CAPACITOR IN BJT
We need to stabilize the Q-point,so,
we include the RE resistor,
which is a feedback resistor.
Let's do the DC analysis of the Common Emitter amplifier:
Without RE,IC=Beta*IB=Beta*[(VBB-0.7)/RB]
whose value depend on Beta,which is a variant parameter;
With RE,IC=Beta*IB
=Beta*[(VBB-0.7)/(RB+Beta*RE)]
=Beta*[(VBB-0.7)/Beta*RE]
=(VBB-0.7)/RE
it's a fixed value.
O.K. we get DC Q-point stabilization now,
how about the ac gain?
Do the ac analysis:
Without RE:
A=-ic*RC=-Beta*ib*RC
=-Beta*[(1/(RB+r-phi)]*RC
=-Beta*[1/RB]*RC
=-Beta*(RC/RB) --->a large value
With RE:
A=-ic*RC=-Beta*ib*RC
=-Beta*[1/(r-phi+Beta*RE)]*RC
=-Beta*[1/Beta*RE]*RC
=-RC/RE
=-1*(RC/RE)---------> a smaller value
yean,
with RE ,we get stabilization but we lose ac gain.
How to improve ?
If we connect a capacitor CE across the RE.
At DC analysis,the CE will be open circuit.
we still get the stabilization.
At AC analysis,the CE will be short circuit and
"short out" RE (Bypass)
we get the ac gain.