John Payson wrote:

> That reminds me... one of the design features of a BJT is that,
> unlike a MOSFET, it will only allow current to flow in one
> direction while it is switched on.  So I was wondering...

  That's a extraordinarily broad statement John!

> If you have a supply of base current, what are the limitations
> of using the BJT as a diode [between collector and emitter].

  The prime limitation as I see it, is that the first statement is
entirely wrong!  In fact, I seem to recall deliberate use of transistors
with collector and emitter transposed specifically to obtain a reduced
Vec(sat) (sic.)

  And a transistor with its collector capacitively coupled to a signal
line (with no other collector bias components) as a gate certainly
pre-dates the use of a MOSFET in the same configuration.
--
  Cheers,
        Paul B.