[sdiy] Left over transistor in a transistor array

[Magnus Danielson]

Scott Nordlund skrev:
> I wouldn't think you'd have to do anything- it's not like a logic gate or op amp where floating inputs can lead to undesirable stuff.  If it's just a single transistor, it can't really do anything.

Since the transistors share the substrate, you want to make sure the 
substrate is biased such that conduction does not occur in an 
"interesting way".

The collectors is an NP-junction away from the substrate. That is how 
transistors may share the same substrate without having collectors tied 
together, your substrate is of opposite type than your collectors, 
considering that this is an NPN transistor array, the substrate is 
P-doped. Hooking the substrate to the lowest voltage makes the 
NP-junctions reversed biased and that is how you want it.

Cheers,
Magnus