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The target 17-dB gain can be achieved with either a cascade of two common source stages or one cascade stage. The latter arrangement was selected for its lower current—i.e., the cascade configuration’s series-connected transistors consume one-half the current of a cascade configuration. The cascode configuration is biased by connecting the upper gate to a resistor divider, while the lower gate is connected to active bias via off-chip inductors L1 and L2. On-chip inductors were not used in these positions since they cannot meet the low RF loss characteristics of off-chip inductors as needed to meet the target NF performance.

In addition to supplying bias, these inductors form highpass networks (in conjunction with capacitors C3 and C16), to roll off excessive low-frequency gain. These input networks need not perform impedance transformation because the ePHEMT device periphery and source inductance Ls have been optimized for impedance matching and low noise in the 2-to-4-GHz band. Likewise, internal prematching by the drain inductance, Ld, simplifies the output networks, L3-C9 and L4-C19.

The on-chip shutdown circuit consists of a transistor switch in series with the active bias. Shutdown is initiated by applying high logic (≥2 V) at Vsd1/2 to open the switch. Conversely, a low logic signal—i.e., Vsd1/2 ≤ 500 mV)—enables the amplifiers. The transition from normal operation to shutdown takes less than 32 ns if the large (≥ 0.1 μF) decoupling capacitors—C8, C22, C23, and C24—are omitted. However, these capacitors are generally recommended because they prevent low-frequency instability and dampen supply transients.

Compact LNA Drives 2.5-GHz Base Stations, Fig. 2

Although the monolithic integration of Lange couplers can achieve the smallest circuit dimensions, the resulting noise figure (e.g., about 7 dB,6)  is too high for TMA applications. For this reason, the signal splitting and combining functions in the current design were performed using commercially available surface-mount couplers, X1 and X2. These backward-wave couplers are fabricated on high-dielectric-constant ceramic substrates to achieve compact dimensions: They are about halfway in size between Lange and branch-line couplers. The 2.6-GHz versions of these couplers have enough bandwidth to straddle Fourth-Generation (4G) WiMAX and cellular Long-Term-Evolution (LTE) bands.

To minimize input loss, a larger (6.4 × 5.1 mm) coupler is used at the input port, while a smaller (2.0 × 1.3 mm) coupler is used at the output port to save space and cost. To ensure that a better than 18-dB input match can be consistently met in volume production, controls were instituted on the two most critical parameters: (a) correlating the amplifiers’ input match to |S11a - S11b| < 0.025 dB and (b) specifying an input coupler with better than 23-dB isolation.16  The output coupler is noncritical since the TMA output match requirement is more relaxed. 

Compact LNA Drives 2.5-GHz Base Stations, Table

The amplifier PCB consists of a 10-mil-thick layer of RO4350B™ printed-circuit-board (PCB) material from Rogers Corp. with an FR-4 layer added to increase the stack height to 1.6 mm (Fig. 2). RF test connections are made via edge-launched SMA receptacles and the measurements are referenced to these connectors. The occupied circuit area, which is populated mostly with 0402-sized passive components, is 450 × 450 mm. The occupied area can possibly be reduced by about 20% if the empty area between input coupler X1 and the input matching networks (L1 - C3) is removed. The table lists the bill of materials for the amplifier.

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