RF5112 Datasheet, PDF (6/19 Pages) RF Micro Devices – 3V TO 5V, 2.4GHz TO 2.5GHz LINEAR POWER AMPLIFIER

RF5112
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
through two control pins (V_REG1 and V_REG2). For the best performance and lower current, there is a 68 resistor (not required
at 5V operation) placed in series with V_REG2 control line. If customer desires, V_REG1 and V_REG2 can be connected together prior
to the series resistor on V_REG2 (see applications schematic for details).
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.
The RF5112 had been primarily characterized with a V_REG voltage of 2.85V. However, the RF5112 will operate from a wide
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.
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