Operational Amplifiers

Operational amplifiers are standard building blocks for analogue circuits
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 generic features of an operational amplifier are:

  • Very high input resistance
  • Low output resistance
  • Very high open loop gain ( A0 )

The operation of an operational amplifier is very simple
The output voltage is the difference between the input voltages multiplied by the open loop gain
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-

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. Two common op-amps are the 741 and the 081, both of which have the same pin layout and essentially the same characteristics making them interchangeable

  op-amp pinout

The 741 is a cheap and robust general purpose operational amplifier (Op-amp). The 741 Op-amp is usually in the form of an 8 pin DIL package although other varieties are available. The block diagram, pin layout and specification are given here. 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 pin layout741 pin layout

741 Specifications

Supply voltage± 22v max
Input impedance2 Megohm typically
Input current100 nA approx
Output impedance75 ohms approx
Output Current13 mA max
Output voltage
(Vs = ±15v)
± 13v
Open loop gain> 100,000
  Offset adjustment allows the output to be set to zero when the differential input is zero. The following circuit is considered standard. Offset adjustment is not usually necessary.

offset circuit