What is the importance of Bypass capacitor of Common-emitter amplifier? Will the gain be reduced when it is removed?

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    \$\begingroup\$ You should be more clear what exactly you are asking. "Bypass" capacitor is a rather vague term. We can't know what you think it means and how exactly it relates to whatever common emitter amplifier you have in mind. SHOW A SCHEMATIC. \$\endgroup\$ – Olin Lathrop Mar 12 '13 at 18:03
  • \$\begingroup\$ For others wondering about schematic: See google images for "Common-emitter amplifier". Note many schematics with emitter cap, often labelled CE. \$\endgroup\$ – gwideman Mar 12 '13 at 21:01
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    \$\begingroup\$ @gwideman There are an infinite number of permutations and combinations. If the OP cannot provide a specific example, a specific answer cannot be provided. \$\endgroup\$ – Adam Lawrence Mar 12 '13 at 21:14
  • \$\begingroup\$ @ Madmanguruman The term ""Common emitter amplifier" is pretty unambiguous, as is the term "emitter bypass capacitor". Can you find any examples where Phil's answer would not be correct? Perhaps not everyone is familiar with common emitter amp, and could become familiar thanks to a schematic. But so far as being ambiguous per se, the question is pretty clear. \$\endgroup\$ – gwideman Mar 12 '13 at 22:42
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    \$\begingroup\$ How is it vague? How can you interpret the question in more than one way? Previous commenters thought that "bypass cap" was ambiguous, yet OP specifies emitter bypass cap, right in the title. He uses standard terminology regarding a standard transistor configuration, which he specified by its standard name. \$\endgroup\$ – gwideman Mar 13 '13 at 3:23

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.

  • \$\begingroup\$ Thanks for it, will it effect on voltage gain or current gain? please clarify \$\endgroup\$ – Ali Khan Mar 12 '13 at 17:06
  • \$\begingroup\$ @AliKhan It sounds like either you have a more fundamental question about common-emitter amplifiers which would merit another question, or you are looking for homework answers. \$\endgroup\$ – Phil Frost Mar 12 '13 at 17:17
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    \$\begingroup\$ +1 for a well written answer, but a schematic would be helpful. Between you and the OP, there is way too much hand waving such that if you don't already know what is being talked about, you're unlikely to learn it. \$\endgroup\$ – Olin Lathrop Mar 12 '13 at 18:08
  • \$\begingroup\$ @Ali: The key is to separate your thinking about DC conditions of the circuit, (baseline V's and I's which put the transistor at a particular part of its operating range), versus the AC behavior of the circuit, where presumably the AC signal of some significant frequency above zero (such as audio, or RF signal) is the signal of interest to be amplified. At DC, emitter cap is invisible, so DC conditions are set primarily by VB and RE, assuming the collector load R is suitable. At AC, em cap looks like a short circuit, so for AC gain, the transistor behaves as though E connected to ground. \$\endgroup\$ – gwideman Mar 12 '13 at 22:48
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    \$\begingroup\$ @Phil Frost and gwideman, great explanation. Now I have started to understand behaviour of Amplifier circuits. \$\endgroup\$ – Ali Khan Mar 13 '13 at 16:56

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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