across R11. The zener diode VR3 will conduct at approximately 22V and forward bias
the transistor.
Set up Procedure
•
Set up a power supply to 30V; current limited to 3.5A.
•
Connect the power supply to E1 of the IPA. The current drawn should be
approximately 1.75A.
•
Remove the coax jumper.
•
Apply a sweep signal to the preamplifier input, being careful not to
overdrive the test equipment. The gain should be at least 18dB to 20dB
and the return loss better than 15dB in the frequency range of interest.
•
Connect a 30dB attenuator to the output of the amplifier.
•
Apply a low level sweep to the amplifier and measure the gain (be sure to
take into account the 30dB pad). Gain should be about 13dB to 14dB.
POWER AMPLIFIER MODULE
CIRCUIT DESCRIPTION
The PA modules used in the 250W amplifier employ dual FET RF transistor
packages in push-pull configuration. The transistors operate Class AB and are biased for
0.6A static drain current per transistor.
The input to the PA module is applied at J1 from the power splitter board.
Transformer T1 divides the input into two equal and opposite signals. Capacitors C3, C4
and the associated strip line circuitry form an impedance matching network to the gates
of the FETs. C1 and C2 are DC isolation (blocking) capacitors. Capacitors C13 and C16
and associated strip line inductors form an output impedance matching network and T2
combines the out of phase signals at the output J2. C14 and C15 also act as blocking
capacitors.
The 50V DC supply is connected to each drain through the 5A fuses and
decoupling inductors L4 and L5. Two resistive divider networks comprising R1 through
R10 apply gate bias voltage to the FETs to set the static drain current. Zener diodes CR1
and CR2 provide a constant voltage 20V for the bias voltage adjustment. The
FET gate bias required for the recommended 0.6A of quiescent drain current may be
anywhere between 2V and 5V depending on the individual FET. Capacitors C6, C7, C9
through C12, C17 and C18 are RF bypass capacitors.
Set up Procedure
•
Set up a 50V power supply; current limited to 1.2A.
•
Turn both bias potentiometers for maximum resistance.
•
Apply the supply to the one transistor at a time (one- half package) and
adjust the corresponding bias resistor for 0.6A drain current. This setting
