Vhf (136-174Mhz 25W) Information; Transmitter Circuit; Power Amplifier; Pre-Driver Power Amplifier - Hytera MD78X Service Manual

VHF (136-174MHz 25W) Information

14. VHF (136-174MHz 25W) Information
14.1

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).
14.1.1

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 version (including version D).
14.1.2

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.

Figure 14-1 Diagram of Transmitter Circuit

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Service Manual