Common emitter amplifier, voltage divider bias (CEVDB) circuit configuration

A common emitter is a type of electronic amplifier stage based on a bipolar transistor in series with a load element such as a resistor. The term "common emitter" refers to the fact that the emitter node of the transistor (indicated by an arrow symbol) is connected to a "common" power rail, typically the 0 volt reference or ground node. The collector node is connected to the output load, and the base node acts as input. Image File history File links Common emitter amplifier. ... The term amplifier as used in this article can mean either a circuit (or stage) using a single active device or a complete system such as a packaged audio hi-fi amplifier. ... The schematic symbols for pnp_ and npn_type BJTs. ... Resistor symbols (US and Japan) Resistor symbols (Europe, IEC) A pack of resistors A resistor is a two-terminal electrical or electronic component that resists an electric current by producing a voltage drop between its terminals in accordance with Ohms law. ... Josephson junction array chip developed by NIST as a standard volt. ... Ground symbols The term ground or earth usually means a common return path in electrical circuits. ...

Explanation of circuit

The electronic circuit diagram (right) shows a common emitter configuration with voltage divider bias (CEVDB). In the figure, the common emitter circuit comprises the load resistor R_C and NPN transistor with the output connected as shown; the other circuit elements are used for biasing the transistor and signal coupling/decoupling. The circuit diagram for a 4 bit TTL counter, a type of state machine A circuit diagram (also known as an electrical diagram or electronic schematic) is a pictorial representation of an electrical circuit. ... In electronics, a voltage divider or resistor divider or potential divider is a design technique used to create a voltage (Vout) which is proportional to another voltage (Vin). ... R1, R2 and RE provide bias in this common emitter amplifier (CEVDB) circuit configuration Biasing in electronics is the method of applying a predetermined voltage and current to the junction of a transistor to set the appropriate quiescent point. ... In electronics and telecommunication, coupling is the desirable or undesirable transfer of energy from one medium, such as a metallic wire or an optical fiber, to another medium, including fortuitous transfer. ...

The resistor R_E between the emitter node and the shared ground appears at first glance to contradict the strict definition of "common emitter", but the term is still appropriate here because, for all frequencies of interest, the capacitor C_E acts as a low impedance by decoupling the emitter to ground. The emitter resistor provides a form of negative feedback called emitter degeneration, which increases the stability and linearity of the amplifier, especially in response to temperature changes. Negative feedback is the process of feeding back to the input a part of a systems output, so as to reverse the direction of change of the output. ...

For the common emitter circuit on the right this is necessary to ensure the transistor is in the active mode and thus prevent it from acting as a rectifier which would cause clipping on the negative portion the input signal, resulting in a distorted output. Assorted transistors A transistor is a semiconductor device that uses a small amount of voltage or electrical current to control a larger change in voltage or current. ... AC, half-wave and full wave rectified signals A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. ...

The resistors R₁ and R₂ are chosen to ensure the base-emitter voltage is approximately 0.7 volts, which is the "on" voltage for a BJT transistor. These resistors, along with R_E, also determine the quiescent current flowing through the transistor and therefore its gain.

Application

Common emitter circuits are used to amplify weak voltage signals, such as the faint radio signals detected by an antenna. They are also used in a special analog circuit configuration known as a current mirror, where a single shared input is used to drive a set of identical transistors, each of whose current drive output will be nearly identical to each other, even if they are driving dissimilar output loads. A yagi antenna Most simply, an antenna is an electronic component designed to send or receive radio waves. ... A current mirror is a circuit designed to copy a current flowing through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. ...

Small-signal characteristics

(The parallel lines indicate components in parallel.) Parallel is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. ... Left: Series / Right: Parallel Arrows indicate direction of current. ...

Inherent voltage gain: In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ...

With C_E present or R_E = 0:

Without C_E and R_E > 0:

Transistors have widely varying transconductances (g_m), even among the same model, and affected strongly by temperature changes. Depending purely on the transconductance of the transistor to set the gain can have unpredictable effects. Emitter degeneration acts like negative feedback to minimize the effect this has on the overall gain of the amplifier. When R_E is included, if and , the above formula can be approximated as:

Input resistance: Transconductance, also known as mutual conductance, is a property of certain electronic components. ... Negative feedback is the process of feeding back to the input a part of a systems output, so as to reverse the direction of change of the output. ... The input impedance or load impedance of a circuit or electronic device is the impedance actually experienced by a signal which is connected to it. ...

With C_E present or R_E = 0:

Without C_E and R_E > 0:

Current gain:

Output resistance: The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ...

The variables not listed in the schematic are:

• g_m is the transconductance in siemens, calculated by , where:
  • is the collector bias current
  • is the thermal voltage, calculated from Boltzmann's constant, the charge on an electron, and the transistor temperature in kelvins. At room temperature this is about 25 mV (Google calculator).
• is the current gain at low frequencies (commonly called h_FE). This is a parameter specific to each transistor, and can be found on a datasheet.
• 

Transconductance, also known as mutual conductance, is a property of certain electronic components. ... The siemens (symbol: S) is the SI derived unit of electric conductance. ... Ludwig Boltzmann The Boltzmann constant (k or kB) is the physical constant relating temperature to energy. ... The Boltzmann constant (k or kB) is the physical constant relating temperature to energy. ... The elementary charge (symbol e or sometimes q) is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron. ... The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero—the lowest possible temperature where nothing could be colder and no heat energy remains in a substance—is defined as zero kelvin (0 K). ...

See also

Transistor amplifiers
Bipolar junction transistor: Common emitter • Common collector • Common base Field effect transistor: Common source • Common drain • Common gate Multiple transistors: Darlington transistor • Cascode • Parallel circuit

Assorted transistors A transistor is a semiconductor device that uses a small amount of voltage or electrical current to control a larger change in voltage or current. ... Generally, an amplifier is any device that uses a small amount of energy to control a larger amount of energy. ... Image File history File links BJT_symbol_NPN.svg File links The following pages link to this file: Bipolar junction transistor NPN ... A bipolar junction transistor (BJT) is a type of transistor. ... Typical common collector or emitter follower circuit. ... Common base amplifier In electronics, common base refers to a type of bipolar transistor circuit configuration in which a transistor is connected such that its base terminal is wired to the ground or common rail of the circuit (or can be considered so for AC signals). ... Large power N-channel field effect transistor The field-effect transistor (FET) is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel in a semiconductor material. ... Common source amplifier with input bias and capacitively coupled input and output. ... A common-drain (commonly called a source follower) amplifier is one of the common configurations of FET electronic amplifier. ... Common gate amplifier A common-gate amplifier is one of the common configurations of FET electronic amplifier. ... It has been suggested that this article or section be merged with darlington pair. ... A cascode is an arrangement of electronic devices where the output of a common emitter (or cathode) circuit is coupled directly to the input of a common base (or grid) circuit (usually with no intervening components). ... Left: Series  / Right: Parallel Arrows indicate direction of current flow. ...

External links

• Basic BJT Amplifier Configurations
• NPN Common Emitter Amplifier — HyperPhysics
• The Common Emitter Amplifier, Physics Lecture Notes, D.M. Gingrich, University of Alberta Department of Physics