common source
A common-source (often abbreviated to CS) amplifier is one of the three possible amplifier circuit configurations that use a single field effect transistor (FET) as the active device. The name derives from the fact that the FET's source terminal is connected to neither the input nor the output path and hence is 'common'.
The CS amplifier is conceptually a voltage amplifier. The input voltage modulates the amount of current flowing through the FET, changing the voltage across the output resistance according to Ohm's law. Alternatively, the CS amplifier can be treated as a transconductance amplifier, with the input voltage modulating the current going to the load. However, the FET device's output resistance is not typically high enough for a decent voltage amplifier, or low enough for a reasonable transconductance amplifier. Another major drawback is the amplifier's limited high-frequency response. Therefore, in practice the output is routed through a common drain (CD) or common gate (CG) stage, which have more favorable output and frequency characteristics. Hybrid circuit arrangements are possible; an example is the cascode, which (in its FET incarnation) is a consolidated CS-CG amplifier.
Characteristics
At low frequencies, the common source (see Figure 1) has the following small signal characteristics. (The parallel lines indicate components in parallel.)
Current gain:
The variables not listed in the schematic are:
- gm is the transconductance in siemens
Bandwidth
The bandwidth of the common source amplifier tends to be low, due to high capacitance resulting from the Miller Effect. The gate-drain capacitance is effectively multiplied by the factor 1 - Av, thus increasing the total input capacitance and lowering the overall bandwidth.
The bandwidth is approximately given by the following expression:
![BW=f_{H_{3dB}}=\frac {1}{2\pi R_{source} [ C_{GS} + C_{GD}(1+g_m R_D)]}](http://content.answers.com/main/content/wp/en/math/e/5/e/e5ea5ebb93bea2a2c7c6a71d1555115e.png)
See also
| Transistor amplifiers | |
|---|---|
| Bipolar
junction transistor: Common emitter • Common collector • Field effect transistor: Common source • Common drain • Common gate Multiple transistors: Darlington pair • Sziklai pair • Cascode • Long-tailed pair |
 |
External links
- JFET Common Source Amplifier, Physics Lecture Notes, D.M. Gingrich, University of Alberta Department of Physics
- Common-Source Amplifier Stage
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
