Audio Engineering

Audio Amplifi ers 303

from localized thermal breakdown in the MOSFET is that the mobility of the electrons
(or holes) in the channel decreases as the temperature increases, which gives all FETs a
positive temperature coeffi cient of channel resistance.

Although it is possible to propose a mathematical relationship between gate voltage and
drain current, with MOSFETs as was done in the case of the JFET, the manufacturers
tend to manipulate the diffusion pattern and construction of the device to linearize its
operation, which leads to the type of performance (quoted for an actual device) shown
in Figure 9.21. However, as a general rule, the gfs of a MOSFET will increase with drain
current, and a forward transconductance (slope) of 10 S/A is quoted for an IRF140 at an
ID value of 15 A.

9.9 Power BJTs vs Power MOSFETs as Amplifi er Output Devices ..........................

Some rivalry appears to have arisen between audio amplifi er designers over the relative
merits of power BJTs, as compared with power MOSFETs. Predictably, this is a mixture
of advantages and drawbacks. Because of the much more elaborate construction of the
MOSFET, in which a multiplicity of parallel connected conducting channels is fabricated
to reduce the conducting “ on ” resistance, the chip size is larger and the device is several
times more expensive both to produce and to buy. The excellent HF characteristics of the
MOSFET, especially the N-channel V and U MOS types, can lead to unexpected forms

10

1

0.1

0.01

1.0 10 100

Collector voltage

(V)

Device voltage limit

Thermal dissipation limit

I amperesc

Current limit

Figure 9.20 : Power MOSFET SOAR limits.