Editing Common emitter (section)

== Emitter degeneration ==
[[File:NPN common emitter degeneration.svg|thumb|130px|Figure&nbsp;2: Adding an emitter resistor decreases gain, but increases linearity and stability]]

Common-emitter amplifiers give the amplifier an inverted output and can have a very high [[gain (electronics)|gain]] that may vary widely from one transistor to the next. The gain is a strong function of both temperature and bias current, and so the actual gain is somewhat unpredictable. [[BIBO stability|Stability]] is another problem associated with such high-gain circuits due to any unintentional [[positive feedback]] that may be present.

Other problems associated with the circuit are the low input [[dynamic range]] imposed by the [[small-signal model|small-signal]] limit; there is high [[distortion]] if this limit is exceeded and the transistor ceases to behave like its small-signal model. One common way of alleviating these issues is with ''emitter degeneration''. This refers to the addition of a small [[resistor]] between the emitter and the common signal source (e.g., the [[ground (electricity)|ground reference]] or a [[power supply rail]]). This impedance <math>R_\text{E}</math> reduces the overall [[transconductance]] <math>G_m = g_m</math> of the circuit by a factor of <math>g_m R_\text{E} + 1</math>, which makes the [[gain (electronics)#Voltage gain|voltage gain]]

:<math>A_\text{v} \triangleq \frac{v_\text{out}}{v_\text{in}} = \frac{-g_m R_\text{C}}{g_m R_\text{E} + 1} \approx -\frac{R_\text{C}}{R_\text{E}},</math>

where <math>g_m R_\text{E} \gg 1</math>.

The voltage gain depends almost exclusively on the ratio of the resistors <math>R_\text{C}/R_\text{E}</math> rather than the transistor's intrinsic and unpredictable characteristics. The [[distortion]] and stability characteristics of the circuit are thus improved at the expense of a reduction in gain.

(While this is often described as "[[negative feedback]]", as it reduces gain, raises input impedance, and reduces distortion, it predates [[Harold Stephen Black|the invention of the negative feedback amplifier]] and does not reduce output impedance or increase bandwidth, as a true negative feedback amplifier would do.<ref>{{Cite web|url=http://sound.whsites.net/articles/distortion+fb.htm|title=Distortion and Feedback|website=sound.whsites.net|access-date=2016-01-27|quote=Although it is commonly accepted that emitter ... degeneration is feedback, this is only partially true. ... it has no effect on effective bandwidth or output impedance. Harold Black invented negative feedback, not degeneration (which pre-dated his invention).|archive-date=2016-12-20|archive-url=https://web.archive.org/web/20161220123657/http://sound.whsites.net/articles/distortion+fb.htm|url-status=dead}}</ref>)