The absence of impedance tranformations in the input LC network means that the bandwidth is not constricted by the network’s loaded quality factor (Q) and the insertion loss is less sensitive to component Q. The value of ΓS offset from the chart center is unintentional and is due to the parasitic elements of the lumped components; the noise figure associated with ΓS is 0.28 dB. Any optimization of the noise match Γopt will prove to be insignificant, providing at most an improvement of 0.03 dB and not justifying the effort of performing the optimization.

The balanced LNA circuit has an output third-order intercept point (OIP3) of better than +42 dBm at optimum output match. The area encompassed by the +42-dBm constant linearity circle will require some impedance transformation since it is located away from the center of the chart (Fig. 5, right). The next lower constant linearity circle, at +40 dBm, includes the chart center. Since a +40-dBm OIP3 is adequate for this application, the output network was designed without impedance transformation (i.e., ΓL = 50 Ω). However, ΓL is shown slightly offset from the exact center becauseof component parasitic elements.

Balanced UHF LNA Simplifies Cell Towers, Table 2

The LNA’s PCB area is 40% smaller than the nearest competitor (Table 2). Moreover, its single-voltage supply requires fewer components to implement than most amplifier solutions for this application. To the author’s knowledge, this is the first dual-amplifier MMIC that integrates a shutdown function. Its noise figure is as good as competing designs using larger couplers and more expensive, shorter-gate-length semiconductor processes (Table 2).11-15 The amplifier’s experimental noise figure and gain are 0.5 dB and 18 dB, respectively, at 900 MHz (Fig. 6).

Balanced UHF LNA Simplifies Cell Towers, Fig. 6

The circuit model developed for this design has good predictive value because  the maximum errors for noise figure and gain are 0.1 and 0.2 dB, respectively, over a 1-GHz range. If not for losses from the input coupler and matching network, the balanced LNA’s noise figure would have been equal to its constituent amplifiers.16 Since the individual amplifiers’ noise figures are about 0.3 dB, it is assumed that the combined loss of the input coaxial connector and hybrid coupler is about 0.2 dB. This design can potentially serve multiple wireless communications standards (e.g., GSM and LTE) because its noise figure changes less than 0.1 dB from 600 to 1050 MHz. The noise figure increases abruptly outside this frequency range due to the input coupler’s characteristics.

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