Radio Frequency Integrated Circuit Design - Webs

158 Radio Frequency Integrated Circuit Design
Example 6.9 Simulation of Gain and Input and Output Impedance with Shunt
Feedback
Simulate a cascode amplifier as shown in Figure 6.13 with a load resistor of
R L = 50� and source resistance of R S = 50�. The transistors are in a 12-GHz
f T process with 15x transistors, with � = 100 and 5 mA for a transconductance
of g m = 200 mA/V. Compare gain and input and output impedance with
various values of feedback resistor R f .
Solution
The results are shown in Figure 6.14. It can be seen that noise figure is not
strongly affected if the feedback resistor is larger than about 500�. It can also
be seen that for this design, linearity seems to be limited by the output, which
here remains at a nearly constant OIP3 of about 14 dBm. IIP3 seems to vary
significantly, but this is mainly due to the change of gain. Specifically, gain
decreases by 4 dB and IIP3 improves by 5 dB, but OIP3 improves by only
about 1 dB as the feedback resistance is reduced from 1,500� to 500�.
In Section 6.8 we will conclude the chapter with a major example of a
broadband amplifier design, including considerations of noise and linearity.
Noise and linearity are topics that will be discussed in some of the following
sections.
6.3 Noise in Amplifiers
When the signal is first received by the radio, it can be quite weak and can be
in the presence of a great deal of interference. The LNA is the first part of the
Figure 6.13 Amplifier with shunt feedback.