2-281
RF2174
Rev A8 060918
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 fre-
quency of the gain peak. This supply should be
bypassed individually with 100pF capacitors before
being combined with VCC for the output stage to pre-
vent 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 con-
ditions. The data shown in this data sheet is based on
a 12.5% duty cycle and a 600
μs pulse, unless speci-
fied 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 sys-
tem 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
damaged from too much power dissipation. At 5.0V,
over +36dBm may be produced; however, this level of
power is not recommended, 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 trans-
mit, 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 tem-
peratures and may force early failures. The RF2174
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
compensate the currents due to ambient temperature
variations.
To avoid excessively high currents it is important to
control the VAPC when operating at supply voltages
higher than 4.0V, such that the maximum output power
is not exceeded.
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