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Common source amplifier with input bias (R₁, R₂) and capacitively coupled input and output. R_s and C_s are typically taken to be equal to 0, i.e., the source is grounded. Otherwise the presence of R_s is known as source degeneration. This circuit arrangement would be typical of an implementation using discrete components.

A common-source (often abbreviated to CS) amplifier is one of the possible amplifier circuit configurations that make use of a field effect transistor (FET). The name derives from the fact that the FET source is connected to neither the input nor the output path and hence is 'common'.

Out of the three FET amplifier circuits — common source, common drain, and common gate — the CS amplifier is the only one that can provide a gain greater than one. However it does not typically have high enough input resistance to be a decent voltage amplifier, or low enough output resistance to be a decent transconductance amplifier. Another major drawback is its limited high-frequency response. Therefore in practice the output is routed through a CD or 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.

(The parallel lines indicate components in parallel.)

Inherent voltage gain:

With C_S present at high frequency (1/ωC_S ~ 0) or R_S = 0:

Without C_S and R_S > 0:

Transistors have widely varying transconductances (g_m); manufacturers typically guarantee a range of values. Furthermore, transconductance is affected strongly by temperature changes. Depending solely on the transconductance of the transistor to set the gain can have unpredictable effects. Transconductance is also current dependent. As the input goes up and down the current in the transistor increases and decreases changing the transconductance of the transistor and thus the gain of the amplifier. This is referred to as distortion and can be avoided by using source degeneration. Source degeneration applies negative feedback to minimize the effect of changes in g_m on the overall gain of the amplifier. When R_S is included, if and , the above formula can be approximated as:

Input resistance:

Current gain:

Output resistance:

The variables not listed in the schematic are:

• g_m is the transconductance in siemens

This circuit can also be DC-coupled, in which case wires replace the capacitors and R₁ and R₂ can be removed. The bias point is set by the previous circuit and this circuit provides the bias for the next. In this case the input resistance is infinite and the input impedance comes from the parasitic capacitances from gate to drain and gate to source.

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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

• JFET Common Source Amplifier, Physics Lecture Notes, D.M. Gingrich, University of Alberta Department of Physics
• Common-Source Amplifier Stage