Common Emitter
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In
electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
, a common-emitter
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost th ...
is one of three basic single-stage bipolar-junction-transistor (BJT) amplifier topologies, typically used as a
voltage amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost th ...
. It offers high current gain (typically 200), medium input resistance and a high output resistance. The output of a common emitter amplifier is 180 degrees out of phase to the input signal. 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 reference or a
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), hence its name. The analogous FET circuit is the common-source amplifier, and the analogous tube circuit is the
common-cathode An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a Signal (information theory), signal (a time-varying voltage or Electric current, current). It may increase the power (physics ...
amplifier.


Emitter degeneration

Common-emitter amplifiers give the amplifier an inverted output and can have a very high
gain Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in d ...
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.
Stability Stability may refer to: Mathematics * Stability theory, the study of the stability of solutions to differential equations and dynamical systems ** Asymptotic stability ** Linear stability ** Lyapunov stability ** Orbital stability ** Structural st ...
is another problem associated with such high-gain circuits due to any unintentional
positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in th ...
that may be present. Other problems associated with the circuit are the low input
dynamic range Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base- ...
imposed by the small-signal limit; there is high
distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio s ...
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 A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias activ ...
between the emitter and the common signal source (e.g., the ground reference or a
power supply rail A power supply unit (PSU) converts mains AC to low-voltage regulated DC power for the internal components of a computer. Modern personal computers universally use switched-mode power supplies. Some power supplies have a manual switch for select ...
). This impedance R_\text reduces the overall
transconductance Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciproc ...
G_m = g_m of the circuit by a factor of g_m R_\text + 1, which makes the voltage gain :A_\text \triangleq \frac = \frac \approx -\frac, where g_m R_\text \gg 1. The voltage gain depends almost exclusively on the ratio of the resistors R_\text/R_\text rather than the transistor's intrinsic and unpredictable characteristics. The
distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio s ...
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 Negative feedback (or balancing feedback) occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by othe ...
", as it reduces gain, raises input impedance, and reduces distortion, it predates the invention of negative feedback and does not reduce output impedance or increase bandwidth, as true negative feedback would do.)


Characteristics

At low frequencies and using a simplified
hybrid-pi model The hybrid-pi model is a popular circuit model used for analyzing the small signal behavior of bipolar junction and field effect transistors. Sometimes it is also called Giacoletto model because it was introduced by L.J. Giacoletto in 1969. The m ...
, the following small-signal characteristics can be derived.
If the emitter degeneration resistor is not present, then R_\text = 0\,\Omega, 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 ''R_\text\,'' is increased, the input impedance is increased and the voltage gain A_\text\, is reduced.


Bandwidth

The bandwidth of the common-emitter amplifier tends to be low due to high capacitance resulting from the Miller effect. The
parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
base-collector capacitance C_\, appears like a larger parasitic capacitor C_\text (1 - A_\text)\, (where A_\text\, is negative) from the base to
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. This large capacitor greatly decreases the bandwidth of the amplifier as it makes the
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of the parasitic input RC filter r_\text (1 - A_\text) C_\text\, where r_\text\, is the output impedance of the signal source connected to the ideal base. The problem can be mitigated in several ways, including: * Reduction of the voltage gain magnitude \left, A_\text\\, (e.g., by using emitter degeneration). * Reduction of the output impedance r_\text\, of the signal source connected to the base (e.g., by using an emitter follower or some other voltage follower). * Using a cascode configuration, which inserts a low input impedance current buffer (e.g. a
common base In electronics, a common-base (also known as grounded-base) amplifier is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a current buffer or voltage amplifier. In this circuit the emitter ...
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. * Using a
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like an emitter follower driving a grounded-base amplifier; as long as the emitter follower is truly a common-collector amplifier, the Miller effect is removed. 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. 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. 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. 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.


Applications


Low-frequency voltage amplifier

A typical example of the use of a common-emitter amplifier is shown in Figure 3. The input capacitor C removes any DC component of the input, and the resistors R1 and R2 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''C/''R''E and shifted by an amount determined by all four resistors. Because ''R''C is often large, the output impedance of this circuit can be prohibitively high. To alleviate this problem, ''R''C is kept as low as possible and the amplifier is followed by a voltage buffer like an emitter follower.


Radio

Common-emitter amplifiers are also used in radio frequency circuits, for example to amplify faint signals received by an 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 A low-noise amplifier (LNA) is an electronic amplifier that amplifies a very low-power signal without significantly degrading its signal-to-noise ratio. An amplifier will increase the power of both the signal and the noise present at its inp ...
s.


Audio

Common-emitter amplifiers are also used for audio amplifiers. For example, a
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or hobbyist application of the common-emitter amplifier is presented in.Single-Transistor Audio Amplifier - How the Common Emitter Amplifier Works https://youtube.com/watch/QGInwQa_XEM


See also

*
Common base In electronics, a common-base (also known as grounded-base) amplifier is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a current buffer or voltage amplifier. In this circuit the emitter ...
*
Common collector In electronics, a common collector amplifier (also known as an emitter follower) is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer. In this circuit the base terminal o ...
* Common gate *
Common drain In electronics, a common-drain amplifier, also known as a source follower, is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage buffer. In this circuit (NMOS) the gate terminal of the ...
*
Common source In electronics, a common-source amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage or transconductance amplifier. The easiest way to tell if a FET is common source, comm ...
*
Open collector An open collector is a common type of output found on many integrated circuits (IC), which behaves like a switch that is either connected to ground or disconnected. Instead of outputting a signal of a specific voltage or current, the output sig ...
*
Two-port network A two-port network (a kind of four-terminal network or quadripole) is an electrical network (circuit) or device with two ''pairs'' of terminals to connect to external circuits. Two terminals constitute a port if the currents applied to them satis ...


References


External links


Simulation of The Common Emitter Amplifier Circuit
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simulation of Common Emitter Transistor Amplifier




HyperPhysics ''HyperPhysics'' is an educational website about physics topics. The information architecture of the website is based on HyperCard, the platform on which the material was originally developed, and a thesaurus organization, with thousands of contr ...

ECE 327: Transistor Basics
– Gives example common-emitter circuit with explanation. {{DEFAULTSORT:Common Emitter Single-stage transistor amplifiers