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Operational Amplifiers - Overview


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.

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) 2 V below the supply voltage although it is often enough to assume that the output saturates at the supply voltage. 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 The 741 Op-Amp IC is a cheap and robust general purpose operational amplifier 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: ± 22 V max
Input impedance: 2 MΩ typically
Input current: 100 nA approx
Output impedance: 75 Ω approx
Output Current: 13 mA max
Output voltage (Vs = ±15 V): ± 13 V
Open loop gain: > 100,000