4-26 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
4.9.5 Driver Stage
The driver stage V16 and V17 is voltage driven from the secondary of T2
and with emitter resistors R52 to R55 provides a current drive to the output
stage. L20, C78 and R56 provide damping for a harmonic resonance in T3.
The bias for V16 and V17 is provided by the current of the pre-driver stages
passing through the diode-connected transistor V13. The bias may be
increased by R46, the current through R46 causing a voltage drop across
R47 in addition to the base-emitter voltage of V13. The current in V16 and
V17 may be measured at the link in the DC feed to T3.
4.9.6 Output Stage and Bias Regulator
The output stage V18 and V19 is current fed from the secondary of T3 with
low frequency current feedback through R58 and R59 from a winding on the
output transformer T4. C93, C94 with C116 and C81 with C82 provide
frequency compensation for T3 and T4 respectively.
The balanced output auto transformer also contains a balun winding so that
one output can be AC coupled to ground and the other output capacitively
coupled to the load.
The bias regulator (V11 and V15) provides a constant voltage to the bases of
V18 and V19. V11 and V15 form a feedback voltage regulator, the output
voltage being the base-emitter voltage of V11. R76 and V23 cause the bias
to increase at low supply voltage (10V approx) to reduce intermodulation
distortion.
V11 is mounted on the PA heatsink and provides temperature compensation
based on heatsink temperature.
When low power is selected (8528S only), V21 is turned on by V20
collector going high. This increases the bias voltage via R71 and therefore
the bias current in V16 and V17. This is required to improve the
intermodulation distortion at low power.
4.9.7 Output Filters and Control
The output of the PA is passed through one of six low-pass filters to remove
harmonics generated in the PA. These filters are relay selected (K1 to K5
and K8) by the band lines generated by the microprocessor controller and
buffered by IC1.
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