Common drain

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Basic N-channel JFET source follower circuit (neglecting biasing details).

In electronics, a common drain (also known as a source follower or voltage follower) amplifier is one of three basic single-stage field effect transistor (FET) amplifier topologies, typically used as a voltage buffer. In this circuit the gate terminal of the transistor serves as the input, the source the output and the drain is common to both, hence its name. An analogous circuit called the common collector is constructed using bipolar junction transistors.

In addition, this circuit is used to transform impedances. For example, the Thévenin resistance of a combination of a voltage follower driven by a voltage source with high Thévenin resistance is reduced to only the output resistance of the voltage follower, a small resistance. That resistance reduction makes the combination a more ideal voltage source. Conversely, a voltage follower inserted between a small load resistance and a driving stage presents an infinite load to the driving stage, an advantage in coupling a voltage signal to a small load.

[edit] Characteristics

At low frequencies, the source follower (see Figure 1) has the following small signal characteristics. ^[1] (The parallel lines indicate components in parallel.)

Voltage gain:

${A_\mathrm{v}} = {v_\mathrm{out} \over v_\mathrm{in}} = {g_m R_\mathrm{S} \over g_m R_\mathrm{S} + 1} \approx 1 \quad (g_m R_\mathrm{S} \gg 1)$

Current gain:

${A_\mathrm{i}} = \infty\,$

Input resistance:

$r_\mathrm{in} = \infty\,$

Output resistance:

$r_\mathrm{out} = R_\mathrm{S} \| {1 \over g_m} \approx {1 \over g_m} \quad (g_m R_S \gg 1)$

The variables not listed in the schematic are:

g_m is the transconductance in siemens

[edit] References

^ Common Drain Amplifier or Source Follower - Circuit analysis, low frequency, high frequency, and impedance calculations.

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v • d • e 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