RED PT500-2, Service Manual

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PT500-2 Service Manual
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Figure 5.5 Schematic Diagram for Power Amplifier and Antenna Switch The
modulated RF signals from VCO are amplified by Q101, Q102 and Q105 before the power
amplification in Q107.
Gate bias of Q105 and Q107 is controlled by APC circuit, so the output power of transmitter
can be controlled conveniently by changing the gate bias voltage.
APC
（
Automatic Power Control
）
Figure 5.6 Schematic Diagram for APC Circuit
R130 R131 and R132 are power amplification current detector, IC100A is power
amplification current sampling amplifier and IC00B is power comparison amplifier.
The power amplification current and IC100A output will increase with oversized output power
of transmitter. When the output voltage of IC100B decreases, the bias voltage of Q105 and
Q107 will decrease, finally the output power of transmitter will decrease or vice versa. Thus,
the output power of transmitter will keep stable under any different working condition. MCU
can set the power by changing the voltage input to IC100B.
Audio Signal Processing of Transmitter