Audio Engineering

196 Chapter 7

Devices of this kind, known as V-MOS and T-MOS constructions, are shown in Figure 7.28.

Although these were originally introduced for power output stages, the electrical
characteristics of such components are so good that these have been introduced, in
smaller power versions, specifi cally for use in small signal linear amplifi er stages. Their
major advantages over bipolar devices, having equivalent chip sizes and dissipation
ratings, are their high input impedance, their greater linearity, and their freedom from
“ hole storage ” effects if driven into saturation.

These qualities are increasingly attracting the attention of circuit designers working
in the audio fi eld, where there is a trend toward the design of amplifi ers having a very
high intrinsic linearity rather than relying on the use of negative feedback to linearize an
otherwise worse design.

7.10.2.3 Breakdown

A specifi c problem that arises in small signal MOSFET devices is that, because the gate-
source capacitance is very small, it is possible to induce breakdown of the insulating

Drain and substrate

(a)

N Current flow N

Source Gate metallisation Oxide layer

N

NN

PPN

Figure 7.28 : Power MOSFET constructions using (a) V and (b) T confi gurations. (Practical

devices will employ many such cells in parallel.)

Drain and substrate

(b)

Oxide layer

Polysilicon gate

Source metallisation

Source

Gate

N

N

P N N P