RF5111 Datasheet, PDF (6/14 Pages) RF Micro Devices – 3V DCS POWER AMPLIFIER

RF5111
Theory of Operation and Application Information
The RF5111 is a three-stage device with 28 dB gain at full power. Therefore, the drive required to fully saturate the out-
put is +5dBm. Based upon HBT (Heterojunction Bipolar Transistor) technology, the part requires only a single positive
3V supply to operate to full specification. Power control is provided through a single pin interface, with a separate Power
Down control pin. The final stage ground is achieved through the large pad in the middle of the backside of the package.
First and second stage grounds are brought out through separate ground pins for isolation from the output. These
grounds should be connected directly with vias to the PCB ground plane, and not connected with the output ground to
form a so called “local ground plane” on the top layer of the PCB. The output is brought out through the wide output pad,
and forms the RF output signal path.
The amplifier operates in near Class C bias mode. The final stage is “deep AB”, meaning the quiescent current is very
low. As the RF drive is increased, the final stage self-biases, causing the bias point to shift up and, at full power, draws
about 1500mA. The optimum load for the output stage is approximately 4.5Ω. This is the load at the output collector, and
is created by the series inductance formed by the output bond wires, vias, and microstrip, and 2 shunt capacitors exter-
nal to the part. The optimum load impedance at the RF Output pad is 4.5-j3.9Ω. With this match, a 50Ω terminal imped-
ance is achieved. The input is internally matched to 50Ω with just a blocking capacitor needed. This data sheet defines
the configuration for GSM operation.
The input is DC coupled; thus, a blocking cap must be inserted in series. Also, the first stage bias may be adjusted by a
resistive divider with high value resistors on this pin to V_PC and ground. For nominal operation, however, no external
adjustment is necessary as internal resistors set the bias point optimally.
When the device is driven at maximum input power self biasing would occur. This results in less isolation than one would
expect, and the maximum output power would be about -15dBm. If the drive power to the PA is turned on before the
GSM ramp-up, higher isolation is required. In order to meet the GSM system specs under those conditions, a PIN diode
attenuator connected to the input can be turned on. The figure below shows how the attenuator and its controls are con-
nected.
VCC
RF IN
750Ω 500Ω
APC
AT_EN
5 kΩ
2 kΩ
PIN
From Bias
Stages
The current through the PIN diode is controlled by two signals: AT_EN and APC. The AT_EN signal allows current
through the PIN diode and is an on/off function. The APC signal controls the amount of current through the PIN diode.
Normally, the AT_EN signal will be derived from the VCO ENABLE signal available in most GSM handset designs. If
maximum isolation is needed before the ramp-up, the AT_EN signal needs to be turned on before the RF power is
applied to the device input. The current into this pin is not critical, and can be reduced to a few hundred micro amps with
an external series resistor. Without the resistor, the pin will draw about 700μA.
2-6
Rev A1 060921