6 of 19

RF5112

DS110617

7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical

support, contact RFMD at (+1) 336-678-5570 or sales-support@rfmd.com.

Theory of Operation and Application Information

The RF5112 is a two-stage Power Amplifier designed primarily for IEEE802.11g/n WiFi applications where the available supply

voltage and current are limited. This PA has a minimum gain of 28dB (29.5dB typical) in the 2.4GHz to 2.5GHz ISM band. The

RF5112 has an integrated input and interstage match to 50

. Only the output stage requires matching. This amplifier will

operate to and below the lowest expected voltage made available by a typical CardBus or PCMCIA card slot in a laptop personal

computer.

The RF5112 operates from a single supply voltage of 3.0V to 5.0V to deliver specified performance. Power control is provided

The output match of the RF5112 provides flexibility to optimize performance on the customer board using minimum compo-

nent count and the lowest cost bill of materials (BOM). The output match topology follows that of a low pass network and it

incorporates a series capacitor as the last component which can also serve as a DC block. Depending on the voltage of opera-

tion, the output match will require either one or two shunt capacitors to optimize the return loss and bandwidth. In the case of

a 3.3V operation, only one capacitor is required while the 5V operation requires two. The value of the shunt tuning capacitor in

the 3.3V case is 2.2pF and its placement is approximately 112mils from the package RF output pin. This location is marked

with the reference designator C10 on the evaluation board. For the 5V operation, the first tuning capacitor value is 1.5pF and

its placement approximately 90mils from the package (reference designator C10). The second tuning cap value for the 5V

operation is 1pF and its placement is approximately 200mils from the package (C11 reference designator). DC bias for the last

stage is provided through an RF Choke connected directly at the node between the transistor's collector and output match.

This inductor plays a small role on the output match performance so its value must be carefully chosen as to not detune the

circuit or lower its Q. RFMD recommends to use a high-Q inductor with a range in value between 5.6nH to 7.5nH.

PCB layout and material will affect optimum value and placement for these tuning capacitors. For fastest implementation and

best results when designing with the RF5112, RFMD recommends that the evaluation board be copied as close as possible in

particular the grounds and distance of components from the package pins (refer to schematic for additional details). The initial

PCB layout should include exposed ground area near the shunt tuning capacitors to allow fine tuning of the output match.

Smith Chart-based design tools may be used to assist in determining the desired capacitor value and transmission line physi-

cal characteristics. Note that the use of a single capacitor output circuit match results in a more sensitive match and slightly

reduced RF5112 bandwidth. In this configuration, the RF5112 will exhibit sufficient output spectrum bandwidth to meet

IEEE802.11b/g requirements when matched properly.

Upon request, RFMD can provide Gerber files for the evaluation board and BOM. High-Q tuning components are not required in

every RF5112 based design. However, it is a good practice and highly recommended to start the initial tune with high-Q compo-

nents then substitute with standard parts and compare against the initial performance. RFMD experience indicates that yield

improvements offset the cost difference between "High-Q" and "Low-Q" components.

range of bias control voltages and within a wide range of frequencies (typically 1800MHz to 2800MHz). If a bias control volt-

age other than 2.85V is preferred or if a different frequency range (other than 2.4GHz to 2.5GHz) is desired, please contact

RFMD Sales or Applications Engineering for assistance.