RF Technology PA50
Page 5
5 CIRCUIT DESCRIPTION
5.1 Driver Stage
5
Circuit Description
The following descriptions should be read as an aid to understanding the block and
schematic diagrams.
5.1 Driver Stage
The function of the driver stage is to transform the RF input signal from the exciter to
the appropriate levels and impedance required by the output stage. The RF input signal
is applied to the high pass filter ( C121, C130, C131, C132, C133, L102 and L106 ).
This filter suppresses any residual low frequency spurious that may be present at the
exciter output. T100 is used to transform the nominal 50 ohm output from the exciter
to match the input of Q102. A negative feedback network ( C120, R130, R132,
R131, R133, R125, R129, L107 and R124 ) is used to achieve a reasonably flat gain
across the frequency of operation and to match the drive level requirements of the
PA50 to the output level provided by the exciter. DC bias is applied to the gate of
Q102 via R120 and R121 and is decoupled by C106 and C116. The DC feed for Q102
drain is decoupled by L100, C112, C102, C100 and C118. T101 is used for impedance
matching between the driver and output stage and to provide a balanced feed for the
push-pull output stage
5.2 Output Stage
The output stage amplifies the 5 – 10 W from the driver to in excess of 150 W for final
delivery into the low pass filter. Q100 and 101 are driven in push-pull configuration
by the outputs of T101. L105, R126, C126 and L104 R127 C127 form negative
feedback paths in order to achieve a reasonably flat frequency response across the
frequency of operation. T102 is used to provide the DC feed via the centre tap in the
primary winding to Q100 and Q101. T102 also combines the balanced output of the
transistors into a single ended output as well as provide the necessary impedance
transformation from the transistors into a nominal 50 ohms. A small DC bias is
applied to the gates of Q100 and Q102 via R118, 119, 122 and 123 and decoupled by
C103, 113, 107 and 117. The DC feed to the transformer is decoupled by L103, L101,
C101, 119, 104, 114, 105, 115 and 128.
5.3 Low Pass Filter
The low pass filter attenuates spurious emissions to less than –86dBc.
5.4 Bias Supply
The function of the bias supplies is to provide a temperature compensated bias voltage
to the gates of Q100 Q101 and Q102 for adjustment to their quiescent currents. U205
and R213, R214 and R215 provide the bias voltages. U200D provides a decreasing
output voltage with increasing temperature. The resistive dividers R214, R229 and
Summary of Contents for Pa50 Eclipse Series
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