# Operational Amplifiers

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## Operational Amplifiers

An op-amp has differential input and is a DC-coupled electronic high-gain voltage amplifier. Generally it has single-ended output. The amplifier's differential inputs consist of a V+ input and a V− input. Ideally op-amp amplifies only the difference in voltage between the two inputs, which is called the differential input voltage. In this configuration, an op-amp produces an output potential difference (relative to ground) that is usually hundreds of thousands of times larger than the potential difference between its input terminals. The output voltage of the op-amp is given by the equation: Vout=Aol [(V+) – (V-)] here AOL is the open-loop gain of the amplifier (the term "open-loop" refers to the absence of a feedback loop from the output to the input), V− is the voltage at the inverting terminal and V+ is the voltage at the non-inverting terminal.

Fig (1): Circuit diagram symbol for an Op-amp

In an op-amp even quite small difference between V+ and V− is able to drive the amplifier output nearly to the supply voltage. “Saturation of the amplifier” is the situation in which the output voltage is equal to or greater than the supply voltage. It is impractical to use an op-amp as a stand-alone differential amplifier because the magnitude of AOL is not well controlled by the manufacturing process. With positive feedback for regeneration and without negative feedback, an op-amp acts as a comparator. If the inverting input is held at ground (0 V) directly or by a resistor and the input voltage Vin applied to the non-inverting input is positive, the output will be maximum positive; if Vin is negative, the output will be maximum negative. This is an open loop circuit acting as a comparator because there is no feedback from the output to either input. The circuit's gain is just the AOL of the op-amp.

## Operational Amplifiers Example

Below example calculates the gain of an inverting Op-amp:

Below is the circuit for the inverting Op-amp. This circuit has the output 180 degrees out of phase with that of input:

Fig (2): Basic inverting operational amplifier circuit

The input to the op-amp itself draws no current hence we can say by looking into the diagram that current flowing in R1 and R2 is the same. Hence, it is very easy to derive the op-amp gain equation. Using the ohms law we get: Vout /R2 = -Vin/R1. Hence, the voltage gain (Av) of the circuit can be written as:

If we want an amplifier of gain 10. We can have it by making R2= 47 k ohms and R1= 4.7 k ohms.

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