Understanding transistor requirements

[Bigdrums]

Is it true that one leg of any transistor will need a DC supply voltage in order to amplify (or even pass) a incoming signal on another leg?

[Mick Bailey]

A BJT will act like a diode if its junctions are forward biased. This means the transistor can pass a half-wave rectified signal without any DC being applied - so long as it is greater than the forward voltage drop of the junction, but it won't amplify. B-E and B-C act like diodes.

[g1]

Some jfets will pass (not amplify) signal from source to drain and only need voltage to be turned off.

[Helmholtz]

Jfets typically are symmetrical, so drain and source can be reversed.

[Mick Bailey]

Point contact transistors are also symmetrical, though you aren't likely to find one outside of a museum, unless you make your own.

[R.G.]

Is it true that one leg of any transistor will need a DC supply voltage in order to amplify (or even pass) a incoming signal on another leg?

t
It's not clear what you're asking.
It is true that no transistor will amplify without a DC supply. As to passing signal, it depends on the type of transistor. For bipolar transistors, you can get only the kind of "passing signal" that a diode will do, as already noted. For specifically depletion mode JFETs and MOSFETs , they are normally on, so in the absence of DC and/or bias on the third leg, the drain and source will pass signal through. In the MOSFET, this is complicated by there being a built-in parasitic diode in one direction from drain to source. But JFETs, with nothing else attached, look like a resistor until biased off or until the signal gets big enough that the voltage drop along the drain-to-source resistance starts causing self-bias.

But in general, no, you get no amplification or "passing signal" unless there is a DC supply. There are (always) asterisks and footnotes.