+23dBm 37% Efficiency PA Pre-driver for 2.4GHz WLAN
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This article presents the trade-off design and measured performance of the PA and PA drivers MAX2644 and MAX2242 for FHSS devices in WLAN applications. Bias, RF board layout, and matching are adjusted to improve performance at 2450MHz. Output power can reach +23dBm at -12dBm input power. Supply current is 185mA at 3V supply.
There are many transceivers optimized for 2.4GHz FHSS (frequency hopping spread spectrum) applications such as Bluetooth, HomeRF, and DECT (digital European cordless telephone). Most of these ICs have very limited transmit power, so they are limited to very short distances. Many PC card manufacturers, looking for ways to differentiate their products from the competition, are adding external PAs (power amplifiers) to their products to extend the transmission range. Bluetooth and HomeRF have a maximum output power of +20dBm (100mW) at the antenna, while 2.4GHz DECT can transmit +24dBm (250mW). The PAs in these applications typically need to provide +23dBm to +27dBm to overcome the 3dB loss introduced by the RF switch and duplexer between the PA and the antenna (see Figure 1). To extend battery life, Bluetooth and DECT use GFSK (Gaussian frequency shift keying) modulation while HomeRF uses two-level and four-level FSK modulation, which allow the PA to operate in saturation and therefore have higher power efficiency than linear power amplifiers. 
Figure 1. 2.4 GHz FHSS WLAN transceiver for Bluetooth, HomeRF, and DECT. This application note demonstrates the performance of the MAX2242 2.4GHz PA in a CSP (chip-scale package) (1.5mm x 2.0mm) configuration with an integrated power detector and 0.5μA of power consumption in shutdown mode (see Figure 2). The MAX2644 2.4GHz LNA is also presented as the pre-driver for the PA. The MAX2242 PA was originally designed for 2.4GHz IEEE802.11b DSSS (direct-sequence spread spectrum) applications. In DSSS applications, this PA can provide +22.5dBm of linear output power with an ACPR of -33dBc at 310mA of supply current from a 3.3V supply. In this application, the saturated output power is +26.5dBm, which is ideal for 2.4GHz DECT applications. The MAX2242 uses external resistors to set the bias current of the device, which essentially allows the performance of the device to be tuned to meet the specific application. The purpose of designing +23dBm saturation power is to provide +20dBm output power at the antenna even if there is about 3dB loss between the PA and the antenna. Because many existing FHSS transceiver ICs can only provide -7dBm to -13dBm output power, a PA pre-driver is required to provide the necessary input power to enable the PA to enter saturation. The MAX2644 with 17dB small signal gain and +4dBm output P1dB compression point is an ideal candidate. 
Figure 2. Typical application circuit for the MAX2242. Several changes were made to the existing MAX2242 EV kit circuit and performance. The existing circuit required some changes to achieve the desired performance. The external bias resistors were changed to 91k  . The shunt output capacitor, C2, was changed from 1.8pF to 2.2pF and moved slightly further from the device output (5.5 scale marks on the EV kit design, 230mils from the output pull-up inductor). The interstage RF bypass capacitor, C8, was moved further in (between the pads of C7 and C8, 70mils from pad A2), and C10 was moved outward (135mils from pad A4). Moving these capacitors helped increase the interstage gain, which was reduced in linear mode to improve the device's ACPR performance. See Figure 3 for component placement and refer to the MAX2242 EV kit Quick Look data sheet for help with board design. 
Figure 3. MAX2242 component placement guide for +23dBm output. The MAX2242 was tuned to provide +23dBm saturated output power at room temperature (TA = +25°C) with a 3.0V supply and 175mA supply current (37% power added efficiency, PAE). Output power, supply current, gain, and PAE were measured as a function of input power, supply voltage, and frequency (see Figures 4 to 7). An interesting feature of this PA is that when there is no RF power input, the supply current drops to 29mA, while other PAs in its class maintain bias currents in excess of 150mA (see Figure 8). Finally, the harmonic content of the PA was evaluated using a worst-case stimulus 35dB below the carrier. 
Figure 4. MAX2242 PA output power and supply current versus input power. 
Figure 5. MAX2242 PA gain vs. input power. 
Figure 6. MAX2242 PA power-added efficiency (PAE) versus input power. 
Figure 7. MAX2242 PA output power vs. frequency. 
Figure 8. MAX2242 PA supply current with no RF input power. The MAX2644 2.4GHz LNA was chosen as the PA pre-driver due to its +4dBm output P1dB compression point, 17dB small-signal gain, low current of 8mA, and integrated output matching network. The MAX2644 EV kit can be used directly without any modification and is available from stock. In this application, the MAX2644 output power and supply current are measured as a function of input power and supply voltage (see Figures 9 and 10). When driven with an input power of -13dBm to -7dBm, the MAX2644 provides an output power of 3dBm to 6dBm at 3V supply and 2.45GHz. 
Figure 9. MAX2644 PA predriver output power vs. input power. 
Figure 10. MAX2644 supply current vs. input power. The cascaded performance of the MAX2644 and MAX2242 was tested to determine the output power range that these transmitter ICs can achieve. The cascaded combination provides +23dBm output at input powers of -12dBm to -5dBm, with the output dropping to +22.9dBm at -13dBm, the worst-case expected input power (see Figure 11). The supply current is approximately 185mA at 3V (see Figure 12). The output power is fairly constant over frequency, from +23.3dBm at 2.40GHz to +22.8dBm at 2.48GHz (see Figure 13). 
Figure 11. Cascaded performance of the MAX2644 and MAX2242. 
Figure 12. Total supply current vs. input power for the MAX2644 and MAX2242. 
Figure 13. Cascaded output power of the MAX2644 and MAX2242 versus frequency. The measured performance shows that the MAX2644 and MAX2242 are perfect choices for PA and PA driver stages in any low-cost, low-current FHSS wireless design that requires minimal board area. The MAX2644 was selected as the LNA for the receive path, with its 17dB gain, 2.0dB noise figure at 2.45GHz, and integrated output matching, which can help further reduce costs and improve the dynamic range of the system.
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