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| {{More footnotes|date=December 2008}}
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| [[File:NPN common emitter.svg|thumb|130px|Figure 1: Basic NPN common-emitter circuit (neglecting [[biasing]] details).]] | |
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| In [[electronics]], a '''common emitter''' [[electronic amplifier|amplifier]] is one of three basic single-stage [[bipolar junction transistor|bipolar-junction-transistor]] (BJT) amplifier topologies, typically used as a [[Electronic amplifier#Input and output variables|voltage amplifier]].
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| In this circuit the base terminal of the transistor serves as the input, the collector is the output, and the emitter is ''common'' to both (for example, it may be tied to [[ground (electricity)|ground reference]] or a [[power supply rail]]), hence its name. The analogous [[field-effect transistor]] circuit is the [[common source]] amplifier, and the analogous [[vacuum tube|tube]] circuit is the [[common cathode]] amplifier.
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| == Emitter degeneration ==
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| [[File:NPN common emitter degeneration.svg|thumb|130px|Figure 2: Adding an emitter resistor decreases gain, but increases linearity and stability]]
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| Common emitter amplifiers give the amplifier an inverted output and can have a very high [[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.
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| 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.
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| One common way of alleviating these issues is with the use of [[negative feedback]], which is usually implemented with ''emitter degeneration''. Emitter degeneration refers to the addition of a small [[resistor]] (or any [[Electrical impedance|impedance]]) 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#Voltage gain|voltage gain]]
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| :<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}} } \qquad (\text{where} \quad g_m R_{\text{E}} \gg 1). \,</math>
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| So 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.
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| == Characteristics ==
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| At low frequencies and using a simplified [[hybrid-pi model]], the following [[small-signal model|small-signal]] characteristics can be derived.
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| <div align="center">
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| {| class="wikitable" style="background:white;text-align:center"
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| ! !!style="width:2in"| Definition !!style="width:2in"| Expression (with emitter degeneration) !!style="width:2in"| Expression (without emitter degeneration, i.e., ''R''<sub>E</sub> = 0)
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| |-
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| ! '''[[gain#Current gain|Current gain]]'''
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| |<math>A_{\text{i}} \triangleq \frac{i_{\text{out}} }{ i_{\text{in}} } \,</math>
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| |<math>\beta \,</math>
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| |<math>\beta </math>
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| |-
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| ! '''[[gain#Voltage gain|Voltage gain]]'''
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| |<math>A_{\text{v}} \triangleq \frac{v_{\text{out}} }{ v_{\text{in}} } \,</math>
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| |<math>\begin{matrix}-\frac{ \beta R_{\text{C}} }{ r_{\pi} + ( \beta +1 ) R_{\text{E}} }\end{matrix}\,</math>
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| |<math>\approx-g_m R_{\text{C}}</math>
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| |-
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| ! '''[[Input impedance]]'''
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| |<math>r_{\text{in}} \triangleq \frac{v_{\text{in}}}{i_{\text{in}}}\,</math>
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| |<math>r_{\pi} +( \beta +1 ) R_{\text{E}}\,</math>
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| |<math>r_{\pi} </math>
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| |-
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| ! '''[[Output impedance]]'''
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| |<math>r_{\text{out}} \triangleq \frac{v_{\text{out}}}{i_{\text{out}}}\,</math>
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| |<math>R_{\text{C}}\,</math>
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| |<math>R_{\text{C}}</math>
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| |}
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| </div>
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| If the emitter degeneration resistor is not present, then <math>R_{\text{E}} = 0\,\Omega</math>, and the expressions effectively simplify to the ones given by the rightmost column (note that the voltage gain is an ideal value; the actual gain is somewhat unpredictable). As expected, when ''<math>R_{\text{E}}\,</math>'' is increased, the input impedance is increased and the voltage gain <math>A_{\text{v}}\,</math> is reduced.
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| ===Bandwidth===
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| The bandwidth of the common-emitter amplifier tends to be low due to high capacitance resulting from the [[Miller effect]]. The [[parasitic capacitance|parasitic]] base-collector capacitance <math>C_{\text{CB}}\,</math> appears like a larger parasitic capacitor <math>C_{\text{CB}} (1-A_{\text{v}})\,</math> (where <math>A_{\text{v}}\,</math> is negative) from the base to [[Ground (electricity)|ground]].<ref name="TAoE">{{cite book
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| |author=[[Paul Horowitz]] and [[Winfield Hill]]
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| |title=[[The Art of Electronics]]
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| |edition=2nd
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| |year=1989
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| |pages=102–104
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| |publisher=Cambridge University Press
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| |isbn=978-0-521-37095-0}}</ref> This large capacitor greatly decreases the bandwidth of the amplifier as it makes the [[time constant]] of the parasitic input [[RC circuit|RC filter]] <math>r_{\text{s}} (1-A_{\text{V}}) C_{\text{CB}}\,</math> where <math>r_{\text{s}}\,</math> is the [[output impedance]] of the signal source connected to the ideal base.
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| The problem can be mitigated in several ways, including:
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| * Reduction of the voltage gain [[Magnitude (mathematics)|magnitude]] <math>\left|A_{\text{v}}\right|\,</math> (e.g., by using emitter degeneration).
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| * Reduction of the [[output impedance]] <math>r_{\text{s}}\,</math> of the signal source connected to the base (e.g., by using an [[emitter follower]] or some other [[voltage follower]]).
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| * Using a [[cascode]] configuration, which inserts a low input impedance current buffer (e.g. a [[common base]] amplifier) between the transistor's collector and the load. This configuration holds the transistor's collector voltage roughly constant, thus making the base to collector gain zero and hence (ideally) removing the Miller effect.
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| * Using a [[differential amplifier]] [[topology (electronics)|topology]] like an [[emitter follower]] driving a grounded-base amplifier; as long as the emitter follower is truly a [[common collector|common-collector amplifier]], the Miller effect is removed.
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| The [[Miller effect]] negatively affects the performance of the common source amplifier in the same way (and has similar solutions).When an AC signal is applied to the transistor amplifier it causes the base voltage VB to fluctuate in value at the AC signal. The positive half of the applied signal will cause an increase in the value of VB this turn will increase the base current IB and cause a corresponding increase in emitter current IE and collector current IC. As a result, the collector emitter voltage will be reduced because of the increase voltage drop across RL. The negative alternation of an AC signal will cause a decrease in IB this action then causes a corresponding decrease in IE through RL. The output signal of a common- emitter amplifier is therefore 180 degrees out of phase with the input signal.
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| It is also named common- emitter amplifier because the emitter of the transistor is common to both the input circuit and output circuit. The input signal is applied across the ground and the base circuit of the transistor. The output signal appears across ground and the collector of the transistor. Since the emitter is connected to the ground, it is common to signals, input and output.
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| The common- emitter circuit is the most widely used of junction, transistor amplifiers. As compared with the common- base connection, it has higher input impedance and lower output impedance. A single power supply is easily used for biasing. In addition, higher voltage and power gains are usually obtained for common- emitter (CE) operation.
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| Current gain in the common emitter circuit is obtained from the base and the collector circuit currents. Because a very small change in base current produces a large change in collector current, the current gain (β) is always greater than unity for the common-emitter circuit, a typical value is about 50.
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| == Applications ==
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| === Low frequency voltage amplifier ===
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| A typical example of the use of a common-emitter amplifier is shown in Figure 3.
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| [[File:Complete common emitter amplifier.png|thumb|280px|Figure 3: Single-ended ''npn'' common-emitter amplifier with emitter degeneration. The AC-coupled circuit acts as a level-shifter amplifier. Here, the base–emitter voltage drop is assumed to be 0.65 Volts.]]
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| The input capacitor ''C'' removes any constant component of the input, and the resistors ''R''<sub>1</sub> and ''R''<sub>2</sub> bias the transistor so that it will remain in active mode for the entire range of the input. The output is an inverted copy of the AC-component of the input that has been amplified by the ratio ''R''<sub>''C''</sub>/''R''<sub>''E''</sub> and shifted by an amount determined by all four resistors. Because ''R''<sub>''C''</sub> is often large, the [[output impedance]] of this circuit can be prohibitively high. To alleviate this problem, ''R''<sub>''C''</sub> is kept as low as possible and the amplifier is followed by a voltage [[buffer amplifier|buffer]] like an [[emitter follower]]. | |
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| === Radio ===
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| Common-emitter amplifiers are also used in radio frequency circuits, for example to amplify faint signals received by an [[antenna (electronics)|antenna]]. In this case it is common to replace the load resistor with a tuned circuit. This may be done to limit the bandwidth to a narrow band centered around the intended operating frequency. More importantly it also allows the circuit to operate at higher frequencies as the tuned circuit can be used to resonate any inter-electrode and stray capacitances, which normally limit the frequency response. Common emitters are also commonly used as [[low-noise amplifier]]s.
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| == See also ==
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| *[[Common drain]]
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| *[[Common gate]]
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| == References ==
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| {{reflist}}
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| == External links ==
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| * [http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=2116.0 Simulation of The Common Emitter Amplifier Circuit] or [http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=550.0 simulation of Common Emitter Transistor Amplifier]
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| * [http://people.deas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html Basic BJT Amplifier Configurations]
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| * [http://230nsc1.phy-astr.gsu.edu/hbase/electronic/npnce.html NPN Common Emitter Amplifier] – [[HyperPhysics]]
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| * [http://www.tedpavlic.com/teaching/osu/ece327/lab1_bjt/lab1_bjt_transistor_basics.pdf ECE 327: Transistor Basics] – Gives example common-emitter circuit with explanation.
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| {{Transistor amplifiers}}
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| {{DEFAULTSORT:Common Emitter}}
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| [[Category:Single-stage transistor amplifiers]]
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