Radio Frequency Integrated Circuit Design - Webs

170 Radio Frequency Integrated Circuit Design
vno, I B ≈ √ 2qIC
� Z eq g m R L (6.67)
Note that this output voltage is proportional to the collector current raised
to the power of 3/2. Therefore, to improve the noise figure due to base shot
noise, we decrease the current, because the signal-to-noise ratio (in voltage
terms) is inversely proportional to the square root of the collector current.
Therefore, at low currents, collector shot noise will dominate and noise
figure will improve with increasing current. However, the effect of base shot
noise also increases and will eventually dominate. Thus, there will be some
optimum level to which the collector current can be increased, beyond which
the noise figure will start to degrade again. Note that this simple analysis ignores
the fact that r b increases at lower currents, so, in practice, thermal noise due
to r b is more important at low currents than is indicated by this analysis.
6.3.5 Effect of the Cascode on Noise Figure
As discussed in Section 6.1.3, the cascode transistor is a common-base amplifier
with current gain close to 1. By Kirchoff’s current law (KCL) of the dotted box
in Figure 6.23, ic2 = ie2 − i b2 ≈ ie2. Thus, the cascode transistor is forced to
pass the current of the driver on to the output. This includes signal and noise
current. Thus, to a first order, the cascode can have no effect on the noise
figure of the amplifier. However, in reality it will add some noise to the system.
For this reason, the cascode LNA can never be as low noise as a commonemitter
amplifier.
Figure 6.23 A cascode LNA showing noise sources.