Electrical – Importance of slew rate

If you are sampling at 1.5 MHz, the time taken for your analogue amplifier to slew its output has to be a bit quicker than the reciprocal of 1.5 MHz i.e. 0.667 µs. If it has to deliver a change of 10 V in this period then the slew rate, as a minimum must be:

\$\dfrac{10\ V}{0.667\ \mu s}\$ = 15 V per micro second.

You also need to look at the op-amp's settling time - this is not encapsulated within the slew rate figure, and the settle time is usually specified to within the voltage reaching its target to an accuracy of 0.1%. If you are using a 10-bit ADC, 0.1% could be 1 LSB.

Does this effect depend on anything else than the JFET/BJT input stage?

Slewing can happen in any stage of an OpAmp, it can happen in the input stage, the output stage and any of the intermediary stages. It occurs whenever a capacitor is driven by a fixed current source. For a given configuration one stage sets the limit and determines the slew-rate often this is in fact the input stage.

A typical input stage consists of a differential pair with a tail current source. In equilibrium the current of the tail current source splits equally and when driven hard on transistor takes the whole current. The required voltage to turn one of the transistors (almost) off determines the onset of slewing. It's a fixed voltage for BJTs and a variable voltage for FETs.

If slewing happens in the output stage and the output stage is not symmetrical (e.g. class A) it is possible to have different slew-rates for falling and rising edges.

Are there input stages where an even lower differential input voltage leads to the maximum slew rate? Perhaps some sort of hybrid?

I don't know of any off-the-shelf devices that do this, but certainly there are adaptively biased OpAmps that increase the current through the input stage to improve the slewing behavior.

Even if there are only two types, do different op-amps of the same type (say, BJT) have different levels?

BJTs have a fixed level, unless emitter degeneration is used and FETs can have different levels.