# Operational amplifier

Operational amplifier – a direct current amplifier characterized by very high gain coefficient. It is used to amplify the voltage or power from the input , usually operates with an external negative feedback loop – what characterises the function of the entire system. They have a very high input resistance and very low output resistance. The first operational amplifiers were used to perform mathematical operations such as additions, subtractions or integrations, hence the name “operational”.

Graphic symbol of the operational amplifier

The input with the ““sign is called the inverting input (that shifts phase of the input signal for 180 degrees towards output), and the input with the “+” sign is a non-inverting input. To allow the occurrence of positive and negative voltages on the input as well as the output, it is crucial to power it with positive and negative voltage through “x” and “y” ends.

Equivalent circuit of the operational amplifier

U-” signal was applied to the inverting input of the amplifier, and “U+” at the non-inverting input. The signal that occurs between the inputs is a UD differential signal expressed as the difference of U – U + signals. There is also a differential RD input resistance between the inputs of the amplifier. The voltage on UOUT output is comparable to the UD voltage. Ku coefficient is a designation of voltage gain of the open loop amplifier. From the before-mentioned analysis the relationship occurs as follows:

The principle of negative feedback

### During designing and analysing systems on operational amplifiers, it is assumed that the amplifier is ideal, which means that it has the following characteristics:

• Infinitely high gain in open loop (Ku -> ∞)
• Infinitely high input impedance
• Output impedance is zero
• Infinitely wide frequency response
• Output voltage equals zero for the same input voltages
• Zero input current (current is not drawn from the external circuits)
• Infinitely high allowable output current
• No interference of their own
• Its parameters do not depend on temperature

### Operational amplifier – real characteristics:

• Open-loop gain coefficient reaches very high but finite values
• Input impedance has high value, but finite
• Output impedance of several dozen
• Upper frequency limit of several dozens of MHz
• There is an input current of low rate of 10-4-10-15 A
• They produce self-interference
• Parameters of the amplifier are dependent from temperature and they change with time of use of the system

### Basic operating systems of operational amplifiers:

• Inverting and non-inverting amplifier
• Audio/video frequency pre-amplifiers
• Summing and differential amplifier
• Integrator
• Voltage follower
• Current – voltage converter
• Phase shifter

### The operational amplifier has been applied in a number of systems:

• In analogue electronic circuits, where it is responsible for performing mathematical operations,
• in logarithmic amplifiers,
• in filters,
• in some generators,
• in linear detectors and peak detectors,
• in sampling circuits with memory.

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