Chapter 45

Basic concepts

45.1 Conductance

Ohm's law can be expressed in two forms:

  • The direct: V=R ⋅ I
  • The inverse: I=G ⋅ V

45.1.1 Linear device without constant term

For a linear device without constant term: 

I = G . V

G =I /V

45.1.2 Linear device with constant term

For a linear device with constant term:

I = G . V + I0

G = ( I - I0 ) / V = d I / d V ≠ I / V

45.1.3 Non linear device

For a non linear device:

Two conductance expressions can be defined:

  • The static conductance: G=IV
  • The dynamic conductance: g= dI dV

Traditionally, capital letters are used for static values and small letters are used for dynamic values.

Both the static and dynamic trans-conductance values depend on the operating point.

45.1.4 Transconductance

For some devices like bipolar and MOS transistors, current flowing into one pin depends on voltage between other pins. In such cases, conductance is "transferred" and is called transconductance.

Just like for conductance, for a non linear device, a static and a dynamic transconductance can be defined.