Radio Frequency Integrated Circuit Design - Webs

394 Radio Frequency Integrated Circuit Design
Figure 10.50 Temperature compensation.
electrons from the lattice atoms. These secondary electrons in turn generate
more free carriers, resulting in avalanche multiplication. A measure of breakdown
is V CEO,max, which is the maximum allowable value of V CE with the base open
circuited. A typical value in a 3-V process might be 5V. However, under ac
conditions and with the base matched, swings of V CE past 2VCC can typically
be provided safely. In processes where this is not possible, it may be necessary
to drop the supply voltage, use cascode devices, and possibly to make use of
more complex biasing, which is adaptive or at least variable [9].
A complication is that many simulators are too simplistic and do not
model the effects of breakdown. Clearly, while better simulation models are
required, many designers depend on laboratory verifications, common sense,
and experience.
10.20 Packaging
How does one remove heat from a power amplifier? One possible mechanism
is thermal conduction through direct contact, for example, when the die is
mounted directly on a metal backing. Another mechanism is through metal
connections to the bond pads, for example, with wires to the package or directly
to the printed circuit board (called chip on board ). In flip-chip implementation,
thermal conduction is through the solder bumps to the printed circuit board.
10.21 Effects and Implications of Nonlinearity
Linearity of the PA is important with certain modulation schemes. For example,
filtered quadrature phase shift keying (QPSK) is often used largely because it can