- Jan 1, 1970
Ingvar Esk said:Well, as you state in your paper "Despite much literature that implies other
wise..." I think it must be debatable, otherwise I can't see that "much
literature" would imply so
From my point of view. If I feed 1mA into the base the Ic would be some
hundreds (hFE) mA (if available), even though I know that hFE is not very
accurate. On the other hand if I supplied 0.1V to the base, nothing much
will happen at the collector. I can accept that it can be seen as a Voltage
amplifier, but only around its working point (Vb ~0.6-0.7V).
The transistor base voltage varies very little over the range
of collector current. Like you say, the base voltage relative to the
emitter will usually be 600 to 700 mV, or maybe a bit more.
A general rule of thumb to calculate voltage gain is
40 times the difference of supply voltage to collector voltage.
So, if the transistor were biased such that the collector is 3
volts and the supply is 6 volts, you get a voltage gain
around (6-3)*40 = 120. So, for every millivolt change on the
base, the collector should change about 120 millivolts.
Doesn't much matter what the hFE is, other than it effects
the input impedance.