Why doesn't a normal npn transistor with common emitter configuration, in active mode, not behave as a DC amplifier?
The &quot;normal npn BJT transistor&quot; is a DC amplifier; only its output quantity is a (collector) current. To make it a voltage amplifier, we have somehow to convert this current to a voltage. For this purpose, we connect a (collector) resistor between the collector and Vcc... and take the voltage drop VRc across it as an output quantity. The problem is that this voltage is not referenced to ground but to Vcc. So, we take its complement to Vcc (Vce = Vcc - VRc) that is grounded. That is why this circuit (aka &quot;common emitter amplifying stage&quot;) is inverting.
Of course, to make a true voltage amplifier, we have to bias the transistor input (to add an initial input voltage about 0.7 V in series to Vin).