Common emitter

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Image:Common emitter.png

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.

The electronic circuit diagram (right) shows a common emitter configuration with voltage divider bias (CEVDB). In the figure, the common emitter circuit is comprised of load resistor RC and transistor NPN with the output connected as shown; the other circuit elements are used for biasing the transistor and signal coupling/decoupling. The resistor RE 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 CE 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.

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.

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Small-signal characteristics

(The parallel lines indicate components in parallel.)

Inherent voltage gain:

With CE:
<math>

-g_m (R_\mathrm{C} \| R_\mathrm{load})\, </math>

Without CE:
<math>

{-\beta_0 (R_\mathrm{C} \| R_\mathrm{load}) \over r_\pi + (1 + \beta_0)R_\mathrm{E}} </math>

Input resistance:

With CE:
<math>

R_1 \| R_2 \| r_\pi\, </math>

Without CE:
<math>

R_1 \| R_2 \| (r_\pi + (1 + \beta_0)R_\mathrm{E})\, </math>

Current gain:

<math>

A_\mathrm{vm} {r_\mathrm{in} \over R_\mathrm{load}} </math>

Output resistance:

<math>

R_\mathrm{C}\, </math>

The variables not listed in the schematic are:

  • gm is the transconductance in siemens, calculated by <math>g_m = I_\mathrm{C} / V_\mathrm{T}</math>, where:
  • <math>\beta_0 = I_\mathrm{C} / I_\mathrm{B}</math> is the current gain at low frequencies (commonly called hFE). This is a parameter specific to each transistor, and can be found on a datasheet.
  • <math>r_\pi = \beta_0 / g_m = V_\mathrm{T} / I_\mathrm{B}</math>
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Transistor amplifiers

One transistor:

Two transistors:

pt:Emissor comum lt:Bendras emiteris

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