In
electronics
The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
, a common-drain
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost th ...
, also known as a source follower, is one of three basic single-stage
field-effect transistor
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs ( JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs con ...
(FET) amplifier topologies, typically used as a
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge t ...
buffer. In this circuit (NMOS) the gate terminal of the transistor serves as the signal input, the source is the output, and the drain is ''common'' to both (input and output), hence its name. The analogous
bipolar junction transistor
A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipola ...
circuit is the
common-collector amplifier. This circuit is also commonly called a "stabilizer".
In addition, this circuit is used to transform
impedances
In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit.
Quantitatively, the impedance of a two-terminal circuit element is the ratio of the comp ...
. 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 driving stage and a high load (i.e. a low resistance) presents an infinite resistance (low load) to the driving stage—an advantage in coupling a voltage signal to a large load.
Characteristics
At low frequencies, the source follower pictured at right has the following
small-signal characteristics.
[Common Drain Amplifier or Source Follower](_blank)
��Circuit analysis, low frequency, high frequency, and impedance calculations.
The variable ''g
m'' that is not listed in the schematic is the
transconductance of the device (usually given in units of
siemens).
See also
*
Negative feedback amplifier
*
Buffer amplifier
A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another, with the aim of preventing the signal source from being affected by whatever currents (or voltages, for a cu ...
*
Two-port network
A two-port network (a kind of four-terminal network or quadripole) is an electrical network (circuit) or device with two ''pairs'' of terminals to connect to external circuits. Two terminals constitute a port if the currents applied to them satis ...
s
*
Hybrid-pi model
*
Common collector
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Common emitter
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Common base
In electronics, a common-base (also known as grounded-base) amplifier is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a current buffer or voltage amplifier.
In this circuit the emitter ...
*
Common source
*
Common gate
*
Emitter degeneration
References
Single-stage transistor amplifiers
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