User's Manual

AT7120 Owner Manual Rev. 1.1 – March 2009 36/76
Cell A – 35W Amplifier
The amplification cell A is obtained by a Push-Pull 35W amplifier, class AB. Transistor T1 is in this
cell. The polarization circuit of cell A is made of a VGS timer circuit with temperature compensation,
differential amplifier and gate impedance reducer. TPO-1 trimmer adjusts the VGS to the T1 transistor.
Adjusting the VGS1, also adjusts the quiescent current (automatic adjustment of the T1 transistor’s
current). This current is measured indirectly, by dividing the reduction in voltage measured between
resistors R34 and R35 (that are in parallel) and its resulting ohm value, that is 0.06Ω.
Trimmers C7, C10 and C13 adjust input paring up and this cell’s gain. The circuit RL made of resistors
R27 and inductor L8, plus resistor R26 and inductor L7 avoid transistor T1’s auto-oscillation in
frequencies below 200 MHz, for which it is not pared up. Resistors R25 and R28 together with transistor
T3 reduce gate impedance for DC. Capacitors C37, C14 and C15 work as DC non-couplers in transistor
T1’s gate.
Cell B – 35W Amplifier
Just as cell A, cell B’s amplification is obtained by a Push-Pull 35W amplifier, class AB. Transistor T2
is in this cell. The polarization circuit in cell B also is made of a VGS timer circuit with temperature
compensation, differential amplifier and gate impedance reducer. TPO-2 trimmer adjusts the VGS to the
T2 transistor. After adjusting the VGS2, an automatic adjustment of the T2 transistor’s current. This
current is measured indirectly, by dividing the reduction in voltage measured between resistors R36 and
R37 (that are in parallel) and its resulting ohm value, that is 0.06Ω.
Trimmers C23, C26 and C29 adjust input paring up and this cell’s gain. The circuit RL made of resistors
R30 and inductor L6, plus resistor R31 and inductor L5 avoid transistor T2’s auto-oscillation in
frequencies below 200 MHz, for which it is not pared up. Resistors R29 and R32 together with transistor
T4 reduce gate impedance for DC. Capacitors C38, C30 and C31 work as DC non-couplers in transistor
T2’s gate.
Output Coupler
It is a hybrid coupler built with a special semi-rigid coaxial cable, better known as “wireline”. This type
of cable has 2 internal conductors interlinked; this set has the characteristics of a hybrid coupler. As the
input coupler, this coupler also has four gates (input/output; isolated, -3dB 0˚, -3 dB 90˚).
The same concept used with the input coupler is used in the output coupler, what differs is that its
function now is to add amplified signals by the 35W cells. The unbalanced output charge is represented
by resistor R38 (50Ω/60W). This charge guarantees that the module’s output impedance is always 50Ω,
even if one of the cells comes to fail.
Temperature Alarm
Module 4442 has a protection circuit that disarms the module in case the temperature rises above 65˚ C.
This circuit is basically made of thermal sensor S1, DC amplifier and voltage comparator CI-1. The
module temperature is converted into voltage, DC, by the temperature sensor (10 Mv/˚C). This voltage
value is amplified and then compared with a reference voltage, thus getting an alarm voltage, which can
be measured on the green cable connected to the module. This voltage is sent to pin 2 of connector
CON-7 in the subordinate feeding power supply corresponding to the amplification module. In this