Op-amps, or operational amplifiers, are high gain differential voltage amplifiers of which one or more are packaged in a chip.

Operational amplifiers can be utilized as basic gain blocks, and used in a lot of applications needing only few additional components. Filters, amplifiers, comparators of analogue signals, and even analog computers (summers, differentiators, integrators) can be made using op-amps.

Op-amps have two inputs commonly referred to as Vin+ for the so-called non-inverting input, and Vin- for the inverting input. Of course they also have an output referred to as Vout. Other terminals are the two power supply connections commonly called VDD for the high or ‘+’ value and VSS for the low 0V or ‘-‘ value. For some applications a symmetrical power supply is useful (e.g. -5V and +5V) whereas for other applications a single power supply is sufficient (e.g. 0V and +5V). Power terminals may be shared in case of more op-amps on a chip.

What an ideal op-amp does, is multiplying the difference of the input voltages by a large number Aol, the open loop gain (100000 or more).
In equation. $V_{out}=A_{ol}*(V_{in+} – V_{in-})$.
In practice the output voltage can never exceed the voltage of the power supply.

This characteristic of op-amps may not seem useful at all, because a tiny voltage difference between the inputs will always lead to an output limited by the power supply voltages. The op-amp becomes much more useful when feedback is used (i.e. the output is connected to one of the inputs through other components).

Most important specifications

  • Supply voltage: limitation on power supply voltages
    • Minimum and maximum voltage for single supply (V)
    • Maximum and minimum, maximum difference¬† voltage for dual supplies (V)
  • Maximum input voltage limitations, absolute value and difference (V)
  • Open loop gain given in ($\frac{V}{mV}$ or dB (decibel); gain in dB $A_{ol} = \log \frac{V_{out}}{V_+ – V_-}$)
  • Maximum output current: the maximum current that van be supplied through the output (mA)
  • Gain Bandwidth Product (the product of the op-amp’s bandwidth and the gain at which the bandwidth is measured. Used to determine the maximum gain that can be extracted from the op-amp for a given frequency (or bandwidth) and vice versa).