7.2 Amplifers with Feedback

Chapter 7.2 Amplifers with Feedback

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 7.2 Amplifiers with Feedback 181Solution:Solving for the various components, it can be shown that low-frequency output resistance is 5.5W and high-frequency output impedance is 55W. The equivalent inductance is 0.2 nH, the zero frequency is 4.59 GHz, and the pole frequency is 45.9 GHz.7.2  Amplifiers with FeedbackThere are numerous ways to apply feedback in an amplifier and it would be almost a book in itself to discuss them all. Only a few of the common feedback techniques will be discussed here.7.2.1  Common-Emitter/Source with Series Feedback (Emitter/Source Degeneration)The two most common configurations for RF LNAs are the common-emitter con-figuration and the cascode configuration shown in Figure 7.8. In most applications, the cascode is preferred over the common-emitter or common-source topology be-cause it can be used up to higher frequencies (the extra transistor acts to reduce the Miller effect) and has superior reverse isolation (S12). However, the cascode also suffers from reduced linearity due to the stacking of two transistors, which reduces the available output voltage swing.Most common-emitter/source and cascode LNAs employ the use of degenera-tion (usually in the form of an inductor in narrowband applications) as shown in Figure 7.8. The purpose of degeneration is to provide a means to transform the real part of the impedance seen looking into the base to a higher impedance for match-ing purposes. This inductor also trades gain for linearity as the inductor is increased in size.Figure 7.8  Narrowband LNAs with inductive degeneration: (a) common emitter, (b) common source, (c) cascode common emitter, and (d) cascode common source.