1-5
1-18.
RF AMPLIFIER CIRCUIT BOARD.
1-19.
Each RF power module is equipped with two RF amplifier circuit boards: 1) power amplifiĆ
er 1 and 2) power amplifier 2. The circuit boards are designed with Class E power am-
plifier circuitry. Each circuit board is designed to produce approximately 550 watts of RF
power. Figure 1-2 presents the RF amplifier circuit board circuitry. The RF amplifier
circuit boards are identical, therefore only power amplifier 1 will be explained.
1-20.
PRE-DRIVER CIRCUIT.
A +15 volt peak-to-peak square-wave signal at the carrier freĆ
quency is applied to a transformer on the power block motherboard assembly. The transĆ
former outputs two signals to inverter U5A.
1-21.
The output from U5A is applied through inverters U5B/U5C to high/low side driver U7
and U8. U7/U8 output high and low drive signals to driver circuit transistors Q3/Q5 and
Q4/Q6.
1-22.
DRIVER CIRCUIT.
The driver circuit consists of: 1) transistors Q3 and Q5 and 2) Q4 and
Q6. Q3/Q5 and Q4/Q6 are MOSFET transistors configured as a push-pull driver circuit.
The outputs of Q3/Q5 and Q4/Q6 are applied to MOSFET power transistors Q1 and Q2.
Operating potentials for the driver circuitry is provided by the RF 30 volt supply.
The supply is protected from over-voltage conditions by a regulator Q7. The regulator
limits the voltage to approximately 47 volts dc. Fuse F2 protects the +30 volt supply from
over-current conditions. Fuse F3 protects the driver circuit components from over-cur-
rent conditions.
1-23.
RF AMPLIFIER CIRCUIT.
The RF amplifier circuit consists of switching MOSFET transisĆ
tors Q1 and Q2. Q1 and Q2 are configured as a Class E switching amplifier network.
Class E power amplifier characteristics consist of: 1) the transistor drain-to-source voltĆ
age must be nominally zero immediately prior to the turn-on of the transistor and 2) the
time slope of the drain-to-source voltage waveform must be nominally zero prior to the
turn-on of the transistor. The Class E circuit results in: 1) reduced device dissipation
and lowers the transistor operating temperature which greatly increases the life of the
components, 2) an operating efficiency of 95% or greater, and 3) increased reliability when
operated into VSWR conditions.
1-24.
Additional characteristics of a Class E amplifier design is the application of dc power to
the amplifier transistors. The B+ and B- supplies are applied to RF choke L1 on the comĆ
biner assembly. The choke is connected to the primary center tap winding of combiner
transformer T1. The transistors are connected to the primary winding of the transform-
ers. The B- supply for the power amplifier is provided by the modulator circuit board.
The modulator outputs a dc voltage which varies with audio modulation and functions as
the RF ground for transistors Q1 and Q2. The RF ground potential will change in re-
sponse to the applied audio. Fuse F1 protects the power amplifiers from over-current
conditions.
1-25.
Transistors Q1 and Q2 operate together to generate approximately 550 watts of RF power.
Q1 operates 180 degrees out-of phase with transistor Q2. Inductors L1 through L7 imĆ
prove the efficiency of the drive circuit by storing the energy required to charge the input
capacitance of the transistors. Transzorbs D9/D10 prevent the gates of Q1/Q2 from damĆ
age by transients during power on and off. Capacitors C 44 through C50 and C51 through
C57 provide shaping for the Class E waveform. The RF power from power amplifier 1 is
combined with the 550 watts of RF power from power amplifier circuit board 2 at a com-
biner transformer on the power block motherboard assembly to generate 1100 watts of RF
power.
