RF2174
Theory of Operation and Application Information
The RF2174 is a three-stage device with 28 dB gain at full power. Therefore, the drive required to fully saturate the output 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 external to the part. The opti-
mum load impedance at the RF Output pad is 4.5-j3.9Ω. With this match, a 50Ω terminal impedance 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 resis-
tive divider with high value resistors on this pin to VPC 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 connected.
VCC
RF IN
PIN
From Bia
750 Ω 500 Ω
5 kΩ
Stages
APC
2 kΩ
AT_EN
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 sig-
nal 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.
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Rev A8 DS060918
RFMD [ RF MICRO DEVICES ]
