how much voltage to open up transistor?

[xplod1236]

how much voltage do i need to feed to the base to open up the transistor so that it will conduct (c-e) with the least resistance?
Let's say that this is the output stage of an amp with +/- 50v rails.
How much do i feed the base to get 50v (the max voltage i can get) out of the transistor?

[sreten]

The simple answer is not much at all - though your question
suggests you really don't know the right question to ask.

An output transistor has around 0.6V to 0.8V across the
base/emitter when conducting, but if its supplying a fair
amount of current, the base must be driven by a fair
amount of current, much lower level but still appreciable.

As output stages are nearly always unity gain (no voltage gain)
the actual voltages at the bases are within 1V of the output
voltage, so ~ 51V in = 50V out.

But its unlikely the driver stage can swing to the voltage rails so
for a +/- 50V amp, maximum output voltage swing is ~ +/- 47V.

sreten.

[xplod1236]

you are right, i didnt know the right question to ask.

so if the voltage on the base is the voltage going out of the transistor, how can i make a voltage gain stage? add a resistor from the output to the base?

[Richard C]

A common emitter configuration is the simplest way to get voltage gain from a single transistor.

Buy or borrow "The Art of Electronics" by Horowitz and Hill for a friendly and conceptual introduction to how transistors work.

Don't bother with the physics and remember these 3 simple approximations:

During conduction the potential at the emitter of an NPN device is ALWAYS 0.7V lower than the base (0.7V higher for PNP)

Collector current = emitter current

Base current = collector current/Beta

Combine this knowledge with Ohms law and you have everything you need to start designing and analysing transistor circuits.

[sreten]

Originally posted by xplod1236
you are right, i didnt know the right question to ask.

so if the voltage on the base is the voltage going out of the transistor, how can i make a voltage gain stage? add a resistor from the output to the base?

For an amplifier output stage the load is placed in the emitter of
the transistor, hence no voltage gain.

When the load is in the collector of the transistor its very different.
0.7V is enough to fully turn on ther transistor.

sreten.