Audio Engineering

Preamplifi ers and Input Signals 197

layer, which destroys the device, as a result of transferred static electrical charges arising
from mishandling.

Although widely publicized and the source of much apprehension, this problem is
actually very rarely encountered in use, as small signal MOSFETs usually incorporate
protective zener diodes to prevent this eventuality, and power MOSFETs, where such
diodes may not be used because they may lead to inadvertent “ thyristor ” action, have
such a high gate-source capacitance that this problem does not normally arise.

In fact, when such power MOSFETs do fail, it is usually found to be because of circuit
design defects, which have either allowed excessive operating potentials to be applied
to the device, or have permitted inadvertent VHF oscillation, which has led to thermal
failure.

7.11 Noise Levels ..........................................................................................................

Improved manufacturing techniques have lessened the differences between the various
types of semiconductor devices in respect to intrinsic noise level. For most practical
purposes it can now be assumed that the characteristics of the device will be defi ned by
the thermal noise fi gure of the circuit impedances. This relationship is shown in the graph
of Figure 7.29.

bandwidth

200 kHz

20 kHz bandwidth2 kHz bandwidth 4 KT.δF.R

whereK 1.38 10 ^23

T absolute temperature

andδF  bandwidth

V

Circuit impedance (Ω)

1

0

100

200

300

10 100 1K 10 K

RMS noise voltage (nV)

Figure 7.29 : Thermal noise output as a function of circuit impedance.