Pre-Driver Power Amplifier; Driver Power Amplifier; Final Power Amplifier; Antenna Switch - Hytera MD78X Service Manual

UHF2 (450-520MHz 45W) Information
11.1.3

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.
11.1.4

Driver Power Amplifier

The driver power amplifier is formed by Q804, with variable gain controlled by voltage VGG. The
maximum output power is 38.5dBm, and the maximum variable gain is 11dB.
11.1.5

Final Power Amplifier

The final power amplifier consists of Q802 and Q803, with the maximum output power of 48.5dBm,
and the maximum variable gain of 10dB. The input matching circuit is composed of C815, C839, R804,
R806, R807, R813, R818 and R861, while the power combining circuit is composed of C820, C8330,
C831, C819, C812, C841 and C842.
11.1.6

Antenna Switch

The antenna switch is used to switch between the transmitter circuit and the receiver circuit.
In TX mode, the switching transistor Q810 is controlled by TX signal, so as to supply 9V 3A to the PIN
diodes D801 and D802. Quiescent bias current of the PIN diode is controlled by the resistors R821
and R825. In this mode, D801 and D802 are turned on; RF signals are applied to the low-pass filter
(composed of C871, C816, C836, C837, C838, L809, L813, and L810) and then transmitted via the
antenna port.
In RX mode, TX signal controls the switching transistor Q810, to further control the power supply for
the PIN diodes D801 and D802, which are not applied with DC bias current. When the two PIN diodes
are turned off, signals feed into the RX path through the low-pass filter (composed of L871, C852 and
C862).
11.1.7

Low-pass Filter

A low-pass filter that consists of inductors (L809, L813 and L810) and capacitors (C836, C837, C830
and C838), is used to attenuate harmonic energy from the transmitter.
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Service Manual