Common emitter
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Common_emitter.png
In this graphic the common emitter circuit is comprised of load resistor RC and transistor NPN with the output connected as shown; the other circuit elements are incidental. The imposition of resistor RE between the emitter node and the shared ground might appear to contradict the strict definition of "common emitter", but the term is still appropriate here because an electrical engineer would conceptually recognize this circuit as a refinement of a simple common emitter circuit
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. Common emitter circuits are notable because, within certain limits, the output voltage Vout is proportional to the input voltage Vin. 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 shown at right shows one use of a common emitter configuration (CEVDB).
Common emitter circuits are used to amplify weak voltage signals, such as the faint radio signals detected by an antenna. Common emitter circuits 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.
Alternative simple transistor amplifier circuits are also possible - see common collector and common base.
Characteristics
Inherent voltage gain:
- With CE:
- <math>
-g_m (R_C || R_{load}) <math>
- Without CE:
- <math>
{-\beta_0 (R_C || R_{load}) \over r_\pi + (1 + \beta_0)R_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_E) <math>
Current gain:
- <math>
A_{vm} {r_{in} \over R_{load}} <math>
Output resistance:
- <math>
R_C <math>