of cookies are accepted. More explanation can be found in our Privacy Policy