6: Power Amplifier Board
6-2
PRC1099A-MS
6.1.1
Predriver Amplifier
Predriver amplifier U1 is a class A amplifier with fixed bias. The predriver
amplifier stage includes a gain leveling network that consists of C59, R60,
R61, R62, and R64. The output from the collector is coupled through
broadband transformer T1. The stage is powered from the regulated T8 line
and is switched off in receive mode.
6.1.2
Driver Amplifier
The driver amplifier stage consists of linear broadband amplifiers Q2 and Q3
in a push-pull amplifier configuration. The driver amplifiers operate in class
AB with a regulated bias supply and grounded emitters. The input from the
predriver amplifier is coupled through broadband transformer T1. The driver
amplifiers uses collector-to-base feedback provided through R7A and C14,
and R10 and C10. The emitters are grounded. The output is coupled to the
final amplifier through broadband tubular-type, ferrite-loaded transformer T2
with a push-pull primary and secondary. The driver amplifiers operate directly
from the 12V supply and is switched off by the bias supply (DBIAS).
6.1.3
Final Amplifier
The final RF amplifier stage consists of RF power amplifiers Q4 and Q5 in a
push-pull configuration. The RF power amplifier transistors are designed for
SSB service in the HF range. The input from the driver amplifiers is coupled
through T2. The transistors operate in class AB using collector-to-base
feedback R16/C20 and R13/C23. The transistor bias is derived from the bias
regulator circuits.
Output transformer T3 is a tubular-type, ferrite-loaded transformer with a
push-pull primary and a single-ended 50 ohm secondary. The final amplifier
stage operates directly from the 12V supply and is switched on and off by
controlling the bias supply. The final amplifier is capable of power outputs of
30 to 40W.
6.1.4
Bias Regulators
Driver amplifier bias regulator circuit U3 compensates for temperature
variations by adjusting the driver amplifier bias voltage. Driver amplifier Q3
temperature is sensed by diode D1. The current through D1 increases as
temperature increases, which decreases the DT SENS voltage. DT SENS is
scaled by R48 and R33 and then applied to the non-inverting input of op-amp
U3. As the voltage decreases, the output of Q6 (DBIAS) decreases, lowering
the base bias to driver amplifiers Q2 and Q3. The quiescent operating point
for the driver stage is set by adjusting R19.