7: Important characteristics explained: Input Bias current, Input Offset current, Input Offset voltage, and "headroom"
Fig 6 represents the input stage of a conventional bipolar op amp. You can see that the inputs Vin+, Vin- need to provide base current Ib1, Ib2 for the transistors. This is the input bias current.
If the gains of the transistors are not identical they will not take the same base current. The difference between them is the input offset current.
Suppose Vin+ = Vin- ; then Vout SHOULD be zero. However if the transistors are not perfectly matched there will be a difference in their Vbe - that is the input offset voltage.
Looking at this circuit you will see that if Vin+ or Vin- was at -Vcc the transistors would be turned off and the circuit could not work; most op amps require the input voltages to be well within the supply voltages;- and in particular, op amps with bipolar input stages - like this - generally need Vin to be about 3V clear of -Vcc or +Vcc. The amount by which the supply voltage must exceed the input voltage range is called "headroom".
Gain Bandwidth product and Slew Rate
Both of these relate to the frequency response of the amplifier. Few op amps will have very high gain-bandwidth product - mostly around 5 - 10MHz.
Suppose you build an amplifier using a NE5532, which has a gain-bandwidth product of 10MHz and set it for a gain of 100 the amplifier bandwidth will be 10MHz / 100 = 100kHz.
The slew rate is the speed at which the output voltage can change in response to a step voltage at its input. Looking at the table (Reply 5) you will see that to get a fast slew rate you need an amp with a good GBw product.
Faster is not always better
Very fast op amps like the AD797 above are useful in specific applications, but may prove less stable and prone to oscillate, sometimes needing careful decoupling and pc layout. Op amps with a GBW of 1 - 10 MHz are easier to work with, and for signals that are only changing slowly consider op amps with a GBw less than 1.
Bandwidth: the highest frequency that can be handled by the amplifier.