RF5110_1 Datasheet, PDF (6/14 Pages) RF Micro Devices – 3V GSM POWER AMPLIFIER

RF5110
Theory of Operation and Application Information
The RF5110 is a three-stage device with 32 dB gain at full power. Therefore, the drive required to fully saturate the output is
+3dBm. 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 2000mA.
The optimum load for the output stage is approximately 2.6Ω. 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 2.6-j1.5Ω. 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 V_PC and ground. For nominal operation, however, no external adjustment is
necessary as internal resistors set the bias point optimally.
VCC1 and VCC2 provide supply voltage to the first and second stage, as well as provides some frequency selectivity to tune to
the operating band. Essentially, the bias is fed to this pin through a short microstrip. A bypass capacitor sets the inductance
seen by the part, so placement of the bypass cap can affect the frequency of the gain peak. This supply should be bypassed
individually with 100pF capacitors before being combined with V_CC for the output stage to prevent feedback and oscillations.
The RF OUT pin provides the output power. Bias for the final stage is fed to this output line, and the feed must be capable of
supporting the approximately 2A of current required. Care should be taken to keep the losses low in the bias feed and output
components. A narrow microstrip line is recommended because DC losses in a bias choke will degrade efficiency and power.
While the part is safe under CW operation, maximum power and reliability will be achieved under pulsed conditions. The data
shown in this data sheet is based on a 12.5% duty cycle and a 600μs pulse, unless specified otherwise.
The part will operate over a 3.0V to 5.0V range. Under nominal conditions, the power at 3.5V will be greater than +34.5dBm at
+90°C. As the voltage is increased, however, the output power will increase. Thus, in a system design, the ALC (Automatic
Level Control) Loop will back down the power to the desired level. This must occur during operation, or the device may be dam-
aged from too much power dissipation. At 5.0V, over +38dBm may be produced; however, this level of power is not recom-
mended, and can cause damage to the device.
The HBT breakdown voltage is >20V, so there are no issue with overvoltage. However, under worst-case conditions, with the RF
drive at full power during transmit, and the output VSWR extremely high, a low load impedance at the collector of the output
transistors can cause currents much higher than normal. Due to the bipolar nature of the devices, there is no limitation on the
amount of current de device will sink, and the safe current densities could be exceeded.
High current conditions are potentially dangerous to any RF device. High currents lead to high channel temperatures and may
force early failures. The RF5110 includes temperature compensation circuits in the bias network to stabilize the RF transistors,
thus limiting the current through the amplifier and protecting the devices from damage. The same mechanism works to com-
pensate the currents due to ambient temperature variations.
To avoid excessively high currents it is important to control the V_APC when operating at supply voltages higher than 4.0V, such
that the maximum output power is not exceeded.
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Rev A0 DS050318