RF5111_1 Datasheet, PDF (7/14 Pages) RF Micro Devices – 3V DCS POWER AMPLIFIER

RF5111
Because of the inverting stage at the APC input, the current through the PIN diode is inverted from the APC voltage. Thus, when
V_APC is high for maximum output power, the attenuator is turned off to obtain maximum drive level for the first RF stage. When
V_APC is low for maximum isolation, the attenuator is be turned on to reduce the drive level and to avoid self-biasing.
The PIN diode is dimensioned such that a low V_APC the impedance of the diode is about 50 Ohm. Since the input impedance
of the first RF stage become very high when the bias is turned off, this topology will maintain a good input impedance over the
entire V_APC control range.
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 1.5A 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 +32dBm at
+85°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 +36dBm 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 is 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 the 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 RF5111 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.
Rev A1 DS060921
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