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Operational Amplifiers - Overview

Introduction

symbolOperational amplifiers are standard building blocks for analogue circuits.

The circuit symbol shows that they have two inputs and a single output and usually operate from a dual rail power supply i.e. a positive and a negative power supply. This allows the output to have both positive and negative values.

The Input labelled with a '+' sign is called the Non-Inverting Input
The Input labelled with the '-' sign is the Inverting Input


The generic features of an operational amplifier are:

Basic Operation

The operation of an Operational Amplifier (Op Amp) is very simple. The Output voltage is the difference between the Input voltages multiplied by the open loop gain. In other words, the Op Amp increases the difference between the two inputs by a large multiplying factor. This can be written as:

Vout = A0 (V+ - V-)

Where V+ is the voltage at the non-inverting input of the amplifier and V- is the voltage at the inverting input

Note that the output voltage is positive when V+ > V- and the output voltage is negative (or zero) when V+ < V-

The output saturates (reaches its maximum value) 2v below the supply voltage although it is often enough to assume that the output saturates at the supply voltage. Obviously, as the op-amp has a very high open loop gain, even a very small difference of just a few mV (or even µV) will cause the output to saturate

The output of a standard op-amp can either source or sink sufficient current to drive an LED. The output is usually internally protected and, although most op-amps are robust, care should be taken to provide suitable current limiting resistors for the load.

741 Pin Layout

pinout layout 741 is a cheap and robust general purpose operational amplifier (Op Amp) although it is an old design and there are many much better Op Amps available. The 741 Op Amp is usually in the form of an 8 pin DIL package although other varieties are available.

The pin layout is common to most Op Amps and can be treated as generic for the 8 pin packages. It should be noted that the 741 is not really suitable for audio applications.


741 Specifications

741 Supply voltage: ± 22V max
Input impedance: 2 Megohm typically
Input current: 100 nA approx
Output impedance: 75 ohms approx
Output Current: 13 mA max
Output voltage (Vs = ±15V): ± 13V
Open loop gain> 100,000