Circuit Description:
Balun T1 splits the phase of the input signal for push-pull operation. Tuned L-
network (C2, L1, L2) matches the exciter's 50-Ohm input to the composite load
presented by the gates of Q1 and Q2. VR1 provides a regulated bias source for
Q1 and Q2 and trimpots R5, R9 set Idq individually. T2 with C6,C8 transform
output of Q1, Q2 to up 50-ohms. A half-wave low-pass filter L3,L4 suppresses
harmonics. Two 150-Ohm resistors provide negative feedback to enhance
linearity and stabilizes input impedance. Five watts drive typically yields 140
watts key-down output. Spurious emissions meet the -60 dBc FCC requirement
(part 97.307e). Because MOS-FETs are not prone to thermal runaway, no bias
compensation has been provided. Heat-sink temperature should be monitored
periodically during prolonged SSB and CW operation, and a muffin fan is
required for FM and other high duty-cycle modes. The load-fault protection
circuit's strip-line detector drives a thyristor to cut power to T/R switching when
SWR exceeds 2.5:1. The load-fault circuit remains latched off until the problem is
identified and the main power switch is cycled. A hash filter on the DC line
suppresses unwanted RF noise that may occur when using marginally filtered
switching supplies.
Important Operating Precautions:
1. Always connect a 50-Ohm load or antenna exhibiting 2:1 SWR or better
before applying power and keying the amplifier.
2. Do not exceed 5-Watts drive unless an appropriate attenuator has been
configured inside the amplifier. Also, never use the exciter's RF-output control to
reduce amplifier drive to 5 Watts -- its ALC circuitry cannot react fast enough to
suppress momentary spikes that can damage the FET gates.
3. Always install a fan or other convective air source to cool the heat sink when
transmitting on FM, AM, FSK, AFSK, or high duty-cycle digital modes.
4. Never exceed 14.5 volts when powering the amplifier.
5. Do not attempt to amplify signals without first connecting a key line between
your exciter and amplifier*.
*Transmitting through the amplifier with no key-line connection will not damage
the radio or amplifier -- the exciter signal simply passes through to the antenna
un-amplified. An accessory RF-detection board with adjustable sensitivity may be
used in conjunction with transceivers lacking external amplifier keying.
