Circuit-wise, the NPN's T1 thru T6 are emitter-followers. In order to turn them on, the base-emitter junction needs to be forward-biased, which is about +0.7 volts. So, if the base is driven to +5v (the max possible from the 74HC device), the max-possible voltage at the emitter will be 5.0-0.7 = 4.3 volts. It makes no difference how high the collector voltage is (unless it's SO high that you get breakdown).
You are correct that the 74HC will source base current, but that will only happen as long as the base-emitter junction is forward-biased. For the sake of argument, let's imagine the base is at +5V and the emitter is around +160V. In this case, the base-emitter junction is reverse-biased, which means the transistor is off and no collector current (other than leakage) is possible. Stated another way, it's not possible for the circuit as drawn to produce more than +4.3V at the emitter. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/720ce346-1029-435a-ae8e-20407b78e987%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
