Common emitter

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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 (for example, it may be tied to ground reference or a power supply rail), hence its name. The analogous field-effect transistor circuit is the common-source amplifier.

1 Emitter degeneration
2 Characteristics
- 2.1 Bandwidth
3 Applications
- 3.1 Radio
4 See also
5 References
6 External links

Emitter degeneration

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

Common-emitter amplifiers can have a very high gain which can vary widely from one transistor to the next. The gain is a strong function of both temperature and bias current, and so the actual gain is 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; there is high distortion if this limit is exceeded and the transistor ceases to behave like its small-signal model. One common way of alleviating these issues is with the use of negative feedback, which is usually implemented with emitter degeneration. Emitter degeneration refers to the addition of a small resistor (or any impedance) between the emitter and the common signal source (e.g., the ground reference or a power supply rail). This impedance $R E$ reduces the overall transconductance $G m = g m$ of the circuit by a factor of $g m R E + 1$ , which makes the voltage gain

$A_{\text{v}} \triangleq \frac{ v_{\text{out}} }{ v_{\text{in}} } = \frac{ -g_m R_{\text{C}} }{ g_m R_{\text{E}}+1 } \approx -\frac{ R_{\text{C}} }{ R_{\text{E}} } \qquad (\text{where} \quad g_m R_{\text{E}} \gg 1). \,$

So the voltage gain depends almost exclusively on the ratio of the resistors $R C / R E$ rather than the transistor's intrinsic and unpredictable characteristics. The distortion and stability characteristics of the circuit are thus improved at the expense of a reduction in gain.

Characteristics

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

	Definition	Expression
Current gain	$A_{\text{i}} \triangleq \frac{i_{\text{out}} }{ i_{\text{in}} } \,$	$\beta \,$
Voltage gain	$A_{\text{v}} \triangleq \frac{v_{\text{out}} }{ v_{\text{in}} } \,$	$\begin{matrix}-\frac{ \beta R_{\text{C}} }{ r_{\pi} + ( \beta +1 ) R_{\text{E}} }\end{matrix}\,$
Input impedance	$r_{\text{in}} \triangleq \frac{v_{\text{in}}}{i_{\text{in}}}\,$	$r_{\pi} +( \beta +1 ) R_{\text{E}}\,$
Output impedance	$r_{\text{out}} \triangleq \frac{v_{\text{out}}}{i_{\text{out}}}\,$	$R_{\text{C}}\,$

If the emitter degeneration resistor is not present, $R_{\text{E}} = 0\,\Omega$ . As expected, when $R_{\text{E}}\,$ is increased, the input impedance is increased and the voltage gain $A_{\text{v}}\,$ is reduced.

Bandwidth

The bandwidth of the common-emitter amplifier tends to be low due to high capacitance resulting from the Miller effect. The parasitic base-collector capacitance $C_{\text{CB}}\,$ appears like a larger parasitic capacitor $C_{\text{CB}} \times (1-A_{\text{v}})\,$ (where $A_{\text{v}}\,$ is negative) from the base to ground^[1]. This large capacitor greatly decreases the bandwidth of the amplifier as it makes the time constant of the parasitic input RC filter $r_{\text{s}} (1-A_{\text{V}}) C_{\text{CB}}\,$ where $r_{\text{s}}\,$ is the output impedance of the signal source connected to the ideal base.

The problem can be mitigated in several ways, like

Reduction of the voltage gain magnitude $\left|A_{\text{v}}\right|\,$ (e.g., by using emitter degeneration).
Reduction of the output impedance $r_{\text{s}}\,$ of the signal source connected to the base (e.g., by using an emitter follower or some other voltage follower).
Using a cascode configuration, which inserts a common-base amplifier with its emitter tied to the common-emitter transistor's collector; this configuration to holds the common-emitter transistor's collector voltage constant and removes the Miller effect.
Using a differential amplifier topology like an emitter follower driving a grounded-base amplifier; as long as the emitter follower is truly a common-collector amplifier, the Miller effect is removed.

The Miller effect negatively affects the performance of the common-source amplifier in the same way (and has similar solutions).

Applications

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.

References

^ Paul Horowitz and Winfield Hill (1989). The Art of Electronics (Second edition ed.), Cambridge University Press. p. pp. 102–104. ISBN 9780521370950.

External links

Basic BJT Amplifier Configurations
NPN Common Emitter Amplifier — HyperPhysics
ECE 327: Transistor Basics — Gives example common-emitter circuit with explanation.

Transistor amplifiers

	Bipolar junction transistor: Common emitter • Common collector • Common base Field-effect transistor: Common source • Common drain • Common gate Multiple transistors: Darlington pair • Sziklai pair • Cascode • Long-tailed pair