Parts List

2-2 VHF Amplifi er Instruction Manual VT-3 132 - 174 MHz
2.3 RF Circuitry
The RF c ircuitry consists of several blocks: a 7 dB input pad (R1, R2, and R3), an RF amplifier
module (U4), an output power boosting transistor (Q7), three directional couplers (TL1, TL3, and
TL4), and the VT-3/150 Lowpass Filter. The heart of the VT-3/150 Ampl ifier is RF amplifier
module U4. The output of U4 is further amplified by Q1 to a maximum of 8.0 Watts at the antenna
connector. The frequency band of the VT-3/150 Amplifier is determined by the operating frequency
range of the RF amplifier module U4. D irectional couplers (TL1, TL3, and TL4) are used to sample
forward and reverse power. The sampled power is used by the sensing and power control circuits to
control the amplifier's operation. The final step in the RF path is output filtering and, as ment ioned
earlier, this is done by the VT-3/150 Lowpass Filter.
2.3.1 VT-3/150 Lowpass Filter
The VT-3/150 Lowpass Filter is a 50 ohm, 9 pole, reciprocal filter with a 3 dB cutoff frequency of
approximately 185 MHz. The lowpass filter assembly attenuates the desired signal's harmonics as
well as any other out-of-band emissions so that a 'clean' RF signal is output to the antenna
connector.
2.4 Power Control Circuitry
The VT-3/150 Amplifier employs a closed loop power control which uses a sample of the forward
RF power to control the gain of RF amplifier U4. Op-amp U1b compares the sampled RF voltage
to the output power setpoint and generates an error signal which Q3 uses to control the voltage on
U4's gain control pin. The output power setpoint is determined by R7, the Output Power Adjust
potentiometer.
The forward power is sampled by TL1, D5, C14, R13, L6, R12, and C15. The power control
circuitry keeps the output power of U4 constant. Therefore the amplifier's output power will
fluctuate with variat ions in the +13.8 Vdc supply voltage.