Abstract
This application note presents the design of an LNA at 2.45 GHz for WLAN applications meeting 802.11b/g needs. It presents curves that show gain, noise figure, input and output return loss, and linearity.
The MAX2645 is a Silicon Germanium (SiGe) Low-Noise Amplifier (LNA) that features a 25dB gain step, shutdown mode, and adjustable IP3. The LNA has been optimized to improve the sensitivity of CMOS receivers for 2.4GHz, 802.11b and 802.11g applications. The device features +18.7dB of gain, a noise figure of 1.9dB, and an Input IP3 of +0.5dB in High-Gain Mode. In Low-Gain Mode, the LNA has -7.0dB of insertion loss and an Input IP3 of +15.3dB. Supply current is a low 8.9mA in High-Gain Mode, 2.7mA in Low-Gain Mode and typically 0.1uA in Shutdown Mode. The LNA also features an externally adjustable bias control, set with a single resistor, which allows the user to meet minimum linearity requirements while minimizing current consumption. Table 1 below summarizes MAX2645 performance. The schematic for optimizing the MAX2645 for 2.45GHz is represented in Figure 1. Figures 2-6 demonstrate High-Gain, and Low-Gain performance of the LNA versus frequency. For further information, consult the MAX2645 SiGe LNA and MAX2645 EV kit datasheets.
| Mode | Supply Current (mA) | Gain (dB) | Noise Figure (dB) | Input IP3 (dBm) | Input Return Loss (dB) | Output Return Loss (dB) |
| High-Gain | 8.9 | +18.7 | 1.9 | +0.5 | -8.3 | -15.2 |
| Low-Gain | 2.7 | -7.0 | 14.1 | +15.3 | -11.3 | -8.5 |
(f = 2.45GHz, VCC = 3.0V, RBIAS = 20kΩ)
Figure 1. MAX2645 SiGe LNA 2.45GHz LNA schematic.
Figure 2. MAX2645 Gain versus frequency (VCC = 3.0V, RBIAS = 20kΩ).
Figure 3. MAX2645 Noise figure versus frequency (VCC = 3.0V, RBIAS = 20kΩ).
Figure 4. MAX2645 Input IP3 versus frequency (VCC = 3.0V, RBIAS = 20kΩ).
Figure 5. MAX2645 Input Return Loss Versus Frequency (VCC = 3.0V, RBIAS = 20kΩ).
Figure 6. MAX2645 Output return loss (VCC = 3.0V, RBIAS = 20kΩ).
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