Service Manual
17. VHF (136-174MHz 25W) Information
17.1 Transmitter Circuit
The transmitter circuit is composed of power amplifier circuit and power control circuit. The former is
used to amplify the VCO signal to the desired output power level, while the latter can keep the output
power at the desired level, so as to protect the power amplifier from damage caused by over heat,
antenna mismatch, and out-of-range voltage (over voltage or under voltage may result in damage to
Q801, Q805, Q804, Q802 and Q803).
Figure 17-1 Diagram of Transmitter Circuit
17.1.1 Power Amplifier
The 3-stage power amplifier amplifies the signal output from VCO to the required output power level. The
first stage is a pre-driver power amplifier circuit with variable gain (formed by Q805); the second stage is
a driver power amplifier circuit with variable gain (formed by Q804); and the final stage is the final power
amplifier circuit with fixed gain (formed by Q802 and Q803). In addition, this power amplifier contains a
TX/RX switch and a low-pass filter.
Note: only a final power amplifier (Q803) is available in later versions (including version D).
17.1.2 Pre-driver Power Amplifier
The pre-driver power amplifier is formed by Q805, with variable gain controlled by voltage VGG. The
input matching circuit is composed of C826, L826, L814, C843 and R824.
17.1.3 Driver Power Amplifier
The driver power amplifier is formed by Q804, with variable gain controlled by voltage VGG. The
maximum output power is 34.5dBm, and the maximum variable gain is 15dB.
17.1.4 Final Power Amplifier
439
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