Radio Frequency Integrated Circuit Design - Webs

152 Radio Frequency Integrated Circuit Design
6.2 Amplifiers with Feedback
There are numerous ways to apply feedback in an amplifier, and it would be
almost a book in itself to discuss them all. Only a few of the common feedback
techniques will be discussed here.
6.2.1 Common-Emitter with Series Feedback (Emitter Degeneration)
The two most common configurations for RF LNAs are the common-emitter
configuration and the cascode configuration shown in Figure 6.9. In most
applications, the cascode is preferred over the common-emitter topology because
it can be used at higher frequencies (the extra transistor acts to reduce the Miller
effect) and has superior reverse isolation (S12). However, the cascode also suffers
from reduced linearity due to the stacking of two transistors, which reduces the
available output swing.
Most common-emitter and cascode LNAs employ the use of degeneration
(usually in the form of an inductor in narrowband applications) as shown in
Figure 6.9. The purpose of degeneration is to provide a means to transform
the real part of the impedance seen looking into the base to a higher impedance
for matching purposes. This inductor also trades gain for linearity as the inductor
is increased in size.
Figure 6.9 Narrowband common-emitter and cascode LNAs with inductive degeneration.