Radio Frequency Integrated Circuit Design - Webs

LNA Design
Figure 6.14 Sample plots using shunt feedback.
radio to process the signal, and it is therefore essential that it amplify the signal
while adding a minimal amount of additional noise to it. Thus, one of the
most important considerations when designing an LNA is the amount of noise
present in the circuit. The following sections discuss this important topic.
6.3.1 Input-Referred Noise Model of the Bipolar Transistor
Note that the following two sections contain many equations, which may be
tedious for some readers. Reader discretion is advised. Those who find equations
disturbing can choose to go directly to Section 6.3.3.
In Chapter 2, we made use of an idealized model for an amplifier with
two noise sources at the input. If the model is to be applied to an actual LNA,
then all the noise sources must be written in terms of these two input-referred
noise sources, as shown in Figure 6.15. Starting with the model shown in Figure
6.15(a), and assuming that the emitter is grounded with base input and collector
output, the model may be determined with some analysis.
When the input is shorted in Figure 6.15(b), then vn is the only source
of noise in the model, and then the output noise current ion_tot would be
(assuming that r b is small enough to have no effect on the gain)
i 2 on_tot = v 2 n g 2 m
159
(6.41)
If instead the actual noise sources in the model are used as in Figure
6.15(a), then the output noise current can also be found. In this case, the effect