Radio Frequency Integrated Circuit Design - Webs

172 Radio Frequency Integrated Circuit Design
vo_ncs ≈ R S + Z�
1 + g m Z� incs ≈ re incs
(6.71)
Therefore, the collector shot noise current sees re , a low value, and output
voltage is low. Thus, the common-collector adds little noise to the signal except
through r b .
6.4 Linearity in Amplifiers
Nonlinearity analysis will follow the same basic principles as those discussed in
Chapter 2, with power series expansions and nonlinear terms present in the
amplifier. These will now be discussed in detail.
6.4.1 Exponential Nonlinearity in the Bipolar Transistor
In bipolar transistors, one of the most important nonlinearities present is the
basic exponential characteristic of the transistor itself, illustrated in Figure 6.25.
Source resistance improves linearity. As an extreme example, if the input
is a current source, R S =∞, then i c = �i b . This is as linear as � is. It can be
shown that a resistor in the emitter of value R E has the same effect as a source
or base resistor of value R E �. The transistor base has a bias applied to it and
an ac signal superimposed. Summing the voltages from ground to the base and
assuming that ie = i c ,
vs + VS = v be + VBE + R E (IC + ic ) (6.72)
where VBE and v be are the dc and ac voltages across the base emitter junction
of the transistor.
Figure 6.25 Bipolar common-emitter amplifier for linearity analysis.