Sources
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Common drain
From Wikipedia, the free encyclopedia
A common-drain (commonly called a source follower) amplifier is one of the common configurations of FET electronic amplifier and is similar in operation to a BJT emitter follower (common collector circuit) in that the source voltage closely follows the voltage at the gate terminal.
Contents |
(The parallel lines indicate components in parallel.)
Inherent voltage gain:
The voltage gain is smaller than 1 (typical values are 0.7 to 0.9).
If 
, this approximates to:
- Vout = Vin
This is called a source follower.
As the gate current is ideally zero, the values of the resistors R1 and R2 can be made very high. Therefore the input resistance can be in the range of several hundreds of kiloohms.
Current gain:
The output resistance will be quite low; several tens or hundreds of ohms are typical.
The variables not listed in the schematic are:
- gm 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. The source will follow the drain, but the voltage offset between the two is more current density dependent than for a common collector.
Rs can be replaced by a current source.
| 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 Drain Amplifier, Physics Lecture Notes, D.M. Gingrich, University of Alberta Department of Physics