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WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation
Diamond Series™
4.9.3.2 Push-Pull Pairs
The amplification is performed by common-source push-pull pair of LDMOS MRF-
183 transistors. There are two push-pull pairs, Q1/Q3 and Q2/Q4, in the UHF PA pallet,
making a total of 4 RF transistors. For proper operation, isolation between each push-
pull pair is needed.
4.9.3.3 Output Circuit
The output circuit is formed by:
a. Drain Bias Circuit. Provides the drain with the high-current 32 Volt supply, while
keeping the DC and RF circuitry’s “RF-decoupled”. It includes a resistive net-
work to avoid possible instabilities.
b. Output matching circuit. Consisting primarily of stepped microstrip lines and
capacitors, this circuit’s main function is to up-transform the output impedance’s
of the devices.
c. Output Balun transformer. Converts the two 180 degrees out-of-phase signals
coming from each drain into an unbalanced 50 ohm output.
d. Output 3dB hybrid. This hybrid combines the amplified signal from the two sepa-
rate push-pull pairs, isolates the output circuit of each push-pull pair and
improves output return loss.
4.9.4 Hybrids
Hybrids are a special RF circuit, whose properties are useful for combining RF power
from two sources, and for attenuating and phase-shifting RF signals.
The combiners and dividers incorporate hybrids. Therefore, it is important to
understand the rules under which hybrids operate, before trying to analyze the
subsystems in which they are applied.
A 3 dB quadrature hybrid is the most commonly encountered type. It is typically drawn
as shown in the diagram below. The four ports are (1) input, (2) 0
o
output, (3) -90
o
output, and (4) reject. This type of hybrid is symmetrical, which means that the device
operates by the same rules for waves incident on either pair of ports.
Other types of hybrids, with different amplitude and phase relationships, also exist and
are useful for non-symmetrical combining, subtracting one signal from another, etc.
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