User's Manual

044-05077 Rev. A
4-3
The primary function of the first loop is to provide an error signal for the second loop. The primary
function of the second loop is to amplify the error signal to cancel out spurious products developed
in the main amplifier. The input signal is amplified by a preamplifier and fed to a coupler and delay
line. The signal from the coupler is fed to the attenuator and phase shifter in the 1st loop. The
first loop control section phase shifts the main input signals by 180 degrees and constantly
monitors the output for correct phase and gain.
Figure 4-2. G3S-1900-80 Power Amplifier Module Functional Block Diagram
The 2nd loop control section obtains a sample of the distortion added to the output signals by the
main amplifiers, phase shifts the signals by 180 degrees, then feeds it to the error amplifier.
There it is amplified to the same power level as the input sample and coupled on to the main
output signal. The final output is monitored by the 2nd loop and adjusted to ensure that the signal
distortion and IMD on the final output is canceled out.
4-5.1. MAIN AMPLIFIER
The input and output of the amplifier employ two-stage, class AB amplifiers, which provide
approximately 25 dB of gain in the 60 MHz frequency band from 1930 to 1990 MHz. The amplifier
operates on +27 Vdc, and is mounted directly on a heat sink, which is temperature monitored by a
thermal sensor. If the heat sink temperature exceeds 85 °C, a high temperature fault occurs. The
alarm logic controls the transistor bias voltage, which shuts down the amplifier.
4-5.2. ERROR AMPLIFIER
The main function of the error amplifier is to sample and amplify the signal distortion level
generated by the main amplifier, to a level that cancels out the distortion and IMD when the error
signal is coupled onto the main signal at the amplifier output. The error amplifier is a balanced
multistage, class AB amplifier, has 75 dB of gain, and produces up to a 65-watt output. The
amplifier operates on +27 Vdc and is mounted directly on a heat sink.
4-5.3. AMPLIFIER MONITORING
In the main and error amplifier modules, all normal variations are automatically compensated for by
the feedforward loop control. However, when large variations occur beyond the adjustment