AT7120 Owner Manual Rev. 1.1
– March 2009 32/76
The trimpot TPO-2 adjusts VGS of T2 and, consequently, the drain bias current (ID) of this transistor.
This adjust is made by measuring the loss of voltage in resistor R18 (2 x 10Ω/5W in parallel), as the
voltage is adjusted to 1V, which corresponds to 1A in the ID.
Note:
Because this is Class A stage, the measurement of T2’s drain current must be the same with or without
input RF signal in the module.
Trimmer C32 adjust the gain associate with T2 and is optimized for the highest value possible.
Third Stage – Class AB
This final amplifier device is the LDMOS transistor T3 (MRF373). This amplifier stage is configured as
a Class AB to assure superior efficiency without sacrificing linear performance.
The drain voltage in transistor T3 is obtained from the +28V main power supply – Module 4401. This
voltage is available at the pins 1, 2, 3, 4, 5 and 6 out of the CON-3 connector found in Module 4407.
The trimpot TPO-3 simultaneously adjusts the VGS for transistor T3 and, consequently, the drain bias
current (ID) of this transistor. This adjust is made by measuring the loss of voltage in resistor R40
(0.12Ω/5W), as the voltage is adjusted to 50mV, which corresponds to 416mA in the ID (in standby).
Note:
Because this is Class AB stage, the measurement of T3’s drain current increases as the input RF level
increases in the module. The correct measurements are obtained when the module is in standby, in other
words, there is no input RF signal in the module.
VGS Polarization Circuit for T2 and T3
Integrated circuit CI-5 is responsible for providing VGS to transistors T2 and T3. This circuit is powered
by the voltage regulator CI-3, which is powered by the +18V main power supply, Module 4401. This
voltage is present at pin-4 of the connector CON-5, Module 4407.
Temperature alarm
Module 4407 is protected against overheating. If the temperature rises above 65˚C, the protection circuit
acts and disconnects the module.
The protection circuit is composed basically of the thermal sensor S1 and the CI-2. The temperature of
the module converted into a DC voltage can be measured at pin-3 of connector CON-5. This voltage is
routed to pin-7 of connector CON-6 at CIP8231 - control unit. The lack of this voltage means that the
temperature of Module 4407 exceeded the 65˚ C. Under this circumstance, the control unit CIP8231 will
no longer allow the +28V power supply to feed the module.
RF Power Measurements
Through a PCB type directional coupler at CIP8208 board, a RF output sampling is collected in Module
4407. This RF sampling is sent to the direct and reversed power reading circuits.