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4-3. RF OUTPUT LOAD
The load impedance should be as good as possible (1.5:1 or better) in the working
band for good power transfer to the load.
4-4. AMPLIFIER FUNCTIONAL DESCRIPTION
The NTGS86AA amplifier (figures 1-1 and 4-1) is a linear, single-channel power
amplifier that operates in the 25 MHz frequency band from 869 MHz to 894 MHz
at an output power of 25 watts. Each amplifier is a self-contained module and is
functionally independent of any other amplifier modules in the system. Each
amplifier module has an alarm board that monitors the amplifier performance. If a
failure or fault occurs in an amplifier module, it is transmitted to the host system
via an 18-pin Molex connector.
The amplifier is compliant to the requirements of FCC Part 22 and TIA/EIA IS97A
with respect to spurious emissions (see table 1-1). Constant gain is maintained by
continuously comparing active paths with passive references, and correcting for
small variations through the RF feedback controls. All gain variations, for example
those due to temperature, are reduced to the passive reference variations. The
amplifier module is comprised of:
A three-stage driver amplifier
A main amplifier
Alarm monitoring and control
A spurious emission detection DSP circuit
4-4.1. THREE-STAGE DRIVER AMPLIFIER
The input of the amplifier employs three stages of class AB amplification which
provide approximately 33 dB of gain in the 25 MHz frequency band from 869 MHz
to 894 MHz. The amplifier operates on +26 Vdc, and a bias voltage of +5 Vdc,
and is mounted directly on a heat sink. The alarm logic controls the +5 Vdc bias
voltage that shuts down the amplifier.
4-4.2. MAIN AMPLIFIER
The main amplifier employs class AB amplification for maximum efficiency. The
signal provides approximately 10 dB of gain in the 25 MHz frequency band. The
output from the main amplifier is typically 30 watts. The amplifier operates on +26
Vdc, and a bias voltage of +5 Vdc, and is mounted directly on a heat sink. The
alarm logic controls the +5 Vdc bias voltage that shuts down the amplifier.
