7.7 Low Voltage Topologies for LNAs and the Use of On-Chip Transformers

Chapter 7.7 Low Voltage Topologies for LNAs and the Use of On-Chip Transformers

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

  • 220 LNA DesignIt can be noted that the factor of 3.226 is equivalent to 10.3 dB. This is close to the 9.66 dB that vIP3 is higher than the 1-dB compression voltage v1dB; thus, vL is approximately equal to v1dB. Note that these voltages are all peak differential. Thus, the linearity of a CMOS differential pair can be adjusted with transistor size and bias current. In comparison, the bipolar differential pair has a fixed vIP3 of 4vT or about 100 mV, although it can be improved with resistive degeneration. Note also that the linearity of a differential pair is not equivalent to twice the linear-ity of a single-ended amplifier since the shape of the transfer curve is quite different and hence the power series expansion is quite different. 7.7  Low Voltage Topologies for LNAs and the Use of On-Chip Transformers Of the configurations described so far, the common-emitter amplifier would seem ideally suited to low voltage operation. However, if the improved properties of the cascode are required at lower voltage, then the topology must be modified slightly. This has led some designers to “fold” the cascode as shown in Figure 7.37(a) [6]. With the use of two additional LC tanks and one very large coupling capacitor, the cascode can now be operated down to a very low voltage. This approach does have drawbacks, however, as it uses two additional inductors, which will use a lot of die area. The other drawback present with any folding scheme is that both transistors can no longer reuse the current. Thus, this technique will use twice the current of an unfolded cascode, although it could be used at half the voltage, to result in a comparable power consumption.Figure 7.37  A folded cascode LNA with: (a) capacitive coupling and (b) inductive coupling.