RF5125 Datasheet, PDF (5/12 Pages) RF Micro Devices – 3V TO 5V, 2.4GHz TO 2.5GHz LINEAR POWER AMPLIFIER

RF5125
Theory of Operation and Application Information
The RF5125 is a two-stage (2-stage) power amplifier (PA) with a minimum gain of 26dB (29dB typical) in the 2.4GHz to
2.5GHz Industrial, Scientific, and Medical (ISM) band. The RF5125 has a 50Ω internal input and interstage match. Only the
RF5125 output stage requires matching. The RF5125 is designed primarily for IEEE802.11g/n wireless local area network
(WLAN) applications where the available supply voltage and current are limited. This amplifier will operate to and below the
lowest expected voltage made available by a typical PC Card (PCMCIA or CardBus) slot in a laptop personal computer (PC). The
RF5125 maintains required linearity at decreased supply voltages.
The RF5125 operates from a single supply voltage of 3.0V_DC to 5.0V_DC to deliver specified performance. Power control is pro-
vided through two (2) bias control input pins (V_REG1 and V_REG2). For the best performance and lower current, there is a 68Ω
resistor placed as R2. In most applications these two (2) bias control input pins are connected together (before R1 and R2)
and employed as a single control input.
There is no required matching on the RF5125 input or interstage circuits. Only the RF5125 output stage requires matching
allowing the RF5125 to be implemented in applications requiring the fewest end product bill of materials (BOM) parts count
and lowest BOM cost. In most cases the capacitor used as part of the RF5125 output matching circuit is also employed to
accomplish DC-blocking. The RF5125PCBA evaluation board (available from RF Micro Devices, Inc. (RFMD)) is optimized for
3.3V_DC supply input.
For best results, the RF5125PCBA evaluation board circuit layout should be copied as closely as possible. In particular, the
RF5125PCBA evaluation board ground layout, ground vias, and output matching components and location should be copied
without deviation. Other PCB layout configurations may provide acceptable RF5125 performance; however, the end product
design process will be faster and manufacturing first time yield (FTY) better if the RF5125PCBA evaluation board design is fol-
lowed. RFMD provides RF5125PCBA design and Gerber files upon request.
Though not difficult to implement to achieve state-of the-art performance, the RF5125 is employed at frequencies greater than
2GHz, where care in circuit layout and component selection is advisable. Of primary concern with RF5125 PCB layout is the
selection and placement of output matching components (RF5125PCBA bill of materials (BOM) is available upon request).
High-Q (quality factor) capacitors and inductors are not required in every RF5125 based design; however, it is highly recom-
mended that the RF5125PCBA evaluation board BOM be followed exactly for all initial end product designs. Upon initial base-
line of RF5125 based PCB performance, less costly (Lower-Q) output matching circuit components may be substituted and
evaluated against the initial design performance. RFMD experience indicates that end product FTY improvements more than
offset the cost difference between “High-Q” and “Low-Q” components.
The output matching capacitor is C10 (or C11) which is connected between the RF5125 and the connector J2 (RFOUT) as
shown on the RF5125PCBA Evaluation Board Schematic. This capacitor is selected in value and positioned with reference to
the 50Ω transmission line. This matching capacitor is a single 2.2pF shunt capacitor whose placement depends on the supply
voltage applied. With the nominal 3.0V_DC to 3.6V_DC supply voltage, the distance of the transmission line should be 115mils
(this is marked as the placement for C10). For a supply voltage of 5V, the length of the transmission line should be 101mils
(which is marked as the placement for C11). Notice, only one of the shunt output capacitors is placed at one time (C10 or
C11). The transmission line length and thickness should be duplicated as closely as possible in any customer PCB layout. Due
to PCB material variation (e.g., FR4) and PCB manufacturing variations, the customer may benefit from small adjustments
made to the length of the transmission line when the RF5125PCBA evaluation board is duplicated to produce an end product
PCB design. The initial PCB layout should include exposed ground area near C10 or C11 to allow ease of RF5125 output circuit
optimization; Smith Chart-based design tools may be used to assist in determining the desired capacitor value and transmis-
sion line physical characteristics. Note that the use of a single capacitor output circuit match results in a more sensitive match
and slightly reduced RF5125 bandwidth. In this configuration, the RF5125 will exhibit sufficient output spectrum bandwidth to
meet IEEE802.11b/g requirements when matched properly.
Rev A23 DS070515
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