Transconductance

Template:Short description Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciprocal of resistance.

Transadmittance (or transfer admittance) is the AC equivalent of transconductance.

Definition

File:Image for Transconductance.svg

Model transconductance device

Transconductance is very often denoted as a conductance, g_m, with a subscript, m, for mutual. It is defined as follows:

$g_{\text{m}}=\frac{\mathrm{\Delta }{I}_{\text{out}}}{\mathrm{\Delta }{V}_{\text{in}}}$

For small signal alternating current, the definition is simpler:

$g_{\text{m}}=\frac{i_{\text{out}}}{v_{\text{in}}}$

The SI unit for transconductance is the siemens, with the symbol S, as in conductance.

Transresistance

Transresistance (for transfer resistance), also infrequently referred to as mutual resistance, is the dual of transconductance. It refers to the ratio between a change of the voltage at two output points and a related change of current through two input points, and is denotated as r_m:

$r_{\text{m}}=\frac{\mathrm{\Delta }{V}_{\text{out}}}{\mathrm{\Delta }{I}_{\text{in}}}$

The SI unit for transresistance is simply the ohm, as in resistance.

Transimpedance (or, transfer impedance) is the AC equivalent of transresistance, and is the dual of transadmittance.

Devices

Vacuum tubes

For vacuum tubes, transconductance is defined as the change in the plate (anode) current divided by the corresponding change in the grid/cathode voltage, with a constant plate (anode) to cathode voltage. Typical values of g_m for a small-signal vacuum tube are 1 to . It is one of the three characteristic constants of a vacuum tube, the other two being its gain Template:Mvar (mu) and plate resistance r_p or r_a. The Van der Bijl equation defines their relation as follows:

$g_{\mathrm{m}}=\frac{\mu }{r_{\mathrm{p}}}$

Field-effect transistors

Similarly, in field-effect transistors, and MOSFETs in particular, transconductance is the change in the drain current divided by the small change in the gate–source voltage with a constant drain–source voltage. Typical values of g_m for a small-signal field-effect transistor are .

Using the Shichman–Hodges model, the transconductance for the MOSFET can be expressed as (see Template:Slink)

$g_{\text{m}}=\frac{2I_{\text{D}}}{V_{\text{OV}}},$

where I_D is the DC drain current at the bias point, and V_OV is the overdrive voltage, which is the difference between the bias point gate–source voltage and the threshold voltage (i.e., V_OV ≡ V_GS – V_th).Template:Rp The overdrive voltage (sometimes known as the effective voltage) is customarily chosen at about 70–200 mV for the 65 nm process node (I_D ≈ 1.13 mA/μm × width) for a g_m of 11–32 mS/μm.Template:RpTemplate:Rp

Additionally, the transconductance for the junction FET is given by

$g_{\text{m}}=\frac{2I_{\text{DSS}}}{|V_{\text{P}}|}(1-\frac{V_{\text{GS}}}{V_{\text{P}}}),$

where V_P is the pinchoff voltage, and I_DSS is the maximum drain current.

Bipolar transistors

The g_m of bipolar small-signal transistors varies widely, being proportional to the collector current. It has a typical range of . The input voltage change is applied between the base/emitter and the output is the change in collector current flowing between the collector/emitter with a constant collector/emitter voltage.

The transconductance for the bipolar transistor can be expressed as

$g_{\mathrm{m}}=\frac{I_{\mathrm{C}}}{V_{\mathrm{T}}}$

where I_C is the DC collector current at the Q-point, and V_T is the thermal voltage, typically about at room temperature. For a typical current of , g_m ≈ . The input impedance is the [[Bipolar_junction_transistor#Transistor_parameters:_alpha_(α)_and_beta_(β)|current gain (Template:Mvar)]] divided by the transconductance.

The output (collector) conductance is determined by the Early voltage and is proportional to the collector current. For most transistors in linear operation it is well below .

Amplifiers

Transconductance amplifiers

A transconductance amplifier (g_m amplifier) puts out a current proportional to its input voltage. In network analysis, the transconductance amplifier is defined as a Template:Dfn (VCCS). These amplifiers are commonly seen installed in a cascode configuration, which improves the frequency response.

An ideal transconductance amplifier in a voltage follower configuration behaves at the output like a resistor of value 1/g_m, between a buffered copy of the input voltage and the output. If the follower is loaded by a single capacitor C, the voltage follower transfer function has a single pole with time constant C/g_m, or equivalently it behaves as a 1st-order low-pass filter with a bandwidth of g_m/2πC.

Operational transconductance amplifiers

Template:Main article An operational transconductance amplifier (OTA) is an integrated circuit which can function as a transconductance amplifier. These normally have an input to allow the transconductance to be controlled.

Transresistance amplifiers

Template:Main article A transresistance amplifier outputs a voltage proportional to its input current. The transresistance amplifier is often referred to as a transimpedance amplifier, especially by semiconductor manufacturers.

The term for a transresistance amplifier in network analysis is current controlled voltage source (CCVS).

A basic inverting transresistance amplifier can be built from an operational amplifier and a single resistor. Simply connect the resistor between the output and the inverting input of the operational amplifier and connect the non-inverting input to ground. The output voltage will then be proportional to the input current at the inverting input, decreasing with increasing input current and vice versa.

Specialist chip transresistance (transimpedance) amplifiers are widely used for amplifying the signal current from photo diodes at the receiving end of ultra high speed fibre optic links.

See also

• Transistor
• Vacuum tube
• Electronic amplifier
• Transimpedance amplifier
• Fontana bridge
• Operational transconductance amplifier
• MOSFET

External links

• Transconductance — SearchSMB.com Definitions
• Transconductance in audio amplifiers: article by David Wright of Pure Music [1]