User's Manual

AT7120 Owner Manual Rev. 1.1 – March 2009 39/76
5. To adjust T2’s quiescent current, slowly adjust trimpot TPO-2 clock wise until T2 is set, when
then some loss in voltage, few mV, will be observed. Continue adjusting TPO-2 until its voltage
measures 60mVdc, what translates to a drain current of 900mA for the BLF861A transistor
Notes
The BLF861A transistor is type Gemini; this means there are two identical transistors inside the capsule.
That is why one trimpot was omitted when balancing the VGS voltage. Another characteristic of this
transistor is that the drains are connected internally, so when a 900mA current is measured, it really
means that each side of the transistor has a drain of 450mA.
Because this is a phase amplifier class AB, the measured drain current of the LDMOS transistors
increases as the RF level increases in the input of the module. The correct values are obtained with the
module in standby, in other words, without input RF signal.
In the case that these adjustments are not possible, measure the gate resistance with a multimeter (this
resistance got to be at least 330 ohms). If the gate resistance is close to 0 ohm, it is possible that the
transistor is broken.
Check for temperature alarm
Module 4442 has a thermal sensor (S1) that when connected to operational circuits (CI-1), generate
alarms that detect when the module temperature is above 65˚C.
1. Power Module 4442 with +28V and wait for the pre-heating to be done for around 5 minutes. It
is not necessary for Module 4442 to be in maximum power; actually it is better to do this adjust
with Module 4442 in standby, in other words, without input RF signal.
2. Measure voltage on pin-2 of thermal sensor S1.
3. Measure voltage on pin-2 of operational circuit CI-1; this voltage is exactly ten times higher than
the value of the voltage measured on pin 2 of the thermal sensor, and it relates to the temperature
value in Module 4442 (i.e.: for a temperature of 50˚C, it should measure 5V).