Uhf3 (350-400Mhz 45W) Information; Transmitter Circuit; Power Amplifier; Tx Buffer Amplifier - Hytera MD78X Service Manual

Service Manual

13. UHF3 (350-400MHz 45W) Information

13.1

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).
13.1.1

Power Amplifier

The 4-stage power amplifier amplifies the signal output from VCO to the required output power level.
The first stage is a buffer amplifier circuit with fixed gain; the second stage is a pre-driver power
amplifier circuit with variable gain (formed by Q805); the third 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.
13.1.2

TX Buffer Amplifier

The first stage (Q801) provides about 16dB power gain, and adjusts its bias circuit to get a quiescent
bias current of 28mA (for Q801). Power supply for the switching transistor Q810 is controlled by TX
signal (enabled by antenna switch), so as to further control the power supply for the whole circuit.

Figure 13-1 Diagram of Transmitter Circuit

391
UHF3 (350-400MHz 45W) Information