Assembly Instructions

MXi501/1002U POWER AMPLIFIER HEATSINK ASSEMBLY
4 PA PALLET ASSEMBLY 21B2729G1
Drawing references: Figure 6 and Figure 7.
The Power Amplifier pallet is a FET amplifier assembly containing a stripline balun system. The baluns are used
for impedance matching and coupling signals at the input and output of the pallet. It is a practical implementation
of the coaxial-based solution on a broadside-coupled horizontal stripline structure.
The amplifier circuit is based on the Laterally Double-Diffused MOSFET Q1. Capacitors C1, C2, C3 and C4 along
with the associated printed circuit traces, form the matching network to the gates of Q1. Similarly, C5 through C14
along with associated stripline traces provide output matching on the drain of the device. C22, C23, C26, C27,
C28, C29 provide DC blocking of the supply. The rest of the capacitors provide DC blocking for bias voltage.
DC power +50VDC enters the module through a screw terminal connection VDD. This DC voltage is fed to the
drains of the device via L1 and L2 with bypass capacitors. Bias for the devices is provided via combination of
resistors, capacitors and stripline traces.
The bias circuit is located on the 21B2729 board at the input side of the pallet. It provides two functions; precision
gate bias setting and external shutdown. The pallet's shutdown function is used in certain applications where the
pallet is used as a driver amplifier.
The bias voltage is adjusted using R11 from a regulated source provided by U1. Thermistor R18 provides thermal
stability for the bias.
The source of the bias voltage is a reference IC, U1, which has its ground reference terminal connected to the
collector of Q5. Q5 is biased using R10 and R16 to provide thermal compensation.
When no Shutdown (5V) is applied at R7, the precision reference is active and provides 6.2VDC to the level
adjustment circuit R21, R11 and R17. When R11 has been set to achieve 1.4A bias current to the pallet, a bias
voltage of approximately 2.6 to 2.7VDC is applied to the gates of the FETs.
PUB11-014 Rev 1 July 2011 11-014-4 MXi501/1002U Power Amplifier Heatsink Assembly