Common emitter

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Figure 1: Basic NPN common-emitter circuit (neglecting biasing details).
Figure 1: Basic NPN common-emitter circuit (neglecting biasing details).

In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor (BJT) amplifier topologies, typically used as a voltage amplifier. 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, hence its name. An analogous circuit called the common source is constructed using field-effect transistors.

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[edit] Emitter degeneration

Figure 2: Adding an emitter resistor decreases gain, but increases linearity and stability
Figure 2: Adding an emitter resistor decreases gain, but increases linearity and stability

Common-emitter amplifiers generally have a very high gain which can vary widely from one transistor to the next, as it is a strong function of both temperature and bias current, making the actual gain somewhat unpredictable. Stability is another problem associated with such high gain circuits, due to any unintentional positive feedback that may be present. Other problems associated with the circuit are the low input dynamic range imposed by the small-signal limit and the high distortion resulting if this is exceeded. One common way of alleviating these issues is with the use of negative feedback, particularly with emitter degeneration. Emitter degeneration typically refers to the addition of a small resistor (or any impedance) between the emitter of the transistor and ground. The effect of this is to reduce the overall transconductance Gm = gm of the circuit by a factor of gmRE + 1, making the voltage gain depend more on the ratio of the resistors than the transistor's characteristics:

{A_\mathrm{v}} = {v_\mathrm{out} \over v_\mathrm{in}} = \frac{-g_m R_\mathrm{C}}{g_m R_\mathrm{E}+1} \approx -{R_\mathrm{C} \over R_\mathrm{E}} \quad (g_m R_\mathrm{E} \gg 1)

The distortion and stability characteristics of the circuit are thus improved, but at the expense of a reduction in gain.

[edit] Characteristics

At low frequencies and using a simplified hybrid-pi model, the following small signal characteristics can be derived.

Definition Expression
Current gain {A_\mathrm{i}} = {i_\mathrm{out} \over i_\mathrm{in}} \beta \
Voltage gain {A_\mathrm{v}} = {v_\mathrm{out} \over v_\mathrm{in}} \begin{matrix}- \frac { \beta R_\mathrm{C}} {r_{\pi} + ( \beta +1) R_\mathrm{E}} \end{matrix}
Input resistance r_\mathrm{in} = \frac{v_{in}}{i_{in}} r_{\pi} +( \beta +1) R_\mathrm{E} \
Output resistance r_\mathrm{out} = \frac{v_{out}}{i_{out}} R_\mathrm{C} \

If the emitter degeneration resistor is not present, RE = 0 Ω.

According to these formulas and in agreement with the previous discussion, when RE is increased the input resistance is increased and the gain is reduced.

[edit] Bandwidth

The bandwidth of the common emitter amplifier tends to be low, due to high capacitance resulting from the Miller effect. The base-collector capacitance is effectively multiplied by the factor 1 − Av, thus increasing the total input capacitance and lowering the overall bandwidth. The discussion of bandwidth parallels that in the article on the common source amplifier. This bandwidth problem is less severe with emitter degeneration, but a better fix is the cascode amplifier.

[edit] Applications

[edit] Radio

Common-emitter circuits are used to amplify weak voltage signals, such as the faint radio signals detected by an antenna. When used in radio frequency circuits, it is common to replace the load resistor with a tuned circuit. This is 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 amplifiers.

[edit] See also

v • d • e
Transistor amplifiers

[edit] External links