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Module 3.3
Conductance
- After studying this section, you should be able to:
- Describe the property of Conductance (G)
- Describe the property of Mutual transconductance (gm)
The Opposite of Resistance
The Ohms Law formula for resistance is R = V/I. If this for R formula is inverted it would become R = I/V. This is still a useful formula, but NOT for resistance. Resistance is a property that, as it increases, reduces current flow. I/V therefore must give a unit that, as it increases also INCREASES current flow, exactly the opposite effect to resistance. This unit must be proportional to current. (Resistance is INVERSELY proportional to current).
Conductance
This property given by I/V is called CONDUCTANCE because the larger its value, the more a circuit conducts (passes more current). The property of Conductance is given the letter G and is measured in units of Siemens (S). As conductance is the opposite of resistance it can also be calculated as the RECIPROCAL of resistance.
Enter the resistance of a circuit (in Ohms) into a scientific calculator and simply press the reciprocal button (labelled 1/x or sometimes x −1) and you have Conductance in Siemens, note that the symbol for Siemens a capital S (small s is used for seconds). Conductance is not widely used in electronics calculations, resistance being generally a more useful property.
Transconductance
Conductance is used however in connection with Field Effect Transistors (FETs) used as amplifiers and with operational amplifier integrated circuits (Op Amp ICs). In these devices a change in output current is related to a change in input voltage by a ratio called the Transconductance or mutual Transconductance of the (amplifier) device.
Mutual Transconductance is given the symbol gm and gives an indication of the gain of a device (i.e. how much it amplifies a signal). The formula for gm is given below and relates a change (Δ) in output Current (Iout) to a change of input Voltage (Vin).
