Radio Frequency Integrated Circuit Design - Webs

LNA Design
Results are shown in Table 6.3. Figure 6.29 shows a continuous curve of
IIP3 and shows the approximation of (6.85).
This example clearly shows how IP3 improves with degeneration resistance.
From the fundamental equations, it can also be seen that for larger IC and
hence larger IB , the improvement will be higher. It can also be seen from the
equation that it is possible to cancel the third-order term if R E = r e /2, which
in this example requires a degeneration of 12.5�. It can be seen that for lower
degeneration, k 3 is positive, resulting in gain expansion, while for larger values
Table 6.3
Calculations of IP3 Versus Degeneration Resistance
R E 0 5 10 15 20
A (peak) (mV) 1 1 1 1 1 mV
k 0 (mA) 1 1 1 1 1
k 1 (mA/V) 4 33.3 28.6 25 22.2
k 2 (A/V 2 ) 0.8 0.463 0.2915 0.1953 0.1372
k 3 (A/V 3 ) 10.667 2.572 0.3967 −0.2035 −0.3387
Fundamental (� A) 40 33.34 28.57 25.0 22.22
IM3 (nA) 8.0 1.929 0.297 −0.153 −0.254
IIP3 (dBm) −13.01 −7.62 −0.175 2.144 −0.581
Figure 6.29 Input IP3 as a function of degeneration resistance.
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