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6
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The 2W UHF Amplifier is a variable gain, class A module that provides amplification to the
selected UHF channel. The amplifier is a three-stage, class A, microstrip design capable of 38dB
gain and 2 watts peak visual output. The first stage is centered around broadband monolithic
amplifier U1 rated for 10dB of gain. U1 is provided bias from 9 volt regulator U2 through filter FB1
and parallel resistors R1/R2. Input and output signal coupling is supplied by capacitors C1/C2 with
B+ bypass furnished by C3 through C6 and C16/C17. At the output of amplifier U1 is 90
(
hybrid
coupler HY1 configured as a variable gain control using PIN diodes CR1/CR2 and the GAIN
ADJUST voltage applied to E3. As the E3 GAIN ADJUST voltage is increased (using the
AMPLIFIER GAIN ADJUST A2PC4R7 on the Control/Interface board), PIN diodes CR1/CR2 begin
to conduct, creating a low impedance at the 0
(
and 90
(
ports of the hybrid. Due to mismatch
generated at these two ports, the resulting reflected power is added in phase at the hybrid’s ISO
port affording lower attenuation for the through signal. Conversely, as the GAIN ADJUST voltage
decreases, the PIN diodes conduct less generating a higher impedance at the 0
(
and 90
(
ports
making 47 ohm resistors R5/R6 electrically more apparent as power absorbing loads. With less
power reflected back into the hybrid, less power will appear at the ISO port making for increased
attenuation of the through signal. Variable gain adjust HY1 is capable of 3dB to 18dB of
attenuation.
From HY1 the UHF signal is coupled into field-effect transistor Q1 via capacitor C9 and an input
matching circuit made up of capacitor C10 and a tuned microstrip network. Biased from 9V
regulator U2 through resistors R11/R12, Q1 furnishes 12dB of signal amplification and with a drain
current of approximately 140mA. Capacitors C12/C13/C14/C15 supply RF bypassing for the bias
voltage with C11/C18 acting as signal coupling capacitors positioned at the input and output of 3dB
matching attenuator R13/R14/R15. At the input of 8 pin DIP power FET Q2 is a microstrip network
with matching capacitors C19/C20/C24 and variable capacitor C35. Connected directly to the
28 volt supply, Q2 is powered through a current regulator circuit centered on transistor Q5 and
drain resistors R20 through R23. This circuit continuously maintains the drain voltage and current
of Q2 over a wide variation of load and temperature. The required collector voltage and current
of this device is established by potentiometer R19, which controls the conduction of regulator Q5,
ultimately setting the 500mA drain current of Q2. At the output the amplified signal proceeds
through another matching network made up of capacitors C31/C32 and various microstrip tuning
elements. Coupling capacitor C33 passes the signal from Q2 to output connector J2.
Within the 2W UHF Amplifier module, fault circuits monitor the status of RF devices U1, Q1 and
Q2. Centered on transistors Q3/Q4/Q5, these fault circuits detect the presence of an open RF
device within the module and send a signal to the Exciter’s front panel AMPLIFIER STATUS
indicator. Under normal operation, transistors Q3/Q4/Q5 are forward biased generating approx-
imately 8.5 volts at collector resistors R4/R9/R26 pushing diodes CR3/CR4/CR5 into reverse bias.
With CR3/CR4/CR5 not conducting, the voltage at pin 5 of comparator U3 will be approximately
7.5 volts as dictated by resistive divider R27/R28. When compared to the 5.7 volt level provided
by divider R28/R30 at pin 6, the higher voltage at pin 5 will force U3 high, lighting the AMPLIFIER
STATUS LED (A2PC3DS1) to indicate the amplifier is operating properly. Due to the low created
on its collector resistor (R4/R9/R26), one of the back biased diodes would begin to conduct taking
U3-5 lower than the reference voltage at pin 6. This action would immediately create a low at the
output of U3 (pin 7) extinguishing the front panel AMPLIFIER STATUS light indicating the amplifier
module should be repaired or replaced.