Radio Frequency Integrated Circuit Design - Webs

390 Radio Frequency Integrated Circuit Design
Figure 10.43 Transistor with multiple stripes.
to increase current capability, it is possible to use multiple metal layers, for
example, metals 1, 2, and 3. The top metal is often thicker, resulting in a higher
total current capability. In this example, the original 1 mA of current per emitter
stripe might be increased to 4 mA/stripe for 1-�m-wide line. Another point
to keep in mind is that since current flows from collector to emitter, the current
density in the emitters is highest close to the external emitter contact, which
for Figure 10.43 is on the bottom.
10.16 Current Limits in Integrated Inductors
Integrated inductors as used for LNAs and oscillators are typically 10 or 20
�m wide. This means they can probably handle no more than 20 to 40 mA
of dc current, and maybe up to 80 mA or so of ac current. This obviously
limits the ability to do on-chip tuning or matching for power amplifiers.
10.17 Power Combining
For high power, it is possible to combine multiple transistors at the output as
shown in Figure 10.44. This distributes the heat and limits the current density
in each transistor (compared to a single super-huge transistor).
However, with many transistors, the base drive to the outside transistors
can be phase delayed compared to the shortest path, so it is important to keep
the line lengths equal, as illustrated in Figure 10.44. Note also that as with all
RF or microwave circuits, sharp bends are to be avoided. Line delay or phase
shift can be determined by considering that the wavelength of a 1-GHz sine