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-gate

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 the v ...

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 contro ...

(FET) amplifier topologies, typically used as a

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

buffer Buffer may refer to: Science * Buffer gas, an inert or nonflammable gas * Buffer solution, a solution used to prevent changes in pH * Buffering agent, the weak acid or base in a buffer solution * Lysis buffer, in cell biology * Metal ion buffer * ...

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 to m ...

. In this circuit, the source terminal of the transistor serves as the input, the drain is the output, and the gate is connected to ground, or "common," 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 bipolar ...

circuit is the common-base amplifier.

Applications

This configuration is used less often than the

common source In electronics, a common-source amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage or transconductance amplifier. The easiest way to tell if a FET is common source, comm ...

source follower In electronics, a common-drain amplifier, also known as a source follower, is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage buffer. In this circuit (NMOS) the gate terminal of the t ...

. It is useful in, for example, CMOS RF receivers, especially when operating near the frequency limitations of the FETs; it is desirable because of the ease of

impedance matching In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize signal ...

and potentially has lower

noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...

. Gray and Meyer provide a general reference for this circuit.

Low-frequency characteristics

At low frequencies and under

small-signal Small-signal modeling is a common analysis technique in electronics engineering used to approximate the behavior of electronic circuits containing nonlinear devices with linear equations. It is applicable to electronic circuits in which the AC si ...

conditions, the circuit in Figure 1 can be represented by that in Figure 2, where the

hybrid-pi model The hybrid-pi model is a popular circuit model used for analyzing the small signal behavior of bipolar junction and field effect transistors. Sometimes it is also called Giacoletto model because it was introduced by L.J. Giacoletto in 1969. The m ...

for the MOSFET has been employed. Common gate output resistance

The amplifier characteristics are summarized below in Table 1. The approximate expressions use the assumptions (usually accurate) ''r_O'' >> ''R_L'' and ''g_mr_O'' >> 1. In general, the overall voltage/current gain may be substantially less than the open/short circuit gains listed above (depending on the source and load resistances) due to the

loading effect In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of distributing the inp ...

Closed circuit voltage gain

Taking input and output loading into consideration, the closed circuit voltage gain (that is, the gain with load ''R_L'' and source with resistance ''R_S'' both attached) of the common gate can be written as: :

\approx \begin \frac   \end

, which has the simple limiting forms :

A_\mathrm = \begin \frac \end \ \ \mathrm \ \ A_\mathrm = g_m R_L

, depending upon whether ''g_mR_S'' is much larger or much smaller than one. In the first case the circuit acts as a current follower, as understood as follows: for ''R_S'' >> 1/''g_m'' the voltage source can be replaced by its

Norton equivalent In direct-current circuit theory, Norton's theorem, also called the Mayer–Norton theorem, is a simplification that can be applied to networks made of linear time-invariant resistances, voltage sources, and current sources. At a pair of te ...

with Norton current ''v_Thév / R_S'' and parallel Norton resistance ''R_S''. Because the amplifier input resistance is small, the driver delivers by

current division Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

a current ''v_Thév / R_S'' to the amplifier. The current gain is unity, so the same current is delivered to the output load ''R_L'', producing by Ohm's law an output voltage ''v_out = v_ThévR_L / R_S'', that is, the first form of the voltage gain above. In the second case ''R_S'' << 1/''g_m'' and the Thévenin representation of the source is useful, producing the second form for the gain, typical of voltage amplifiers. Because the input impedance of the common-gate amplifier is very low, the

cascode The cascode is a two-stage amplifier that consists of a common-emitter stage feeding into a common-base stage. Compared to a single amplifier stage, this combination may have one or more of the following characteristics: higher input–output ...

amplifier often is used instead. The cascode places a

common-source In electronics, a common-source amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage or transconductance amplifier. The easiest way to tell if a FET is common source, com ...

amplifier between the voltage driver and the common-gate circuit to permit voltage amplification using a driver with ''R_S >> 1/g_m''.

References

External links

A 24GHz CMOS Front-end
{{Transistor amplifiers Single-stage transistor amplifiers

Applications

Low-frequency characteristics

Closed circuit voltage gain

See also

References

External links