What is the importance of Bypass capacitor of Common-emitter amplifier? Will the gain be reduced when it is removed?
Usually, this capacitor is in parallel with a resistor:
In a common-emitter amplifier, any impedance between the emitter and ground (\$R_e\$ and \$C_e\$) serves to reduce the gain of the amplifier: it is a form of negative feedback. By increasing the negative feedback and decreasing the gain, we can make variations in transistors less significant. Perhaps most relevant to this point, by adding \$R_e\$ we make the bias current more dependent on the resistors (which are easy to control) and less dependent on the gain of the transistor (which varies over a large range, even among transistors of the same model).
But what if we still want high gain? Because a capacitor presents an impedance that decreases with frequency, putting \$C_e\$ in parallel with \$R_e\$ serves to decrease the negative feedback, and thus increase the gain, at high frequencies. Effectively, high-frequency signals can bypass the emitter resistor, through the capacitor. Yet, to DC, the capacitor appears as an open circuit, so adding the capacitor does not affect the DC bias current. Thus, if we are interested in amplifying AC signals only, this capacitor allows us to have a stable DC bias current while maintaining high gain for our signals of interest.
What is the importance of Bypass capacitor of Common-emitter amplifier?
I would just like to add a bit to Phil's answer.
To be precise, for a common emitter amplifier, the emitter is tied to the signal common node thus the title "Common Emitter".
This means that, for the signals of interest, the emitter is effectively at zero volts.
If the emitter resistor is not bypassed at signal frequencies, one does not have a genuine common emitter amplifier since there are signal frequencies present at the emitter.
In this case, as Phil points out, there is emitter degeneration present which, essentially, reduces signal (AC) gain while increasing linearity.
Sometimes, you will see a hybrid emitter circuit with two resistors in series, one bypassed by a capacitor and one not. This extra degree of freedom allows more flexibility in choosing the DC operating point and AC gain.
protected by W5VO♦ Mar 12 '13 at 19:02
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