Common emitter

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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.

1 Explanation of circuit
2 Application
3 Small-signal characteristics
4 See also
5 External links

[edit] 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 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.

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 of the input signal, resulting in a distorted output.

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.

[edit] 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.

[edit] Small-signal characteristics

(The parallel lines indicate components in parallel.)

Inherent voltage gain:

With C_E present or R_E = 0:

${V_\mathrm{out} \over V_\mathrm{in}} = -g_m (R_\mathrm{C} \| R_\mathrm{load})\,$

Without C_E and R_E > 0:

${V_\mathrm{out} \over V_\mathrm{in}} = {-\beta_0 (R_\mathrm{C} \| R_\mathrm{load}) \over r_\pi + (1 + \beta_0)R_\mathrm{E}}$

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 $g_m R_E \gg 1$ and $R_\mathrm{load} \gg R_\mathrm{C}$ , the above formula can be approximated as:

${V_\mathrm{out} \over V_\mathrm{in}} = -{R_\mathrm{C} \over R_\mathrm{E}}$

Input resistance:

With C_E present or R_E = 0:

$r_\mathrm{in} = R_1 \| R_2 \| r_\pi\,$

Without C_E and R_E > 0:

$r_\mathrm{in} = R_1 \| R_2 \| (r_\pi + (1 + \beta_0)R_\mathrm{E})\,$

Current gain:

$A_\mathrm{vm} {r_\mathrm{in} \over R_\mathrm{load}}$

Output resistance:

$r_\mathrm{out} = R_\mathrm{C}\,$

The variables not listed in the schematic are:

g_m is the transconductance in siemens, calculated by , where:
- $I_\mathrm{C} \,$ is the collector bias current
- $V_\mathrm{T} = kT / q \,$ 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 26 mV (Google calculator).
$\beta_0 = I_\mathrm{C} / I_\mathrm{B} \,$ 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.
$r_\pi = \beta_0 / g_m = V_\mathrm{T} / I_\mathrm{B} \,$

[edit] 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