RF3108PCBA datasheet(5/10 Pages) RFMD

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RF3108

Rev A3 010702

Theory of Operation and Application Information

The RF3108 is a triple-band, GSM/DCS/PCS power

amplifier with two separate RF inputs and outputs that

are internally matched to 50

Ω. Pins 2 and 14 of the

device provide the RF input and output for the DCS/

PCS band, which is optimized for performance

between 1710 MHz and 1910MHz. Pins 5 and 8 of the

device provide the RF input and output for the GSM

band, which is optimized for performance between

880MHz and 915 MHz. Both bands include an internal

DC-blocking capacitor to protect the device from exter-

nal DC source inputs and block internal DC from exit-

ing the inputs and outputs of the module. The

performance is similar to the performance of the

RF2173 and RF2174 MIMIC devices used in dual- or

triple-band applications. However, the RF3108 module

includes the matching and bypass capacitors required

for operation internal to the 9mmx10mm module.

However, some external components are required to

improve stability, isolation and noise power perfor-

mance. These components are included on the evalua-

tion board and schematic, and will be described in the

following paragraphs.

The GSM 900 MHz band provides 32dB and the DCS/

PCS 1710MHz to 1910 MHz band provides 28dB of

small signal gain at full output power. Therefore, the

drive level required to fully saturate the output is

+4dBm for each band. Based upon HBT (Heterojunc-

tion Bipolar Transistor) technology, the part requires

only a single positive 3 V supply to operate to full spec-

ification. The DCS/PCS band input is located at pin 2

of the device and requires no external components.

The GSM 900MHz band input is located at pin 6 of the

device and also does not require external components.

However, a 180

Ω resistor is included at the input of the

GSM band to improve the input impedance and isola-

tion performance at low VAPC levels. The output for

both high and low bands are internally matched to 50

Ω

at the output of pin 14 and 8. A 50

Ω microstrip should

be used to interface to the input and output connec-

tions.

Power control for the GSM 900MHz band is provided

through pin 5 of the device, and pin 3 for the DCS/PCS

band. The VAPC inputs do not contain any internal

bypass capacitors and will require some external filter-

ing. Because the VAPC filtering capacitor is external to

the device, the user has the option of choosing a

capacitor value that meets the control loop BW and fil-

tering requirement for various applications. In most typ-

ical applications with a closed loop power control, the

recommended bypass capacitor for this input is

approximately 33pF for the GSM band, and 12pF for

the DCS/PCS band. However in open loop operation, a

10nF VAPC bypass capacitor is recommended for both

bands to filter noise from the external VAPC source. A

10nF capacitor is installed on pins 3 and 5 on the cur-

rent evaluation board (see the evaluation board sche-

matic). Noise on the VAPC input will degrade the noise

power performance of the device, so care should be

used to provide a clean VAPC input signal. This is espe-

cially important when measuring noise power or stabil-

ity performance.

The voltage supply VCC contains internal bypass

capacitors and inductors to filter unwanted noise on

the DC supply voltage. However, the main VCC input to

the device at pin 11 requires some additional bypass

capacitors as shown in the evaluation board sche-

matic. C5 (1 uF) and C4 (3.3uF) are required to

improve the stability performance.

All the internal ground connections are connected to a

series of ground pads located on the backside of the

package as shown in the pin out diagram. Pins 4, 9,

10, 12, and 13 are also ground connections. The final

stages of both bands are connected to the ground

pads on the backside of the package. Therefore this

ground connection is essential to dissipate heat and to

provide proper current flow. Refer to the evaluation

board layout as an example of the vias locations and

quantity required for proper connection.

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