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RF5117
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 RF5117 is a two-stage device with a nominal gain of 26dB in the 2.4GHz to 2.5GHz ISM band. The RF5117 is designed pri-
marily for IEEE802.11B/11G WiFi 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 laptop PC, and will maintain
required linearity at decreased supply voltages.
The RF5117 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 (V
REG1
and V
REG2
), 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 applications
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-block-
ing 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 RF5117 evaluation board. Gerber files of our designs can be provided upon request.
The RF5117 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, interstage and out-
put matching components (C1, C5, and C11). In these cases, high-Q capacitors suitable for RF applications 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 recom-
mended 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. Gen-
eral 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 cir-
cuit 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, V
CC
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 transmission 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 con-
cern, 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 pro-
vide initial guidance for value selection and parts placement. Be aware of the self-resonant 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 possible).
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.
