Please take a look here. Tell me if you can why putting an emitter
bypass capacitor on a common emitter amplifier makes the signal the
way it is. (i.e. amplified and clipped). Exactly why does sending
the AC to ground at the emitter increase the gain?
thank u in advance to anyone who helps me understand this.
No emitter cap' and the emitter voltage follows sympathetically whatever
voltage is fed to the base.
This emitter voltage is across the emitter resistor,
Therefore the emitter current also follows in sympathy.
As the emitter current is pretty much the collector current, then the
collector voltage must also follow suit.
As the collector resistor is a bigger resistance than the emitter resistor
then the voltage across the collector resistor must be a bigger version of
the emitter voltage. You've already fixed the current (a.c.) that can flow
down the transistor by using the emitter resistor, so a value of collector
resistor can be chosen to give whatever voltage gain you want (or even
Add an emitter cap' and you lose the ability to develop a voltage across the
external emitter resistor *but* you still have a small resistance inside the
transistor at work. This resistor value is = 25 / emitter ma's.
(In your case, the quiescent transistor emitter current is set about 0.45ma,
so this internal resistor is 25 / 0.45 =56ohm.)
Thus the input signal voltage across this internal emitter resistance sets
the emitter current and hence the collector current and in turn the colector
voltage and overall voltage gain.
Without an emitter cap' your circuit has a fluctuating emitter current of
100mVac / 1500ohms = 67uA. This turns up across the 10k collector resistor
as a fluctuating voltage of 67uA X 10kohms = 0.67Vac. Thus the stage has a
voltage gain of 0.67Vac / 100mVac =6.7 times.
With an emitter cap' the fluctuating emitter current is 100mVac / 55ohms =
1.8ma. This tries to force a fluctuating voltage across the 10k collector
resistor of 1.8ma X 10k = 18Vac!. Thus the voltage gain is massively higher
(consequently clipped at the rails), as can be seen on the scope display.
There's other knock on effects caused by this high gain and in practice it's
better to leave some undecoupled external emitter resistance.
Essentially, go for quality rather than quantity.