2-576
RF5117C
Rev A5 060517
Theory of Operation and Application Information
The RF5117C is a two-stage device with a nominal gain of 26dB in the 2.4GHz to 2.5GHz ISM band. The RF5117C is
designed primarily for IEEE802.11B/11G 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 PCMCIA slot in a lap-
top PC, and will maintain required linearity at decreased supply voltages.
The RF5117C requires only a single positive supply of 3.0V nominal (or greater) to operate to full specifications. Power
control is provided through two bias control input pins (VREG1 and VREG2), but in most applications these are tied
together and used as a single control input.
There is some external matching on the input and output of the part, thus allowing the part to be used in other applica-
tions outside the 2.4GHz to 2.5GHz ISM band (such as MMDS). Both the input and the output of the device need a
series DC-blocking capacitor. In some cases, a capacitor used as a matching component can also serve as the blocking
cap. The circuit used on the evaluation board is optimized for 3.0V nominal applications.
For best results, the PA circuit layout from the evaluation board should be copied as closely as possible, particularly the
ground layout and ground vias. Other configurations may also work, but the design process is much easier and quicker if
the layout is copied from the RF5117C evaluation board. Gerber files of our designs can be provided upon request.
The RF5117C is not a difficult part to implement, but care in circuit layout and component selection is always advisable
when designing circuits to operate at 2.5GHz. The most critical passive components in the circuit are the input, inter-
stage and output matching components (C1, C5, and C11). In these cases, high-Q capacitors suitable for RF applica-
tions are used on our evaluation board (a BOM is available on request). High-Q parts are not required in every design,
but it is very strongly recommended that the original design be implemented with the same or similar parts used on our
evaluation board. Then, less costly components can be substituted in their place, making it easy to test the impact of
cheaper components on performance. General RFMD experience has indicated that the slightly higher cost of better
quality passive components is more than offset by the significant improvements in production yields in large-volume
manufacturing. Using less costly components will typically result in a 1 to 2dB degradation in gain.
The interstage matching capacitor, C11, along with the combined inductance of the internal bond wire, the short length
of circuit board trace, and the parasitic inductance of this capacitor, tunes the peak of the small-signal gain response.
The trace length between C11 and pins 13 and 14 should be kept as short as possible.
In practice, VCC and the supply for the output stage bias will be tied to the same supply. It is important to isolate C11 from
other RF and low-frequency bypass capacitors on this supply line. This can be accomplished using a suitably long trans-
mission line which is RF shorted on the other end. Ideally the length of this line will be a quarter wavelength, but it only
needs to be long enough so that the effects of other supply bypass capacitors on the interstage match are minimized. If
board space is a concern, this isolation can also be accomplished with an RF choke inductor or ferrite bead. Additionally,
a higher-value capacitor than shown on the application schematic can be used if bypass capacitors must be closer. A
Smith Chart can be used to provide initial guidance for value selection and parts placement. Be aware of the self-reso-
nant frequency (SRF) of higher-valued capacitors. The SRF must be above the frequency of operation.
The output matching capacitor is C5, located 130mils from the IC (this distance should be duplicated as closely as pos-
sible). Due to variations in FR-4 characteristics and PCB manufacturer process variations, some benefit will be obtained
from small adjustments to these transmission line lengths when the evaluation board layout is duplicated on another
design. Prior to full rate manufacturing, the board layout of early prototypes should include some additional exposed
ground areas around C5 to optimize this part of the circuit. A Smith Chart can help determine the desired value and
transmission line length, which can be similarly adjusted on the board prior to production.
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