Early effect in BJT

[Connoiseur]

I was wondering what is the significance of Early effect in BJT's from the circuit design point of view. Does it have any pitfalls/advantages? I mean, does it influence the switching ON/OFF time hFE etc. I've never seen it being mentioned in any of off the shelf transistors (like bc547, 2n2222, 2n3055......), yet it is an important topic in almost all of the electronics books.

[c4757p]

It is not strong enough to be of note for most common discrete BJT applications these days. It can absolutely ruin old-fashioned discrete analog stuff though, the stuff that's usually done on a chip. For instance, if you make a simple current mirror (the output impedance of which should ideally be infinite), the Early effect dominates its output impedance. Same with accuracy in differential amplifiers - if you try to make an op amp from scratch and the collector-emitter voltages of the two transistors in the core differential pair are not close to equal, you can have some pretty serious error.

[T3sl4co1l]

By definition, Early effect IS the output impedance of a non-cascode type current source/sink (straight mirror, not Wilson or cascode).

Some geometries and types have more than others; apparently some RF types have it in the extreme, so that even using them as followers, the gain is rather low.  On the other hand, some have it so slight that it's hard to tell if they're positive or negative resistance (if you wait for the tempco to act, they do indeed go negative).

Tim