Emulating Logic-AND gate
That circuit isn't very good.


  ... what is the usual voltage or current requirement for an A/B input to be considered in the 'on' position?


It's whatever you define it to be based on what you consider acceptable as high and low on the output.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. The problem.

The problem is that the Q1 and Q2 will behave differently. If input B is pulled high then current will flow through R2, Q2's base-emitter junction (which is a diode, remember) and to your output.

On the other hand, if B is off and A is on you won't get the same diode behavior.

The circuit might perform adequately for some application but it's not true logic. 
Have a look at how AND gates are done in TTL logic to get a better idea of how it's done.