Basic Electronics (3110016)

Transconductance

• Transconductance is an expression of the performance of a field-effect transistor (FET). Consider Fig. - 1.
• In general, the larger the transconductance figure for a device, the greater the gain(amplification) it is capable of delivering, when all other factors are held constant.
• Formally, For a FET, transconductance is the ratio of the change in drain current to the change in gate voltage over a defined, arbitrarily small interval on the drain-current-versus-gate-voltage curve.
• The symbol for transconductance is gm. The unit is the siemens.
• If $\mathrm{dI}$ represents a change in drain current caused by a small change in gate voltage $\mathrm{dE}$, then the transconductance is approximately : $\mathrm{gm} = \raisebox{1ex}{$\mathrm{dI}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{dE}$}\right.$ .

Switching in FET

• FET can be used as a switch by operating it in two regions, they are cutoff and saturation region. Consider Fig. - 2.
• When the VGS is zero the FET operates in saturation region and maximum current flows through it. Hence it is like a fully switched ON condition.
• Similarly, when the VGS applied is more negative than the pinch off voltage, FET operates in cutoff region and doesn’t allow any current flow through the device. Hence FET is in fully OFF condition.